The Cambridge Natural History Vol. 3 (of 9)

CONTENTS

Scheme of the Classification adopted in this Book.

SCHEME OF THE CLASSIFICATION ADOPTED IN THIS BOOK

MOLLUSCA

BRACHIOPODA

LIST OF MAPS

MOLLUSCS

CHAPTER I INTRODUCTION—POSITION OF MOLLUSCA IN THE ANIMAL KINGDOM—CLASSIFICATION—ORIGIN OF LAND AND FRESH-WATER MOLLUSCA — Origin of the Fresh-water Fauna — Origin of the Land Fauna

It is the generally accepted opinion among men of science that all life originated in the sea. Not that all parts of the sea are equally favourable to the development of forms of life. The ocean surface, with its entire absence of shelter or resting-place, and the deep sea, whose abysses are always dark and cold and changeless, offer little encouragement to plant or animal life, as an original starting-point. True, both the surface and the depths of the sea have become colonised by myriads of forms, Mollusca amongst them, but these quarters are in the truest sense colonised, for the ancestors of those who inhabit them in all probability migrated from elsewhere.

It was no doubt the littoral region and the shallow waters immediately below it, a region of changeable currents, of light and shade, of variation, within definite limits, of temperature and tide effects, which became the scene of the original development of plant life, in other words, of the food-supply which rendered possible its colonisation by higher animals. But the littoral region, besides the advantages of tenancy which it offers to animal life, has also its drawbacks. The violence of the surf may beat its inhabitants in pieces, the retreat of the tide exposes them, not merely to innumerable enemies in the shape of predatory birds and beasts, but also to a change in the atmospheric medium by which they are surrounded. Hence, in all probability, have arisen the various forms of adaptation which are calculated to bring about the ‘survival of the fittest’; hence, to narrow our point of view to the Mollusca, the development of hard shells, or exoskeletons, hence the sand-burrowing, rock-boring, rock-clinging instincts of various genera and species.[1]

What was the primitive form of molluscan life is little likely to be ever positively known, although, on grounds of comparative anatomy, something approaching to the archi-mollusc is often constructed, with more or less probability, by careful observers. From one of the oldest known geological strata, the Cambrian, nearly four hundred species of Mollusca are known, which include representatives of nearly all the great Orders as they exist at the present day, and without the slightest sign of approximation to one another. With regard to the origin of the land and fresh-water Mollusca some definite conclusions can be arrived at, which will be given in their proper place.

Scarcely any portion of the coast-line of the world is destitute of molluscan life, except in regions where extreme cold forbids its existence. Thus along the shores of Northern Asia there is no proper littoral fauna, the constant influence of travelling ice sweeping it all away; animal life begins at about three fathoms. But in every coast region not positively hostile to existence Mollusca make their home. Each description of habitat has its own peculiar species, which there flourish best, and exist precariously, if at all, elsewhere. Thus the sandy waste of estuaries, the loose and shingly beaches, the slimy mud-flats beset with mangroves, the low stretches of jagged rock, and even the precipitous cliffs, from whose base the sea never recedes, have all their own special inhabitants. The same is true of the deep sea, and of the ocean surface. And when we come to examine the land and fresh-water Mollusca, it is found not merely that some Mollusca are terrestrial and others fluviatile, but that certain species haunt the hills and others the valleys, some the recesses of woods and others the open meadow sides, some prefer the limestone rocks, others the sandy or clayey districts, some live only in still or gently moving waters, while others are never found except where the current is rapid and powerful.

It is within the tropics that the Mollusca become most numerous, and assume their finest and quaintest forms. A tropical beach, especially where there is a good tide-fall and considerable variety of station, abounds in molluscan life to an extent which must literally be seen to be believed. The beach at Panama, to select an instance familiar to the present writer, is astonishingly rich in species, which probably amount in all to several hundreds. This is due to the immense variety of habitat. On the rocks at high-water mark, and even above them, occur Truncatella, Melampus, Littorina, and Siphonaria; where a mangrove-swamp replaces the rock, on the branches overhead are huge Littorina, while three species of Cerithidea crawl on the mud, and Cyrena and Arca burrow into it. Lower down, in the rock pools at half-tide mark are Cerithium, Purpura, Omphalius, Anachis (2 sp.), Nassa, and several Crepidula. At low-water mark of ordinary tides, under stones half buried in clean sand, are Coecum and Vitrinella; under the blocks which rest on solid rock are Cypraea (4 or 5 sp.), Cantharus, more Anachis, Columbella (3 sp. including the graceful C. harpiformis), and Nitidella. Where the blocks of rock are rather muddy, Conus lurks, and with it Turritella and Latirus. Where the rocks form a flat-topped platform 2 or 3 feet high, with here and there a deep crack, huge Chitons 3 inches long conceal themselves, with two species of Turbo, Purpura, and Clavella. At extreme low-water mark of spring tides, on the isolated rocks are Monoceros, Leucozonia, and Vermetus, in them are Pholas and a burrowing Mytilus, under them are more Conus, Dolium, and huge frilled Murices. Patches of clean gravelly sand here produce Strombus; on the operculum of the great Str. galea is sure to be a Crepidula, exactly fitting its breadth. On the liquid mud-flats to the north glide about Marginella, Nassa, and Truncaria, in the clean sandstretch to the west Olivella ploughs about by hundreds with several species of Natica, and Tellina and Donax bury themselves deep, while farther down are Artemis, Chione, and, where mud begins to mix with the sand, Mytilus and more Arca. Each of these species has its own habitat, often circumscribed to a few square feet at the most, and it would be utterly useless to seek for it anywhere except in its own special domain.

Equally abundant are the land Mollusca of the tropics. Prof. C. B. Adams relates that within the limits of a single parish in Jamaica, named Manchester, which measures no more than four miles long and one mile broad, he obtained no fewer than one hundred species. Mr. J. S. Gibbons, in a description of the Mollusca he obtained near St. Ann’s, Curaçao, gives a lively picture of their abundance in an exceptionally favoured locality:—[2]

“Near the outskirts of the town a waste piece of ground supplied me with occupation for all the time I had to spare. Neither grass nor water was to be seen, the only vegetation consisting of a few stunted cacti and still fewer acacia bushes. This, however, was so rich in shells that of several species enough specimens could have been collected in a few yards to supply, I should suppose, all the shell cabinets in the world.... The stones, plants, and ground were covered with Strophia uva L., Tudora megacheila, P. and M., was in equal abundance, suspended by its silk-like thread from acacia boughs, or strewed thickly on the ground underneath. A Bulimulus (B. multilineatus var. sisalensis) abounded on the smaller boughs, while under masses of coral Macroceramus inermis Gundl., Pupa parraiana d’Orb, and P. pellucida Pfr., were abundant. In the loose soil Cylindrella Raveni Bland, Cistula Raveni Bland, and a curious Cionella were so numerous that a spade would have been the best instrument with which to collect them. I wasted a good deal of valuable time in separating them from the soil, when by simply taking away a few handfuls of mould, I might have obtained a larger number of specimens. A species of Stenogyra and a Succinea complete a list, all of which might have been gathered from almost any square yard of ground on the hillside.”

Position of Mollusca in the Animal Kingdom.—Up to very recent times it was usual to regard the Mollusca as one of the four subdivisions of a great family known as Malacozoa, the subdivisions being (1) Mollusca, (2) Tunicata, (3) Brachiopoda, (4) Polyzoa or Bryozoa. This classification is still retained in the leading modern manual on the subject.[3] The progress, however, of investigation leads to the belief that the Mollusca are not so closely related to these other groups as such a classification would seem to imply. The Tunicata, for instance, appear, from the whole course of their development, to occupy a position near to the Vertebrata. The relations of the Brachiopoda and Polyzoa will be more particularly referred to in that part of this History which deals especially with those groups. The position of the Mollusca is, in many respects, one of considerable isolation. Any attempt, therefore, definitely to relate them to one group or another, is, in all probability, to go further than the present state of our knowledge warrants. Especially to be deprecated are systems of classification which confidently derive the Mollusca in general from this or that group. The first undisputed traces of animal life, which appear in the Cambrian epoch, exhibit the same phyletic distinctions as now exist. Sponges, Echinoderms, Mollusca, and Worms, formed already, in those immeasurably remote ages, groups apparently as generally distinct from one another as they are at the present time. It would seem that any theory of development, which confidently teaches the derivation of any one of these groups from any other, is, in the present state of the evidence before us, hazardous in the extreme.

Some indications of relationship, which must not be pushed too far, may be drawn from a consideration of embryonic resemblance. An especial characteristic of the Mollusca is the possession of a particular form of larva, which occurs in one of the stages of development, known as the trochosphere (see p. 130). This form of larva is shared with two orders of Annelida, the Chaetopoda and the Gephyrea armata, and, in all probability, with the Polyzoa as well. It may also be significant that the adult form in Rotifera bears a close resemblance to the trochosphere larva in those groups.

Basis of Classification.—The Mollusca are divided into four great Orders—Cephalopoda, Gasteropoda, Scaphopoda, and Pelecypoda.[4] Each name, it will be noticed, bears reference to the ‘foot,’ i.e. to the organ of motion which corresponds in function to the foot in the Vertebrata.

In the Cephalopoda the feet, or, as they are more frequently termed, the ‘arms,’ are arranged symmetrically round the head or mouth. The common forms of ‘cuttle-fish’ (Octopus, Loligo) are familiar examples of Cephalopods.

The Gasteropoda crawl on the flat under-surface or ‘sole’ of the foot. Snails, slugs, sea-hares, whelks, periwinkles, and coats-of-mail or chitons are examples of this Order.

The Scaphopoda possess a long tubular shell open at both ends; with their small and elongated foot they are supposed to dig into the mud in which they live. The common Dentalium or tusk-shell of our coasts is a representative of this Order.

Fig. 1.—Examples of the four Orders: A, Cephalopoda; B, Gasteropoda; C, Scaphopoda, and D, Pelecypoda.

A, Ommastrephes sagittatus Lam., Naples: a, a, arms surrounding the mouth; f, funnel; t, t, the two ‘tentacular’ arms, × ⅖. B, Buccinum undatum L., Britain: f, foot; pr, proboscis. × ½. C, Dentalium entalis L., Norway: f, foot. D, Cardium oblongum Chem., Naples: f, foot; s, efferent or anal siphon; s’, efferent or branchial siphon, × ½.

The Pelecypoda[5] are enclosed in a bivalve shell fastened by a muscular hinge, the adjacent part of the valves being generally more or less toothed; the foot is as a rule roughly comparable to the shape of an axe-head.

To these four Orders is frequently added a fifth, the Pteropoda, whose exact position is at present not absolutely settled. The Pteropoda[6] are ‘pelagic,’ i.e. they live in the open waters of the ocean, rising to the surface at night, and sinking into cooler water by day. They are provided with a pair of wing-like appendages or ‘feet,’ on each side of the head, by means of which they are enabled to swim. Some authorities regard the Pteropoda as a subdivision of Gasteropoda, others as forming a separate Order, of equivalent value to the other four. The question will be further discussed below (see chap. xv.), but for the present it will be sufficient to state that the weight of evidence appears to show that the Pteropoda are modified Gasteropoda, with special adaptations to pelagic life, and are therefore not entitled to rank as a separate Order.

Some writers conveniently group together the first three of these Orders, the Cephalopoda, Gasteropoda, and Scaphopoda, under the title Glossophora,[7] or Mollusca furnished with a radula or ribbon-shaped ‘tongue,’ set with rows of teeth and situated in something of the nature of a head, as distinguished from the Aglossa (or Lipocephala),[8] i.e. those Mollusca which have no radula and no head. To the latter belong only the fourth Order, the Pelecypoda. This view postulates, for the primitive ancestral Mollusc, a body with a more or less developed head, and possibly the rudiments of an apparatus for grinding or triturating food. This form, it is held, either developed or degenerated. In the former case, in consequence of the more active mode of life upon which it may be supposed to have entered, it gave rise to all the more highly organised forms which are grouped under the three great Orders. When, on the other hand, the ancestral form associated itself with an inactive or sedentary life, it was, we may believe, modified accordingly, and either lost by atrophy or failed to acquire those special points of organisation which characterise the highly-developed form. Hence the Pelecypoda, or bivalves, whose characteristic is the absence of any definite cephalic region or masticatory apparatus. It is a remarkable fact in support of this theory of the origin of the Aglossa that certain of their larvae are known to possess traces of higher organisation, e.g. an external mouth and eyes, the former of which becomes covered by the mantle lobes, while the latter disappear long before the adult stage is reached.

Thus we have

Classification of Gasteropoda.—The Gasteropoda are numerically very largely in excess of the two other Orders of the Glossophora, far more complicated as regards classification, and contain a large proportion of those examples of the Mollusca which are most familiar to the ordinary observer. It will therefore be convenient to postpone for the present a fuller discussion of the subdivisions of the Cephalopoda and Scaphopoda, as well as of the Aglossa, returning to them again in special chapters (chaps. xiii. and xvi.), and to devote a few introductory words to the classification and relations of the Gasteropoda.

The Gasteropoda are divided into four Classes, Amphineura, Prosobranchiata, Opisthobranchiata, and Pulmonata.

Fig. 2.—An example of the Polyplacophora: Chiton spinosus Brug.

Fig. 3.—An example of the Aplacophora, Neomenia carinata Tullb.: a, anus; gr, ventral groove; m, mouth.

(1) The Amphineura[9] are bilaterally symmetrical Mollusca, i.e. with organs either single and central, or paired and disposed on either side of the longer axis of the animal. The shell, when present, is never spiral, but consists of eight overlapping plates, kept together by an elliptical girdle. The Amphineura are divided into (a) Polyplacophora,[10] or Chitons, and (b) Aplacophora (Chaetoderma and Neomenia).

(2) The Prosobranchiata[11] are so named from the fact that the breathing organ (branchia or ctenidium[12]) is as a rule situated in front of the heart, the auricle at the same time being in front of the ventricle. They are asymmetrical, almost always furnished with a shell, which is at some time spiral, and with an operculum. The sexes are separate. They are either marine animals, or can be shown to be more or less directly derived from genera which are marine. They are divided into (a) Diotocardia[13] (Haliotis, Fissurella, Trochus, Nerita, Patella), which have, or whose immediate ancestors are believed to have had, two auricles to the heart, two sets of breathing organs, two kidneys, but no proboscis, penis, or siphon, and (b) Monotocardia,[14] in which the heart has only one auricle, the true breathing organ is single, and there is a single kidney. To this division belong the great majority of marine univalve Mollusca, e.g. Cypraea, Buccinum, Murex, Littorina, Ianthina, all the land and fresh-water operculates (Cyclostoma, Melania, Paludina, etc.), as well as the Heteropoda, which are a group of Prosobranchiata which have betaken themselves to a pelagic life.

Fig. 4.—Example of a Heteropod, Carinaria mediterranea Lam., Naples: a, anus; br, branchia; f, foot; i, intestine; m, mouth; p, penis; s, sucker; sh, shell; t, tentacles. × ½. The animal swims foot uppermost.

(3) In the Opisthobranchiata[5] the breathing organs (when present) are behind the heart, and the auricle of the heart is consequently behind the ventricle. They are asymmetrical marine animals; usually, but by no means always, without a shell, scarcely ever with an operculum in the adult state. The sexes are united in the same individual. The Opisthobranchiata fall into two divisions: (a) Tectibranchiata, in which the breathing organ is more or less covered by the mantle, and a shell is usually present, which is sometimes rudimentary, e.g. Bulla, Aplysia, Umbrella, and the whole group of Pteropoda; (b) Nudibranchiata, or sea slugs, which have no shell and no true ctenidia, but breathe either by the skin, or by ‘cerata’ or papilliform organs prominently developed on the back: e.g. Doris, Aeolis, Dendronotus.

Fig. 5.—A, A Tectibranchiate Opisthobranch, Umbrella mediterranea Lam., Naples: a, anus; br, branchia; f, foot; m, mouth; rh, rhinophores; sh, shell.

B, A Pteropod, Hyalaea tridentata Forsk., Naples: sh, shell; l, l, swimming lobes of foot.

C, A Nudibranchiate Opisthobranch, Aeolis peregrina, Naples: f, foot; c, cerata.

Fig. 6.—Examples of—A, Pulmonata Basommatophora, the common Limnaea peregra Müll.: e, e, eyes; t, t, tentacles. B, Pulmonata Stylommatophora, Helix hortensis Müll.: e, e, eyes; t, t, tentacles; p. o, pulmonary orifice (the position of the pulmonary orifice in Limnaea will be seen by reference to Fig. 101).

(4) The Pulmonata[15] are asymmetrical air-breathing non-marine Mollusca, generally, but not always, furnished with a shell. The sexes are always united in the same individual, and the operculum is always wanting, except in Amphibola. They are conveniently divided into Stylommatophora,[16] in which the eyes are at the tip of the upper tentacles, which are retractile (Helix, Limax, Bulimus, and all true land slugs and snails), and Basommatophora, in which the eyes are at the base of the tentacles, which are not retractile (Limnaea, Planorbis, Physa, and all the Auriculidae).

Thus we have

The relation of the four great Orders to one another will be better discussed when we come to deal with each Order separately. The problem of the origin and mutual relationship of the various forms of molluscan life is of extreme subtlety, and its solution can only be approached after a comprehensive survey of many complicated anatomical details. But there is one branch of the Mollusca—the land and fresh-water genera—whose origin is, comparatively speaking, of recent date, and whose relationships are therefore less likely to have suffered complete obliteration.

Origin of the Land and Fresh-water Mollusca.—The ultimate derivation of the whole of the land and fresh-water molluscan fauna must, as has already been remarked, be looked for in the sea. In certain cases the process of conversion, if it may be so termed, from a marine to a non-marine genus, is still in progress, and can be definitely observed; in others the conversion is complete, but the modification of form has been so slight, or the date of its occurrence so recent, that the connexion is unmistakable, or at least highly probable; in others again, the modification has been so great, or the date of its occurrence so remote, that the actual line of derivation is obscured or at best only conjectural.

Fig. 7.—A, the common cockle (Cardium edule L.). B, Adacna plicata Eichw., Caspian Sea. C, Didacna trigonoides Pall., Caspian Sea.

This passage from a marine to a non-marine life—in other words, this direct derivation of non-marine from marine genera—is illustrated by the faunal phenomena of an inland brackish-water sea like the Caspian, which is known to have been originally in connexion with the Mediterranean, and therefore originally supported a marine fauna. The Mollusca of the Caspian, although without exception brackish- or fresh-water species, are in their general facies distinctly marine. Of the 26 univalve species which inhabit it 19 belong to 4 peculiar genera (Micromelania, Caspia, Clessinia, Nematurella), all of which are modified forms of the marine Rissoidae. The characteristic bivalves belong to the genera Adacna, Didacna, and Monodacna, all of which can be shown to be derived from the common Cardium edule. We have here a case where complete isolation from the sea, combined no doubt with a gradual freshening of the water, has resulted in the development of a number of new genera. The singularly marine facies of several of the fresh-water genera now inhabiting Lake Tanganyika, has given rise to the belief, among some authorities, that that lake was at one time an inlet of the Indian Ocean. In the upper waters of the Baltic, marine and fresh-water Mollusca flourish side by side. So complete is the intermixture, that an observer who had lived on no other shores would probably be unable to separate the one set of species from the other.[18] Thus between Dagö and Papen-Wiek[19] Mytilus edulis, Cardium edule, Tellina balthica, Mya arenaria, Littorina rudis, and Hydrobia balthica are the only true marine species; with these live Unio, Cyclas, Neritina, Limnaea, and Bithynia. The marine species and Neritina live down to 15–20 fath., the rest only down to 3 fath. Under stones close to the shore of the Skärgård at Stockholm[20] are found young Cardium and Tellina, and at 3 to 6 fath. Limnaea peregra, and Physa fontinalis. Near Gothland Limnaea is found in the open sea at 8–12 fath., and with it occur Cardium and Tellina. At the Frisches Haff[21] Mya arenaria is the only marine species, and lives in company with 6 sp. Limnaea, 1 Physa, 9 Planorbis, 1 Ancylus, 4 Valvata, 2 Sphaerium. Were the Sound to become closed, and the waters of the Baltic perfectly fresh, it would be inevitable that Mya arenaria, and such other marine species as continued to live under their changed conditions, should in course of time submit to modifications similar in kind to those experienced by the quondam marine species of the Caspian.

It seems probable, however, that the origin, at least in a great part, of the land and fresh-water Mollusca need not be accounted for by such involuntary changes of environment as the enclosure of arms of the sea, or the possible drying up of inland lakes. These cases may be taken as illustrations of the much more gradual processes of nature by which the land and fresh-water fauna must have been developed. The ancestry of that fauna must be looked for, as far as the Gasteropoda are concerned, in the littoral and estuarine species; for the Pelecypoda, in the estuarine alone. The effect of the recess of the tide, in the one case, and the effect of the reduced percentage of salt, in the other, has tended to produce a gradual adaptation to new surroundings, an adaptation which becomes more and more perfect. It may be safely asserted that no marine species could pass into a land or fresh-water species except after a period, more or less prolonged, of littoral or estuarine existence. Thus we find no land or fresh-water species exhibiting relationships with such deep-sea genera as the Volutidae, Cancellariidae, Terebridae, or even with genera trenching on the lowest part of the littoral zone, such as the Haliotidae, Conidae, Olividae, Capulidae. The signs of connexion are rather with the Neritidae, Cerithiidae, and above all the Littorinidae, which are accustomed to live for hours, and in the case of Littorina for days or even weeks, without being moistened by the tide. Similarly the fresh-water Pelecypoda exhibit relationships, not with genera exclusively marine, but with genera known to inhabit estuaries, such as the Mytilidae, Corbulidae, Cardiidae.

It would be natural to expect that we should find this process of conversion still going on, and that we should be able to detect particular species or groups of species in process of emigration from sea to land, or from sea to fresh water. Such species will be intermediate between a marine and a land or fresh-water species, and difficult to classify distinctly as one or the other. Cases of Mollusca occupying this intermediate position occur all over the world. They inhabit brackish swamps, damp places at high-water mark, and rocks only at intervals visited by the tide. Such are Potamides, Assiminea, Siphonaria, Melampus, Hydrobia, Truncatella, among the univalves, and many species of Cyrena and Arca among the bivalves.

(a) Pelecypoda.—Estuarine species, which have become accustomed to a certain admixture of fresh water, have gradually ascended the streams or been cut off from the sea, and have at last become habituated to water which is perfectly fresh.

Fig. 8.—A, The common Mytilus edulis L., a marine genus and species. B, Dreissensia, a fresh-water genus, closely allied to Mytilus.

Fig. 9.—A, Arca navicella Reeve, Philippines, a marine species. B, Arca (Scaphula) pinna Bens., R. Tenasserim, a fresh-water species which lives many miles above the tide-way.

Thus Dreissensia (rivers and canals throughout N. Europe and N. America) and Mytilopsis (rivers of America) are scarcely modified Mytili (Fig. 8); Scaphula is a modified Arca, and lives in the Ganges, the Jumna, and the Tenasserim at a distance of 1600 miles from the sea (Fig. 9). Pholas rivicola is found imbedded in floating wood on the R. Pantai many miles from its mouth. Cyrena, Corbicula, and probably Sphaerium and Pisidium are derived, in different degrees of removal, from the exclusively marine Veneridae; Potamomya (rivers of S. America), and Himella (R. Amazon) are forms of Corbula. The Caspian genera derived from Cardium (Adacna, Didacna, Monodacna), have already been referred to. Nausitora is a form of Teredo, which lives in fresh water in Bengal. Rangia, Fischeria, and Galatea probably share the derivation of the Cyrenidae, while in Iphigenia we have one of the Donacidae which has not yet mounted rivers, but is confined to a strictly estuarine life. The familiar Scrobicularia piperata of our own estuaries is a Tellina, which lives by preference in brackish water.

Fig. 10.—Trigonia pectinata Lam., Sydney, N.S.W.

The great family of the Unionidae is regarded by Neumayr[22] as derived from Trigonia, the points of similarity being the development of a nacreous shell, the presence of a strong epidermis, and the arrangement of the muscular scars. It is remarkable, too, that on many Uniones of Pliocene times there is found shell ornamentation of such a type as occurs elsewhere among the Pelecypoda only on Trigonia.

The genera of fresh-water Pelecypoda are comparatively few in number, and their origin is far more clearly discernible than that of any other group. This is perhaps due to the fact that the essential changes of structure required to convert a marine into a fresh-water bivalve are but slight. Both animals “breathe water,” and both obtain their nutriment from matter contained in water. Similar remarks apply to fresh-water operculate Gasteropoda. But the passage from a marine to an aerial life involves much profounder changes of environment, which have to be met by correspondingly important changes in the organism. This may be in part the reason why the ancestry of all Pulmonata, whether land or fresh-water, is so difficult to trace.

Fig. 11.—A, Cominella, a marine genus, which lives between tide marks, and from which is probably derived B, Clea, a genus occurring only in fresh water.

Fig. 12.—A, Cerithium columna Sowb. (marine). B, Potamides microptera Kien. (brackish water). C, Io spinosa Lea, one of the Pleuroceridae (fresh water).

(b) Gasteropoda.—(1) Operculate. Canidia and Clea are closely allied, with but little modification, to the marine Cominella[23] (Fig. 11), as is also Nassodonta to Nassa. They occur (in fresh water) in the rivers of India, Indo-China, Java, and Borneo, associated with essentially fresh-water species. Potamides, with its various sub-genera (Telescopium, Pyrazus, Pirenella, Cerithidea, etc.), all of which inhabit swamps and mud-flats just above high-water mark in all warm countries, are derived from Cerithium (Fig. 12); Assiminea, Hydrobia, and perhaps Truncatella, from Rissoa. It is a remarkable fact that in Geomelania (with its sub-genera Chittya and Blandiella) we have a form of Truncatella which has entirely deserted the neighbourhood of the sea, and lives in woody mountainous localities in certain of the West Indies. Cremnoconchus, a remarkable shell occurring only on wet cliffs in the ghâts of southern India, is a modified Littorina. Neritina and Nerita form a very interesting case in illustration of the whole process. Nerita is a purely marine genus, occurring on rocks in the littoral zone; one species, however, (N. lineata, Chem.) ascends rivers as far as 25 miles from their mouth, and others haunt marshes of brackish water. Neritina is the fresh-water form, some species of which are found in brackish swamps or even creeping on wet mud between tide marks, while the great majority are fluviatile, one group (Neritodryas) actually occurring in the Philippines on trees of some height, at a distance of a quarter of a mile from any water. Navicella is a still further modified form of Neritina, occurring only on wet rocks, branches, etc., in non-tidal streams (Fig. 13).

Fig. 13.—Illustrating the development of the fresh-water genus Navicella, through the brackish-water Neritina, from the marine Nerita, with corresponding changes in the operculum. 1. Nerita; 2, 3. Neritina; 4. Neritina, intermediate form; 5, 6. Navicella.

The great family of the Melaniidae, which occurs in the rivers of warm countries all over the world, and that of the Pleuroceridae, which is confined to North America, are, in all probability, derived from some form or forms of Cerithium. The origin of the Paludinidae, Valvatidae, and Ampullariidae is more doubtful. Their migration from the sea was probably of an early date, since the first traces of all three appear in the lower Cretaceous, while Melaniidae are not known until Tertiary times. Ampullaria, however, shows distinct signs of relationship to Natica, while the affinities of Paludina and Valvata cannot as yet be approximately affirmed.

(2) Pulmonata.—Intermediate between the essentially fresh-water and the essentially marine species come the group sometimes known as Gehydrophila, consisting of the two families Auriculidae and Otinidae. These may be regarded as Mollusca which, though definitely removed from all marine species by the development of a true lung or lung cavity in the place of a gill, have yet never become, in respect of habitat, genuine fresh-water species. Like Potamides, they haunt salt marshes, mangrove swamps, and the region about high-water mark. In some cases (Otina, Melampus, Pedipes) they live on rocks which are moistened, or even bathed by the spray, in others (Cassidula, Auricula) they are immersed in some depth of brackish water at high tide, in others again (Scarabus) they are more definitely terrestrial, and live under dead leaves in woods at some little distance from water. Indeed one genus of diminutive size (Carychium) has completely abandoned the neighbourhood of the sea, and inhabits swampy ground almost all over the world.

Fig. 14.—Examples of the Auriculidae: A, Auricula Judae Lam., Borneo; B, Scarabus Lessoni Blainv., E. Indies; C, Cassidula mustelina Desh., N. Zealand; D, Melampus castaneus Mühlf., S. Pacific; E, Pedipes quadridens Pfr., Jamaica.

Fig. 15.—An example of Amphibola (avellana Chem.), the only true Pulmonate which possesses an operculum.

To this same section Gehydrophila have been assigned two remarkable forms of air-breathing “limpet,” Siphonaria and Gadinia (see page 151), and the aberrant Amphibola, a unique instance of a true operculated pulmonate. Siphonaria possesses a pulmonary cavity as well as a gill, while Gadinia and Amphibola are exclusively air-breathing. Siphonaria lives on rocks at or above high-water mark, Gadinia between tide marks, Amphibola (Fig. 15) in brackish water at the estuaries of rivers, half buried in the sand. There can be little doubt that all these are marine forms which are gradually becoming accustomed to a terrestrial existence. In Gadinia and Amphibola the process is so far complete that they have exchanged gills for a pulmonary cavity, while in Siphonaria we have an intermediate stage in which both organs exist together. A curious parallel to this is found in the case of Ampullaria, which is furnished with two gills and a pulmonary chamber, and breathes indifferently air and water. It is a little remarkable that Siphonaria, which lives at a higher tide level than Gadinia, should retain the gill, while Gadinia has lost it.

The ultimate affinities of the essentially fresh-water groups, Limnaea, Physa, Chilina, cannot be precisely affirmed. The form of shell in Latia, Gundlachia, and perhaps Ancylus, may suggest to some a connexion with the Otinidae, and in Chilina, a similar connexion with the Auriculidae. But, in a question of derivation, similarities of shell alone are of little value. It is not a little remarkable, for instance, that we should find a simple patelliform shell in genera so completely distinct from one another in all anatomical essentials as Ancylus, Patella, Siphonaria, Propilidium, Hipponyx, Cocculina, and Umbrella.

Some recent authors, on grounds of general organisation, regard the Limnaeidae and their allies as Opisthobranchs adapted to an aerial life. It is held[24] that the Nudibranchiate Opisthobranchs have given birth to the Pulmonata Stylommatophora or land snails, and the Tectibranchiate Opisthobranchs to the Pulmonata Basommatophora or fresh-water snails. Such a view seems at first sight open to some objection from other views than those which deal simply with anatomy. The Opisthobranchiata are not, to any marked extent, littoral genera, nor do they specially haunt the mouths of rivers. On the contrary, they inhabit, as a rule, only the very lowest part of the littoral zone, and are seldom found, except where the water is purely salt. In other cases, when the derivation of land or fresh-water genera is fairly well established, intermediate forms persist, which indicate, with more or less clearness, the lines along which modification has proceeded. It has, however, recently been shown that Siphonaria[25] and Gadinia,[26] which have, as has been already mentioned, hitherto been classified as Pulmonata, are in reality modified forms of Opisthobranchiata, which are in process of adaptation to a life partly marine, partly on land. They may therefore be regarded as supplying the link, hitherto missing, between the land Pulmonata and the marine groups from one or other of which the latter must have been derived. The general consensus of recent opinion inclines towards accepting these views, some writers[27] being content to regard the Pulmonata, as a whole, as derived from the Tectibranchiate Opisthobranchs, while others[28] go further and regard the Stylommatophora as derived directly from the Basommatophora.

Gasteropoda.—(1) Operculate. On a priori grounds, one might predict a double origin for land operculates. Marine species might be imagined to accustom themselves to a terrestrial existence, after a period, more or less prolonged, of littoral probation. Or again, fresh-water species, themselves ultimately derived from the sea, might submit to a similar transformation, after a preliminary or intermediate stage of life on mudbanks, wet swamps, branches overhanging the water, etc. Two great families in this group, and two only, seem to have undergone these transformations, the Littorinidae and the Neritidae. The derivation of almost all existing land operculates may be referred to one or other of these groups.

Fig. 16.—Two rows of the radula of Littorina littorea L., × 72.

The power of the Littorinidae to live for days or even weeks without being moistened by the sea may be verified by the most casual observer. In the tropics this power seems even greater than on our own shores. I have seen, in various parts of Jamaica, Littorina muricata living at the top of low cliffs among grass and herbage. At Panama I have taken three large species of Littorina (varia, fasciata, pulchra), on trees at and above high-water mark. Cases have been recorded in which a number of L. muricata, collected and put aside, have lived for three months, and L. irrorata for four months.[29] These facts are significant, when we know that the land operculates almost certainly originated in a tropical climate.

The Cyclophoridae, Cyclostomatidae, and Aciculidae, which, as contrasted with the other land operculates, form one group, have very close relations, particularly in the length and formation of the radula, or lingual ribbon, with the Littorinidae.

Fig. 17.—Two rows of the radula of Cyclophorus sp., India, × 40.

On the other hand, the Helicinidae, Hydrocenidae, and Proserpinidae are equally closely related to Neritina. The Proserpinidae (restricted to the Greater Antilles, Central America and Venezuela) may perhaps be regarded as the ultimate term of the series. They have lost the characteristic operculum, which in their case is replaced by a number of folds or lamellae in the interior of the shell. It has already been noticed how one group of Neritina (Neritodryas) occurs normally out of the water. This group furnishes a link between the fresh-water and land forms. It is interesting to notice that here we have the most perfect sequence of derivatives; Nerita in the main a purely marine form, with certain species occurring also in brackish water; Neritina in the main fresh-water, but some species occurring on the muddy shore, others on dry land; Helicina the developed land form; and finally Proserpina, an aberrant derivative which has lost the operculum.[30]

Fig. 18.—A, Neritina reticularis Sowb., Calcutta (brackish water); B, Helicina neritella Lam., Jamaica (land); C, Proserpina (Ceres) eolina Ducl., Central America (land).

Gasteropoda.—(2) Pulmonata. The origin of these, the bulk of the land fauna, must at present be regarded as a problem not yet finally solved. Some authorities, as we have seen, regard them as derived from the Nudibranchiate, others, probably more correctly, from the Tectibranchiate Opisthobranchs.

The first known members of the land Pulmonata (Pupa [?], Hyalinia) are from the Carboniferous of North America. Similar but new forms appear in the Cretaceous, from which time to the present we have an unbroken series. The characteristically modern forms, according to Simroth,[31] are Helices with thick shells. According to the same author, Vitrina and Hyalinia are ancestral types, which give origin not only to many modern genera with shells, but to many shell-less genera also, e.g. Testacella is probably derived through Daudebardia from Hyalinia, while from Vitrina came Limax and Amalia. A consideration of the radulae of the genera concerned certainly tends in favour of these views.

Godwin-Austen, speaking generally, considers[32] genera of land Pulmonata with strongly developed mantle-lobes and rudimentary shell as more advanced in development than genera in which the shell is large and covers all or nearly all the animal.

CHAPTER II LAND AND FRESH-WATER MOLLUSCA, THEIR HABITS AND GENERAL ECONOMY

The majority of the Land Mollusca are probably more sensitive than is usually believed. The humidity of the air must affect the surface of their skin to a considerable extent. Every one has noticed how the snails ‘come out’ on a damp evening, especially after rain. As a rule, they wait till rain is over, probably objecting to the patter of the drops upon their delicate tentacles. Snails kept in captivity under a bell-glass are acutely sensitive of a damp atmosphere, and will bestir themselves after rain just as if they were in the open air. Certain Helices which are accustomed to live in moist places, will find their way to water, if removed from their usual haunts. A case is recorded[33] of a specimen of H. arbustorum, kept in a kitchen, which used to find its way directly under the cold water tap, and appeared to enjoy the luxury of a douche. How delicately the conditions of life are balanced in some of these creatures is seen in the case of Omalonyx, a genus akin to Succinea, which is found in Brazil and the northern parts of South America. It lives creeping on plants which overhang the margin of water, but perishes equally, if placed in the water itself, or removed to a distance from it for any length of time.[34]

Endurance of Heat and Cold.—The Mollusca are capable, at least as far as some species are concerned, of enduring severe extremes both of cold and heat. The most northern pulmonate yet observed is a fresh-water species, Physa (Aplecta) hypnorum L. This hardy mollusc, whose shell is so fragile as to need most careful handling, has been noticed on the peninsula of Taimyr, North Siberia, in 73° 30’ N. lat, a region whose mean annual temperature is below 10° F. with a range of from 40° F. in July to -30° F. in January.

It is well known that the Limnaeidae, and probably most fresh-water Mollusca of sub-temperate regions, can continue to live not merely under, but enveloped in ice, and themselves frozen hard. Garnier relates[35] that, during the winter of 1829–30, some large Limnaea auricularia, which had been placed in a small basin, were frozen into a solid mass, experiencing a cold of -2° F. He supposed they were dead, but, to his surprise, when the basin thawed, the Limnaea gradually revived. Paludina vivipara and Anodonta anatina have been known to resist a temperature of 23° F., and the former has produced young shortly after being thawed out of the ice.[36] As far north as Bodø in Norway (67° 37’ N. lat., well within the Arctic circle) there are found no less than fourteen species of terrestrial Mollusca, among them being Balea perversa and Clausilia rugosa.[37]

Vitrina is one of our most hardy molluscs, and may be observed crawling on bright mornings over the frost-covered leaves of a wood or copse. V. glacialis is said by Charpentier to live in the Alps at a height where the stones are covered with snow from nine to ten months of the year. Many of the Hyaliniae are very hardy. Arion, in spite of having no external shell to protect it, is apparently less affected by the cold than Helix, and does not commence hibernation till a later period in the autumn. The operculate land Mollusca, in spite of the protection which their operculum may be supposed to afford, are exceedingly sensitive to cold, and the whole group is without doubt a product of tropical or semi-tropical regions (see map at frontispiece). A species of Helicina which inhabits the southern States of North America has been known to be almost exterminated from certain districts by the occurrence of an unusually severe winter.

One of the highest altitudes at which a land shell is known to live appears to be the Liti Pass (Himalayas, 14,000 ft.). At this enormous altitude, two species of Buliminus (arcuatus Hutt. and nivicola Bens.) live on juniper bushes among patches of snow. An Anadenus is said to have been found in a similar locality at 15,000 ft., while Limnaea Hookeri has been taken from over 16,400 ft. in Landour. In the Andes of Peru and Bolivia, five species of Bulimulus, one of Pupa, and one of Limax occur at an elevation of 10,500 to 15,000 ft. Several fresh-water Mollusca inhabit Lake Titicaca, which stands at a height of 12,550 ft. in the Bolivian table-land.

In certain parts of the desert of Algeria, where there is not a trace of vegetation to be seen, and the temperature at mid-day is 110° F., the ground is sometimes so covered with Helix lactea as to appear perfectly white. Dr. F. H. H. Guillemard has told me that he noticed, in somewhat similar surroundings between Fez and Tangier, H. pisana in such extraordinary abundance that they hung from the low scrub in bunches the size of a man’s two fists. It is singular that Mollusca should live, and not only live, but flourish, in localities apparently so unpromising. Shells which occur in the Algerian Sahara are actually larger and altogether finer than the ordinary European form of the same species. In order to protect themselves to some extent against the scorching heat and consequent evaporation, desert species are frequently modified in one of two ways; the shell becomes either white or a light dusky brown, as in the familiar Helix desertorum, or else it gains immensely in thickness. Specimens of H. pomatia, recently procured from Fez, are of extraordinary thickness as compared with forms from our own chalk downs of Kent and Surrey.

Fresh-water Mollusca are frequently found inhabiting hot springs. Thus Neritina fluviatilis lives at Bagnères de Bigorre in water at about 68° F. In another hot spring in the eastern Pyrenees a Bithynia lives at a temperature of over 73° F.; while Blainville mentions another case of a Bithynia living in water at 122° F.

Hibernation and Aestivation.—As autumn begins to draw on, and the first frosts to nip vegetation, terrestrial species retire beneath stones, into cracks in old walls, holes in tree trunks, deep fissures in rocks, and nooks and crannies of every kind, or else bury themselves deeply in the earth or in moss and heaps of leaves. They thus commence their period of hibernation, which varies in length according to the duration of winter. Frequently masses of Helices may be found attached to one another, probably not so much for the sake of warmth, for their temperature is but low, as to share the comforts of a cosy retreat in common. Slugs generally hibernate alone, excavating a sort of nest in the earth, in which they encyst themselves, contracting their bodies until they are almost round, and secreting a covering of their own slime. The Helices usually close up the mouth of their shell by the formation of a membranous or chalky epiphragm, which will be further described below. Both snails and slugs take care to be in good condition at the time their winter sleep begins, and for this reason the former are said to be most esteemed by foreign epicures if captured just at this period.[38]

During hibernation, the action of the heart in land Pulmonata ceases almost entirely. This appears to be directly due to the effect of cold. Mr. C. Ashford has related[39] some interesting experiments made upon H. hortensis and Hyal. cellaria, with the view of ascertaining the effect of cold upon their pulsations. His observations may be tabulated as follows:—

At low temperatures the character, as well as the number of the pulsations changed; they became imperfect and intermittent, although exceptionally at 31° F. a H. rufescens gave five or six pulsations a minute, very full and deliberate. The result of taking the Hyalinia suddenly into the heat of a greenhouse was to bring on palpitations. Further experiments resulted in evidence of a similar kind. Hyal. radiatula, placed upon a deal table in a room, showed 52 pulsations per minute at 62° F. Placed upon the palm of the hand, the action soon rose to 108. Hyal. alliaria, similarly treated, rose from 72 pulsations to 110. Floated upon water, the action of the heart of the latter suddenly fell to 29.

Fresh-water Pulmonata do not appear to hibernate. Unio and Anodonta, however, bury themselves more deeply in the mud, and Dreissensia casts off its byssus and retires under the mud in deeper water.[40] Limnaea and Planorbis have often been noticed to crawl about under the lower surface of a thick coating of ice. In periods of prolonged drought, when the water in the ponds dries up, the majority of genera bury themselves in the mud. I have known Limnaea peregra bury itself three inches deep, when surprised by a sudden fall of the water in the ditch on Coe Fen, behind Peterhouse, Cambridge. Physa hypnorum frequents by preference ditches which dry up in summer, as does also Planorbis spirorbis, the latter often forming a sort of epiphragm against evaporation. Ancylus has been observed to spend the whole winter out of water, and P. spirorbis has been noticed alive after four months’ desiccation.[41]

True aestivation, however, occurs mainly in the tropics, where there is no winter, but only a period when it is not quite so hot as the rest of the year, or on a coast like the Mediterranean, which is subject to sudden and severe heat. This period is usually rainless, and the heat is therefore a dry heat. At this season, which may last for three or four months, most of the land Mollusca enter upon a period of inaction, either burying themselves deeply in the ground, or else permanently attaching themselves to the stalks of grass and other herbage, or the under sides of rocks. For instance, the large and beautifully painted Orthalicus, Corona, and Porphyrobaphe, which inhabit Brazil, Ecuador, and eastern Peru, bury themselves deeply in the ground during the dry season, while in the rains they climb to the topmost branches of the great forest trees.[42] Thus it may well happen that a visitor to a tropical island, Ceylon for instance, or one of the Greater Antilles, if he times his visit to coincide with the rainless season, may be grievously disappointed at what seems its unaccountable poverty in land Mollusca. But as soon as the weather breaks, and the moisture penetrates their retreats, every bush and every stone, in favoured localities, will be alive with interesting species.

The Epiphragm.—A considerable number of the land Pulmonata (and a very few of the fresh-water) possess the power of closing the aperture of their shell by means of what is known as an epiphragm or covering of hardened mucus. This epiphragm is habitually formed by certain species during hibernation or aestivation, or even during shorter periods of inactivity and retirement, the object being, either to check evaporation of the moisture of the body, or to secure the animal against the cold by retaining a thin layer of slightly warm air immediately within the aperture of the shell.

The epiphragm differs widely in character in different species, sometimes (Clausilia, Pupa, Planorbis) consisting of the merest pellicle of transparent membrane, while at others (Helix aperta, H. pomatia) it is a thick chalky substance, with a considerable admixture of carbonate of lime, with the consistency of a hardened layer of plaster of Paris. Within these extremes every variety of thickness, solidity, and transparency occurs. During long hibernation several epiphragms are not unfrequently formed by the same individual snail, one within the other, at gradually lessening distances. The epiphragm thus performs, to a certain extent, the part of an operculum, but it must be remembered that it differs radically from an operculum physiologically, in being only a temporary secretion, while the operculum is actually a living part of the animal.

The actual mode of formation of the epiphragm would seem to differ in different species. According to Fischer,[43] the mollusc withdraws into its shell, completely blocking all passage of air into the interior, and closing the pulmonary orifice. Then, from the middle part of the foot, which is held exactly at the same plane as the aperture, is slowly secreted a transparent pellicle, which gradually thickens, and in certain species becomes calcareous. Dr. Binney, who kept a large number of Helix hortensis in confinement, had frequently an opportunity of noticing the manner in which the epiphragm was formed.[44] The aperture of the shell being upward, and the collar of the animal having been brought to a level with it, a quantity of gelatinous matter is thrown out [? where from]. The pulmonary orifice is then opened, and a portion of the air within suddenly ejected, with such force as to separate the viscid matter from the collar, and to project it, like a bubble of air, from the aperture. The animal then quickly withdraws farther into the shell, and the pressure of the external air forces back the vesicle to a level with the aperture, when it hardens and forms the epiphragm. In some of the European species in which the gelatinous secretion contains more carbonate of lime, solidification seems to take place at the moment when the air is expelled, and the epiphragm in these is in consequence strongly convex.

Thread-spinning.—A considerable number of fresh-water Mollusca possess the power of stretching a thread, which is no more than an exceedingly elongated piece of mucus, to the surface of the water, and of using it as a means of locomotion. This thread bears no analogy whatever to the fibrous byssus of certain bivalves, being formed in an entirely different manner, without the need of a special gland.

The threads are ‘spun’ by several species of Limnaea, Physa, and Planorbis, by Bithynia tentaculata, and several of the Cycladidae. They are anchored to the surface by a minute concavity at the upper end, which appears to act like a small boat in keeping the thread steady. The longest threads are those of the Physae, which have been noticed to attain a length, in confinement, of 14 inches. They are always spun in the ascent, and as a rule, when the animal descends, it rolls the thread up and carries it down as it goes. A single thread is never spun on the descent, but occasionally, when a thread has become more or less of a permanence, it becomes stronger by the addition of more mucus each time it is used, whether for ascending or descending purposes. Cyclas cornea appears to be an exception to the rule that threads are only spun on the ascent. This species, which is particularly fond of crawling along the under surface of the water, has been noticed to spin a thread half an inch in length while on the surface, and to hang suspended from it for a considerable time.

What the exact use of the thread may be, must to a certain extent be matter of conjecture. The Limnaeidae are, in the great majority of cases, compelled to make periodic visits to the surface in order to inspire oxygen. It is also a favourite habit with them to float just under the surface, or crawl about on its under side, perhaps in pursuit of tiny vegetable organisms. Whatever may be the object of an excursion to the surface, a taut thread will obviously be a nearer way up than any other which is likely to present itself; indeed, without this thread-spinning power, which insures a tolerably rapid arrival at the surface, the animal might find itself asphyxiated, or at least seriously inconvenienced, before it could succeed in taking in the desired supply of oxygen. With the Cycladidae, which do not breathe air, such an explanation is out of place; in their case the thread seems to be a convenient means of resting in one position in the intervals of the periods of active exercise to which several of the species are so much addicted.

The power of suspension by a thread is also possessed by certain of the Cyclostomatidae, by some Cerithidea, several Rissoa and other marine genera, prominent among which is Litiopa bombyx, whose name expresses its power of anchoring itself to the Sargasso weed by a silken thread of mucus. Several species of slugs are known to be able to let themselves down by threads from the branches of trees. Limax arborum is especially noted for this property, and has been observed suspended in pairs during the breeding time. According to Binney, all the American species of Limax, besides those of Tebennophorus, possess this singular property. Limax arborum appears to be the only slug which has been noticed to ascend, as well as descend, its thread. It has also been observed[45] that when this species is gorged with food, its slime is thin and watery, and unable to sustain its weight, but that after the process of digestion has been performed, the mucus again becomes thick and tenacious. It appears therefore that when the animal is hungry and most in need of the power of making distant excursions in search of food, its condition enables it to do so, but that when no such necessity is pressing, the thread-forming mucus is not secreted, or is perhaps held in suspense while the glands assist in lubricating the food before digestion.[46]

Food of Land and Fresh-water Mollusca.—Arion ater, the great black slug, although normally frugivorous, is unquestionably carnivorous as well, feeding on all sorts of animal matter, whether decaying, freshly killed, or even in a living state. It is frequently noticed feeding on earthworms; kept in captivity, it will eat raw beef; it does not disdain the carcases of its own dead brethren. An old man near Berwick-on-Tweed, going out one morning to mow grass, found a black slug devouring, as he supposed, a dead mouse. Being of an inquisitive turn, and wishing to ascertain if it were really thus engaged, he drew the mouse a little back. When he returned in the evening, the mouse was reduced almost to a skeleton, and the slug was still there.[47] Indeed it would seem almost difficult to name anything which Arion ater will not eat. Dr. Gray mentions[48] a case of a specimen which devoured sand recently taken from the beach, which contained just enough animal matter to render it luminous when trodden on in the dark; after a little time the faeces of the slug were composed of pure sand, united together by a little mucus. A specimen kept two days in captivity was turned out on a newspaper, and commenced at once to devour it. The same specimen ate dead bodies of five other species of slugs, a dead Unio, pupae of Adimonia tanaceti, part of the abdomen of a dragon-fly, and Pears’ soap, the latter reluctantly.[49]

According to Simroth[50] and Scharff[51] the food of several of our British slugs, e.g. Limax maximus, L. flavus, Arion subfuscus, A. intermedius, consists of non-chlorophyllaceous substances only, while anything containing chlorophyll is as a rule refused. On the other hand L. agrestis and Amalia carinata feed almost entirely on green food, and are most destructive in gardens. The latter species lives several inches under ground during the day, and comes to the surface only at night. It is largely responsible for the disappearance of bulbs, to which it is extremely partial. L. marginatus (= arborum Bouch.) feeds exclusively on lichens, and in captivity absolutely refuses green leaves and a flesh diet. It follows therefore, if these observations are correct, that the popular notions about slugs must be revised, and that while we continue to exterminate from our gardens those species which have a taste for chlorophyll, we ought to spare, if not encourage those whose tastes lie in the opposite direction.

Limax agrestis has been seen devouring the crushed remains of Arion ater. Five specimens of the same species were once noticed busily devouring a May-fly each, and this in the middle of a large meadow, where it may be presumed there was no lack of green food. The capture and eating of insects by Mollusca seems very remarkable, but this story does not stand alone. Mr. T. Vernon Wollaston once enclosed in a bottle at least three dozen specimens of Coleoptera together with 4 Helix cantiana, 5 H. hispida, and 1 H. virgata, together with an abundant supply of fresh leaves and grass. About a fortnight afterwards, on the bottle being opened, it was found that every single specimen of the Coleoptera had been devoured by the snails.[52] Amalia marginata in captivity has been fed upon the larvae of Euchelia jacobaeae, eating three in two hours.[53]

Limax maximus (Fig. 19) has been seen frequently to make its way into a dairy and feed on raw beef.[54] Individuals kept in confinement are guilty of cannibalism. Mr. W. A. Gain kept three specimens in a box together, and found one of them two-thirds eaten, “the tail left clean cut off, reminding one of that portion of a fish on a fishmonger’s stall.” That starvation did not prompt the crime was proved by the fact that during the preceding night the slug had been supplied with, and had eaten, a considerable quantity of its favourite food. On two other occasions the same observer found one of his slugs deprived of its slime and a portion of its skin, and in a dying condition.[55] An adult L. maximus, kept for thirty-three days in captivity with a young Arion ater, attacked it frequently, denuded it of its slime, and gnawed numerous small pieces of skin off the body and mantle.[56] The present writer has found no better bait for this species on a warm summer night than the bodies of its brethren which were slain on the night preceding; it will also devour dead Helix aspersa. Mr. Gain considers it a very dainty feeder, preferring fungi to all other foods, and apparently doing no harm in the garden.

Fig. 19.—Limax maximus L. PO, pulmonary orifice: × ⅔.

Limax flavus, which is fond of inhabiting the vicinity of cellars, makes its presence most disagreeable by attacking articles of food, and especially by insinuating itself into vessels containing meal and flour.[57] It is particularly partial to cream.

Slugs will sometimes bite their captor’s hands. Mr. Kew relates that a Limax agrestis, on being stopped with the finger, while endeavouring to escape from the attack of a large Arion, attempted to bite fiercely, the rasping action of its radula being plainly felt. According to the same authority, probably all the slugs will rasp the skin of the finger, if it is held out to them, and continue to do so for a considerable time, without however actually drawing blood.[58] While Mr. Gain was handling a large Arion ater, it at once seized one of the folds of skin between the fingers of the hand on which it was placed; after the action of the radula had been allowed to continue for about a minute, the skin was seen to be abraded.[59] Another specimen of Arion ater, carried in the hand for a long time enclosed in a dock leaf, began to rasp the skin. The operation was permitted until it became too painful to bear. Examination with a lens showed the skin almost rasped away, and the place remained tender and sore, like a slight burn, for several days.[60]

Helix pisana, if freshly caught, and placed in a box with other species, will set to work and devour them within twenty-four hours. The present writer has noticed it, in this position, attack and kill large specimens of H. ericetorum, cleaning them completely out, and inserting its elongated body into the top whorls of its unfortunate victims in a most remarkable manner. Amongst a large number of species bred in captivity by Miss F. M. Hele,[61] was Hyalinia Draparnaldi. In the first summer the young offspring were fed on cabbage, coltsfoot, and broadleafed docks. They would not hibernate even in the severest frosts, and, no outdoor food being available, were fed on chopped beef. This, Miss Hele thinks, must have degenerated their appetites, for in the following spring and summer they constantly devoured each other.

Zonites algirus feeds on decayed fruit and vegetables, and on stinking flesh.[62] Achatina panthera has been known to eat meat, other snails (when dead), vegetables, and paper.[63] The common Stenogyra decollata of the South of Europe has a very bad character for flesh-eating habits, when kept in captivity. Mr. Binney[64] kept a number for a long time as scavengers, to clean the shells of other snails. As soon as a living Helix was placed in a box with them, one would attack it, introduce itself into the upper whorls, and completely remove the animal. One day a number of Succinea ovalis were left with them for a short time, and disappeared entirely! The Stenogyra had eaten shell as well as animal. This view of Stenogyra is quite confirmed by Miss Hele, who has bred them in thousands. “I can keep,” she writes,[65] “no small Helix or Bulimus with them, for they at once kill and eat them. They will also eat raw meat.”

Even the common Limnaea stagnalis, which is usually regarded as strictly herbivorous, will sometimes betake itself, apparently by preference, to a diet of flesh. Karl Semper frequently observed the Limnaeae in his aquarium suddenly attack healthy living specimens of the common large water newt (Triton taeniatus), overcome them, and devour them, although there was plenty of their favourite vegetable food growing within easy reach.[66] The same species has also been noticed to devour its own ova, and the larvae of Dytiscus. Limnaea peregra has been detected capturing and partially devouring minnows in an aquarium, when deprived of other food, and Dr. Jeffreys has seen the same species attack its own relatives under similar circumstances, piercing the spire at its thinnest point near to the apex.[67] L. stagnalis, kept in an aquarium, has succeeded in overpowering and partially devouring healthy specimens of the common stickleback.[68]

Powers of Intelligence, Homing, and finding Food.—It is not easy to discover whether land Mollusca possess any faculties which correspond to what we call intelligence, as distinct from their capacities for smell, sight, taste, and hearing. Darwin mentions[69] a remarkable case, communicated to him by Mr. Lonsdale. A couple of Helix pomatia, one of which was sickly, were placed in a small and ill-provided garden. The stronger of the two soon disappeared over the wall into the next garden, which was well furnished with food. It was concluded that the snail had deserted its weakly mate, but after twenty-four hours it returned, and apparently communicated the results of its expedition, for after a short time both started off along the same track, and disappeared over the wall. According to Dr. W. H. Dall,[70] a young girl who possessed a remarkable power over animals succeeded in training a snail (H. albolabris) to come out of its lurking-place at her call. If placed in a room, it would shrink into its shell at the sound of any other voice, but it would always start off in the direction of hers.

Snails and slugs possess to a considerable extent the faculty of ‘homing,’ or returning to the same hiding-place day after day, after their night excursions in search of food. Mr. C. Ashford once marked with a dab of white paint seven Helix aspersa found lurking under a broken flagstone; at 10 P.M. the same evening three had disappeared on the forage; the next morning all were ‘at home.’ The following night at 10 P.M. five were gone out, two being discovered with some difficulty ‘in a small jungle’ six feet away; the next morning six out of the seven were safely beneath the flagstone. According to the same authority, Helix aspersa will find its way across a cinder-path (which it specially detests) to get to its favourite food, and will return by the same way to its old quarters, although it could easily have found new lodgings nearer the food-supply. A snail has been observed to occupy a hole in the brick wall of a kitchen-garden about four feet from the ground. Leaning against the wall, and immediately under the hole, was a piece of wood, the lower end of which rested in a bed of herbs. For months the snail employed this ladder between its food and its home, coming down as soon as it was dark, and retiring to rest during the day.

In greenhouses a slug will forage night after night—as gardeners know to their cost—over the same beat, and will always return to the same hiding-place. Limax flavus has been noticed crawling with great regularity to a sink from a hole near the water-pipe, and keeping to a well-marked circular track. In all probability the scent, either of the desired object of food, or of the creature’s own trail, plays a considerable part in keeping it to the same outward and homeward track, or at least in guiding it back to its hiding-place. Yet even scent is occasionally at fault, for on one occasion a Limax flavus was accustomed to make nightly excursions to some basins of cream, which were kept in a cool cellar. When the basins were removed to a distant shelf, the creature was found the next morning ‘wandering disconsolately’ about in the place where the basins had formerly stood.[71]

A remarkable case of the power of smell, combined with great perseverance on the part of a Helix, is recorded by Furtado.[72] He noticed a Helix aspersa lodged between a column on a verandah and a flower-pot containing a young banana plant, and threw it away into a little court below, and six or seven yards distant. Next morning the snail was in precisely the same place on the flower-pot. Again he threw it away, to the same distance, and determined to notice what happened. Next morning at nine o’clock, the snail was resting on the rail of a staircase leading up to the verandah from the court; in the evening it started again, quickening its space as it advanced, eventually attacking the banana in precisely the same place where it had been gnawed before.

For further instances of the power of smell in snails, see chap. vii.

Slugs have been known to make their way into bee-hives, presumably for the sake of the honey.[73] ‘Sugaring’ the trees at night for moths will often attract a surprising concourse of slugs. Sometimes a particular plant in a greenhouse will become the object of the slugs’ persistent attacks, and they will neglect every other food in order to obtain it. Farfugium grande is one of these favourite foods, “the young leaves and shoots being always eaten in preference to all other plants growing in the houses; where no Farfugiums were kept the slugs nibbled indiscriminately at many kinds.”[74] The flowers of orchidaceous plants exercise a special attraction over slugs, which appear to have some means of discovering when the plants are in bloom. “I have often observed,” says Mr. T. Baines, “that a slug will travel over the surface of a pot in which is growing a Dendrobium nobile, a Cattleya, Vanda, or similar upright plant for a score of times without ever attempting to ascend into the head of the plant unless it is in bloom, in which case they are certain to find their way straight to the flowers; after which they will descend, and return to some favourite hiding-place, often at the opposite end of the house.”[75] Mr. R. Warner has “actually seen many little slugs suspending themselves by slime-threads from the rafters and descending on the spikes of the beautiful Odontoglossum alexandrae; and thus many spikes, thickly wadded round with cotton wool (which the slugs could not travel over), and growing in pots surrounded by water, had been lost.”[76] Perhaps the most singular instance of a liking for a particular food is that related by Mr. E. Step.[77] In a London publishing house, slugs were observed, during a period of nearly twelve months, to have fed almost nightly on the colouring matter in certain bookcovers, and though the trails were often seen over the shelves, and cabbage and lettuce leaves laid down to tempt the creatures, they continued their depredations with impunity for the time above mentioned.

Limnaea peregra has been observed feeding on old fish-heads thrown into a dirty stream, and a large gathering of Limnaea stagnalis has been noticed feeding upon an old newspaper in a pond on Chislehurst Common, ‘so that for the space of about a square foot nothing else could be seen.’[78]

Tenacity of Life.—Land Mollusca have been known to exhibit, under unusual conditions, remarkable tenacity of life. Some of the most noteworthy and best authenticated instances of this faculty may be here mentioned.

The well-known story of the British Museum snail is thus related by Mr. Baird.[79] On the 25th March 1846 two specimens of Helix desertorum, collected by Charles Lamb, Esq., in Egypt some time previously, were fixed upon tablets and placed in the collection among the other Mollusca of the Museum. There they remained fast gummed to the tablet. About the 15th March 1850, having occasion to examine some shells in the same case, Mr. Baird noticed a recently formed epiphragm over the mouth of one of these snails. On removing the snails from the tablet and placing them in tepid water, one of them came out of its shell, and the next day ate some cabbage leaf. A month or two afterwards it began repairing the lip of its shell, which was broken when it was first affixed to the tablet.

While resident in Porto Santo, from 27th April to 4th May 1848, Mr. S. P. Woodward[80] collected a number of Helices and sorted them out into separate pill-boxes. On returning home, these boxes were placed in empty drawers in an insect cabinet, and on 19th October 1850, nearly two and a half years afterwards, many of them were found to be still alive. A whole bagful of H. turricula, collected on the Ilheo de Cima on 24th April 1849, were all alive at the above-mentioned date.

In September 1858 Mr. Bryce Wright sent[81] to the British Museum two specimens of H. desertorum which had been dormant for four years. They were originally collected in Egypt by a Mr. Vernèdi, who, in May 1854, while stopping at one of the stations in the desert, found a heap of thorn-bushes lying in a corner of the building, rather thickly studded with the snails. He picked off fifteen or twenty specimens, which he carried home and locked up in a drawer, where they remained undisturbed until he gave two to Mr. Wright in September 1858.

In June 1855 Dr. Woodward placed specimens of H. candidissima and H. aperta in a glass box, to test their tenacity of life; he writes of their being still alive in April 1859.

Mr. R. E. C. Stearns records[82] a case of Buliminus pallidior and H. Veatchii from Cerros I. living without food from 1859 to March 1865.

H. Aucapitaine mentions[83] a case of H. lactea found in calcinated ground in a part of the Sahara heated to 122° F., where no rain was said to have fallen for five years. The specimen revived after being enclosed in a bottle for three and a half years.

In August 1863, Mr. W. J. Sterland[84] put specimens of H. nemoralis in a box and afterwards placed the box in his cabinet; in November 1866 one specimen was discovered to be alive.

Gaskoin relates[85] a case in which specimens of H. lactea were purchased from a dealer in whose drawer they had been for two years. This dealer had them from a merchant at Mogador, who had kept them for more than that time under similar conditions. One of these shells on being immersed in water revived, and in April 1849 was placed quite alone under a bell jar with earth and food. In the end of the following October about thirty young H. lactea were found crawling on the glass.

Mr. R. D. Darbishire bought[86] some H. aperta in the market at Nice on 18th February 1885. Two specimens of these, placed with wool in a paper box, were alive in December 1888. This is a very remarkable case, H. aperta not being, like H. desertorum, H. lactea, H. Veatchii and Bul. pallidior, a desert snail, and therefore not accustomed to fasting at all.

Age of Snails.—It would appear, from the existing evidence, which is not too plentiful, that five years is about the average age of the common garden snail. Mr. Gain has published[87] some interesting observations on the life of a specimen from the cradle to the grave, which may be exhibited in a tabular form.

According to Clessin, the duration of life in Vitrina is one year, Cyclas 2 years; Hyalinia, Succinea, Limnaea, Planorbis, and Ancylus are full grown in 2 to 3 years, Helix and Paludina in 2 to 4, and Anodonta in 12 to 14. Hazay finds[88] that the duration of life in Hyalinia is 2 years, in Helix pomatia 6 to 8, in Helix candicans 2 to 3, in Paludina 8 to 10, in Limnaea and Planorbis 3 to 4.

Growth of the Shell.—Mr. E. J. Lowe, many years ago, conducted[89] some interesting experiments on the growth of snails. The facts arrived at were—

(1) The shells of Helicidae increase but little for a considerable period, never arriving at maturity before the animal has once become dormant.

(2) Shells do not grow whilst the animal itself remains dormant.

(3) The growth of shells is very rapid when it does take place.

(4) Most species bury themselves in the ground to increase the dimensions of their shells.

Six recently hatched H. pomatia were placed in a box and regularly fed on lettuce and cabbage leaves from August until December, when they buried themselves in the soil for winter; at this period they had gradually increased in dimensions to the size of H. hispida. On the 1st April following, the box was placed in the garden, and on the 3rd the Helices reappeared on the surface, being no larger in size than they were in December. Although regularly fed up to 20th June, they were not perceptibly larger, but on that day five of them disappeared, having buried themselves, with the mouth of the shell downwards, in the soil. After ten days they reappeared, having in that short time grown so rapidly as to be equal in size to H. pisana. On the 15th July they again buried themselves, and reappeared on 1st August, having again increased in size. For three months from this date they did not become perceptibly larger; on 2nd November food was withheld for the winter and they became dormant.

A similar experiment, with similar results, was carried on with a number of H. aspersa, hatched on 20th June. During the summer they grew but little, buried themselves on 10th October with the head upwards, and rose to the surface again on 5th April, not having grown during the winter. In May they buried themselves with the head downwards, and appeared again in a week double the size; this went on at about fortnightly intervals until 18th July, when they were almost fully grown.

Helix nemoralis, H. virgata, H. caperata, and H. hispida bury themselves to grow; H. rotundata burrows into decayed wood; Hyalinia radiatula appears to remain on decaying blades of grass; Pupa umbilicata, Clausilia rugosa, and Buliminus obscurus bury their heads only.

The observations of Mr. W. E. Collinge[90] do not at all agree with those of Mr. Lowe, with regard to the mode in which land Mollusca enlarge their shells. He bred and reared most of the commoner forms of Helix and also Clausilia rugosa, but never saw them bury any part of their shell when enlarging it. While admitting that they may increase their shells when in holes or burrows of earthworms, he thinks that the process of burying would seriously interfere with the action of the mantle during deposition, and in many cases damage the membranaceous film before the calcareous portion was deposited. Mr. Collinge has found the following species under the surface in winter: Arion ater (3–4 in.), Agriolimax agrestis, (6–8 in.), Hyalinia cellaria and H. alliaria (6–8 in.), Hyalinia glabra (5 in.), Helix aspersa (5–6 in.), H. rufescens (4–6 in.), H. rotundata (4–5 in.), H. hispida (7 in.), Buliminus obscurus (4–6 in.), B. montanus[91] (24 in.), and the following in summer, Hyalinia cellaria and alliaria (6–8 in.), Helix rotundata (4–5 in.), Balea perversa (6–8 in.), Cyclostoma elegans (3–4 in.). The same author has found the following species of fresh-water Mollusca living in hard dry mud: Sphaerium corneum (3–14 in.), S. rivicola (5–6 in.), S. lacustre (10–14 in.), all the British species of Pisidium (4–12 in.), Limnaea truncatula (18 in., a single specimen). All our species of Unio, Anodonta, Bithynia, and Paludina bury themselves habitually in fine or thick wet mud, to a depth of from 4 to 14 inches.

This burying propensity on the part of Mollusca has been known to play its part in detecting fraud. When my friend Mr. E. L. Layard was administering justice in Ceylon, a native landowner on a small scale complained to him of the conduct of his neighbour, who had, during his absence from home, diverted a small watercourse, which ran between their holdings, in such a way as to filch a certain portion of the land. The offender had filled up and obliterated the ancient course of the stream, and protested that it had never run but in its present bed. Mr. Layard promptly had a trench sunk across what was said to be the old course, and the discovery of numerous living Ampullaria, buried in the mud, confirmed the story of one of the litigants and confounded the other.[92]

Depositing and Hatching of Eggs: Self-fertilisation.—There appears to be no doubt that Helices, when once impregnated, can lay successive batches of eggs, and possibly can continue laying for several years, without a further act of union. A specimen of Helix aspersa was noticed in company with another on 5th August; on 9th August it laid eggs in the soil, and early in the following summer it laid a second batch of eggs, although its companion had been removed directly after its first introduction. An Arion received from a distance laid 30 eggs on 5th September, and 70 more on the 23rd of the same month, although quite isolated during the whole time.[93] By far the most remarkable case of the kind is related by Gaskoin.[94] A specimen of Helix lactea was kept in a drawer for about two years, and then in another drawer for about two years more. It was then taken out, and placed in water, when it revived, and was placed alone under a bell jar with earth and food. Six months after, about 30 young H. lactea were found crawling on the glass, the act of oviposition not having been observed.

The observations of Mr. F. W. Wotton,[95] with regard to the fertilisation and egg-laying of Arion ater, are of extreme interest and value. A pair of this species, kept in captivity, united on 10th September 1889, the act lasting about 25 minutes. From that date until the eggs were laid, the animals looked sickly, dull of colour, with a somewhat dry skin. Eggs were deposited in batches, one, which we will call A, beginning three days before B. On 10th October A laid 80 eggs; on the 16th, 110; on the 25th, 77; on 8th November, 82; and on 17th November, 47; making a total of 396. Specimen B, which began on 13th October, three days after A, made up for the delay by laying 246 eggs in 40 hours; on 26th October it laid 9, on 10th November, 121; and on 30th November, 101; a total of 477. These eggs weighed 624 to the ounce, and, in excluding the batch of 246, B parted with ⅜ of its own weight in 40 hours, while the whole number laid were rather over ¾ of its own weight!

While depositing the eggs, the slug remained throughout in the same position on the surface of the ground, with the head drawn up underneath the mantle, which was lifted just above the reproductive orifice. When taken into the hand, it went on laying eggs without interruption or agitation of any kind. After it had finished laying it ate half a raw potato and then took a bath, remaining submerged for more than an hour. Bathing is a favourite pastime at all periods. Specimens, says Mr. Wotton, have survived a compulsory bath, with total submersion, of nearly three days’ duration.

Mr. Wotton’s account of the hatching of the eggs is equally interesting. It is noticeable that the eggs of one batch do not hatch by any means simultaneously; several days frequently intervene. The average period is about 60 days, a damp and warm situation bringing out the young in 40 days, while cold and dryness extended the time to 74 days, extremes of any kind proving fatal. Of the batch of eggs laid by B on 30th November, the first 2 were hatched on the following 16th January, and 2 more on the 17th; others, from 10 to 20, followed suit on the succeeding 5 days, until 82 in all were hatched, the remaining 19 being unproductive.[96]

By placing the egg on a looking-glass the act of exclusion can be perfectly observed. For several days the inmate can be seen in motion, until at last a small crack appears in the surface of the shell: this gradually enlarges, until the baby slug is able to crawl out, although it not unfrequently backs into the shell again, as if unwilling to risk itself in the world. When it once begins to crawl freely, it buries itself in the ground for 4 or 5 days without food, after which time it emerges, nearly double its original size. At exclusion, the average length is 9 mm., increasing to 56 mm. after the end of 5 months. Full growth is attained about the middle of the second year, and nearly all die at the end of this year or the beginning of the next. Death from exhaustion frequently occurs after parturition. Death from suffocation is sometimes the result of the formation of small blisters on the margin of the respiratory aperture. The attacks of an internal parasite cause death in a singular way. The upper tentacles swell at the base in such a way as to prevent their extrusion; digestive troubles follow, with rigidity and loss of moisture, and death ensues in 2 or 3 days.

Mr. Wotton isolated newly-hatched specimens, with the view of experimenting on their power of self-fertilisation, if the opportunity of fertilising and being fertilised by others was denied them. One of these, after remaining in absolute solitude for 10½ months, began to lay, scantily at first (11th January, 2; 25th January, 2; 11th February, 2), but more abundantly afterwards (3rd April, 60; 15th and 16th, 70; 29th, 53, etc.), the eggs being hatched out in 42–48 days. The precautions taken seem to have been absolutely satisfactory, and the fact of the power of self-fertilisation appears established as far as Arion ater is concerned.

Braun took young individuals of Limnaea auricularia on the day they were hatched out, and placed them singly in separate vessels with differing amounts of water. This was on 15th June 1887. In August 1888 specimen A had only produced a little spawn, out of which three young were hatched; specimen B had produced four pieces of spawn of different sizes, all of which were hatched; specimen C, which happened to be living with three Planorbis, produced five pieces of spawn distinctly Limnaeidan, but nothing is recorded of their hatching. Self-impregnation, therefore, with a fruitful result, appears established for this species of Limnaea.[97]

Reproduction of Lost Parts.—When deprived of their tentacles, eyes, or portions of the foot, Mollusca do not seem to suffer severely, and generally reproduce the lost parts in a short time. If, however, one of the ganglia is injured, they perish. Certain of the Mollusca possess the curious property of being able to amputate certain parts at will. When Prophysaon, a species of Californian slug, is annoyed by being handled, an indented line appears at a point about two-thirds of the length from the head, the line deepens, and eventually the tail is shaken completely off. Sometimes the Prophysaon only threatens this spontaneous dismemberment; this line appears (always exactly in the same place), but it thinks better of it, and the indentation proceeds no further.[98] According to Gundlach,[99] Helix imperator and H. crenilabris, two large species from Cuba, possess the same property, which is said to be also characteristic of the sub-genus Stenopus (W. Indies). Amongst marine species, Harpa ventricosa and Solen siliqua have been observed to act in a similar way, Harpa apparently cutting off the end of the foot by pressure of the shell. Karl Semper, in commenting on the same property in species of Helicarion from the Philippines (which whisk their tail up and down with almost convulsive rapidity, until it drops off), considers[100] it greatly to the advantage of the mollusc, since any predacious bird which attempted to seize it, but only secured a fragment of tail, would probably be discouraged from a second attack, especially as the Helicarion would meanwhile have had time to conceal itself among the foliage.

Strength and Muscular Force.—The muscular strength of snails is surprisingly great. Sandford relates[101] an experiment on a Helix aspersa, weighing ¼ oz. He found it could drag vertically a weight of 2¼ oz., or nine times its own weight. Another snail, weighing ⅓ oz., was able to drag in a horizontal direction along a smooth table twelve reels of cotton, a pair of scissors, a screwdriver, a key, and a knife, weighing in all no less than 17 oz., or more than fifty times its own weight. This latter experiment was much the same as asking a man of 12 stone to pull a load of over 3¾ tons.

If a snail be placed on a piece of glass and made to crawl, it will be seen that a series of waves appear to pursue one another along the under surface of the foot, travelling from back to front in the direction in which the animal is moving. Simroth has shown that the sole of the foot is covered with a dense network of muscular fibres, those which run longitudinally being chiefly instrumental in producing the undulatory motion. By means of these muscles the sole is first elongated in front, and then shortened behind to an equal extent. Thus a snail slides, not on the ground, but on its own mucus, which it deposits mechanically, and which serves the purpose of lubricating the ground on which it travels. It has been calculated that an averaged sized snail of moderate pace progresses at the rate of about a mile in 16 days 14 hours.[102]

Sudden Appearance of Mollusca.—It is very remarkable to notice how suddenly Pulmonata seem to appear in certain districts where they have not been noticed before. This sudden appearance is more common in the case of fresh-water than of land Mollusca, and there can be little doubt that, wherever a new pond happens to be formed, unless there is something in its situation or nature which is absolutely hostile to molluscan life, Mollusca are certain to be found in it sooner or later. “Some 23 years ago,” writes Mr. W. Nelson,[103] “I was in the habit of collecting shells in a small pond near to the Black Hills, Leeds. At that time the only molluscan forms found there were a dwarf form of Sphaerium lacustre, Pisidium pusillum, Planorbis nautileus, and Limnaea peregra. About 10 years ago I resumed my visits to the locality, and found, in addition to the species already enumerated, Planorbis corneus. These were the only species found there until this spring [1883], when, during one of my frequent visits, I was surprised to find Physa fontinalis and Planorbis vortex were added to the growing list of species. Later on Pl. carinatus, Limnaea stagnalis, and Ancylus lacustris turned up; and during June, Pl. contortus was found in this small but prolific pond.” Limnaea glutinosa is prominent for these remarkable appearances and disappearances. In 1822 this species suddenly appeared in some small gravel pits at Bottisham, Cambs., in such numbers that they might have been scooped out by handfuls. After that year they did not appear numerous, and after three or four seasons they gradually disappeared.[104] Physa (Aplecta) hypnorum is noted in a similar way. In February 1852, for instance, after a wet month, the water stood in small puddles about 3 feet by 2 in a particular part of Bottisham Park which was sometimes a little swampy, though usually quite dry. One of these puddles was found to contain immense numbers of the Aplecta, which up to that time had not been noted as occurring in Cambridgeshire at all.[105] In a few days the species entirely disappeared and was never again noticed in the locality.[106]

Writing to the Zoological Society of London from New Caledonia, Mr. E. L. Layard remarks:[107] “The West Indian species Stenogyra octona has suddenly turned up here in thousands; how introduced, none can tell. They are on a coffee estate at Kanala on the east coast. I have made inquiries, and cannot find that the planter ever had seed coffee from the West Indies. All he planted came from Bombay, and it would be interesting to find out whether the species has appeared there also.”

Sometimes a very small event is sufficient to disturb the natural equilibrium of a locality, and to become the cause either of the introduction or of the destruction of a species. In 1883 a colony of Helix sericea occupied a portion of a hedge bottom twenty yards long near Newark. It scarcely occurred outside this limit, but within it was very plentiful, living in company with H. nemoralis, H. hortensis, H. hispida, H. rotundata, Hyalinia cellaria and Hy. nitidula, and Cochlicopa lubrica. In 1888 the hedge was well trimmed, but the bottom was not touched, and the next year a long and careful search was required to find even six specimens of the sericea.[108]

Showers of Shells.—Helix virgata, H. caperata, and Cochlicella acuta sometimes occur on downs near our sea-coasts in such extraordinary profusion, that their sudden appearance out of their hiding-places at the roots of the herbage after a shower of rain has led to the belief, amongst credulous people, that they have actually descended with the rain. There seems, however, no reason to doubt that Mollusca may be caught up by whirlwinds into the air and subsequently deposited at some considerable distance from their original habitat, in the same way as frogs and fishes. A very recent instance of such a phenomenon occurred[109] at Paderborn, in Westphalia, where, on 9th August 1892, a yellowish cloud suddenly attracted attention from its colour and the rapidity of its motion. In a few moments it burst, with thunder and a torrential rain, and immediately afterwards the pavements were found to be covered with numbers of Anodonta anatina, all of which had the shell broken by the violence of the fall. It was clearly established that the shells could not have been washed into the streets from any adjacent river or pond, and their true origin was probably indicated when it was found that the funnel-shaped cloud which burst over the town had passed across the one piece of water near Paderborn, which was known to contain the Anodonta in abundance.

Cases of Singular Habitat.—Mollusca sometimes accustom themselves to living in very strange localities, besides the extremes of heat and cold mentioned above (pp. 23–24). In the year 1852, when some large waterpipes in the City Road, near St. Luke’s Hospital, were being taken up for repairs, they were found to be inhabited in considerable numbers by Neritina fluviatilis and a species of Limnaea.[110] Dreissensia polymorpha has been found in a similar situation in Oxford Street, and also in Hamburg, and has even been known to block the pipes and cisterns of private houses. In an engine cistern at Burnley, 60 feet above the canal from which the water was pumped into the cistern, were found the following species: Sphaerium corneum, S. lacustre; Valvata piscinalis, Bithynia tentaculata; Limnaea peregra, very like Succinea in form and texture; Planorbis albus, P. corneus, P. nitidus, P. glaber, and thousands of P. dilatatus, much larger than the forms in the canal below, a fact probably due to the equable temperature of the water in the cistern all the year round.[111] In certain parts of southern Algeria the fresh-water genera Melania and Melanopsis inhabit abundantly waters so surcharged with salt that the marine Cardium edule has actually become extinct from excess of brine. The common Mytilus edulis is sometimes found within the branchial chamber and attached to the abdomen of crabs (Carcinus maenas), which are obliged to carry about a burden of which they are powerless to rid themselves (see p. 78). A variety of the common Limnaea peregra lives in the hot water of some of the geysers of Iceland, and has accordingly been named geisericola.

Underground Snails.—Not only do many of the land Mollusca aestivate, or hibernate, as the case may be, beneath the surface of the soil, but a certain number of species live permanently underground, like the mole, and scarcely ever appear in the light of day. Our own little Caecilianella acicula lives habitually from 1 to 3 feet below ground, appearing to prefer the vicinity of graveyards. Testacella, the carnivorous slug, scarcely ever appears on the surface during the day, except when driven by excessive rain, and even then it lurks awhile under some protecting cover of leafage. There is a curious little Helix (tristis Pfr.), peculiar to Corsica, which is of distinctly subterranean habits. It lives in drifted sand above high-water mark, always at the roots of Genista Saltzmanni, at a depth which varies with the temperature and dryness of the air. In hot and very dry weather it buries itself nearly 2 feet below the surface, only coming up during rain, and burying itself again immediately the rain is over. Like a Solen, it often has a hole above its burrow, by which it communicates with the air above, so as to avoid being stifled in the sand. The animal, in spite of its dry habitat, is singularly soft and succulent, and exudes a very glutinous mucus. It probably descends in its burrow until it arrives at the humid stratum, the persistence of which is due to the capillarity of the sand.[112] I am assured by Mr. E. L. Layard that precisely similar underground habits are characteristic of Coeliaxis Layardi, which lives exclusively in sand at the roots of scrub and coarse grass at East London.

Rock-boring Snails.—Cases have sometimes been recorded, from which it would appear that certain species of snails possess the power of excavating holes in rocks to serve as hiding-places. At Les Bois des Roches, ten miles from Boulogne, occur a number of solid calcareous rocks scattered about in the wood. The sides of the rocks which face N.E. and E. are covered with multitudes of funnel-shaped holes, 1½ inch in diameter at the opening and contracting suddenly within to ½ inch. Sometimes the holes are 6 inches deep, and terminate, after considerable windings, in a cup-shaped cavity. Helix hortensis inhabits these holes, and has been observed to excavate them at the rate of ½ inch each hibernation, choosing always the side of the rock which is sheltered from the prevailing rains. It does not form an epiphragm, but protrudes part of its body against the rock. That the snails secrete an acid which acts as a solvent seems probable from the fact that red litmus paper, on being applied to the place where the foot has been, becomes stained with violet.[113] Helix aspersa is said to excavate holes 10 to 12 cm. deep at Constantine,[114] and H. Mazzullii is recorded as perforating limestone at Palermo.[115]

Snails as Barometers.—An American writer of more than thirty years ago[116] gave his experience of Helices as weather-prophets. According to him, H. alternata is never seen abroad except shortly before rain; it then climbs on the bark of trees, and stations itself on leaves. Helix clausa, H. ligera, H. pennsylvanica, and H. elevata climb trees two days before rain, if it is to be abundant and continuous. Succinea does the same, and its body is yellow before rain and bluish after it. Several of the Helices assume a sombre colour after rain, when their bodies are exceedingly humid; after the humidity has passed off they resume a clearer and lighter tint.

Production of Musical and other Sounds.—Certain molluscs are said to be capable of producing musical sounds. Sir J. E. Tennent describes his visit to a brackish-water lake at Batticaloa, in Ceylon, where the fishermen give the name of the ‘crying shell’ to the animal supposed to produce the sounds. “The sounds,” he says,[117] “came up from the water like the gentle thrills of a musical chord, or the faint vibrations of a wineglass when its rim is rubbed by a moistened finger. It was not one sustained note, but a multitude of tiny sounds, each clear and distinct in itself; the sweetest treble mingling with the lowest bass. On applying the ear to the woodwork of the boat, the vibration was greatly increased in volume. The sounds varied considerably at different points as we moved across the lake, and occasionally we rowed out of hearing of them altogether.” According to the fishermen, the shells were Pyrazus palustris and Littorina laevis. It appears uncertain whether the sounds are really due to Mollusca. Fishermen in other parts of India assert that the sounds are made by fish, and, like those in Ceylon, produce the fish which they say ‘sings.’ The same, or a similar sound, has also been noticed to issue from the water in certain parts of Chili, and on the northern shores of the Gulf of Mexico. Dendronotus arborescens, when confined in a glass jar of sea water, has been noticed[118] to emit a sound like the clink of a steel wire. According to Lieut.-Col. Portlock,[119] F.R.S., Helix aperta, a very common species in South Europe, has the property of emitting sounds when irritated. When at Corfu, he noticed that if the animal is irritated by a touch with a piece of straw or other light material, it emits a noise, as if grumbling at being disturbed. He kept a specimen in his house for a considerable time, which would make this noise whenever it was touched.

The Rev. H. G. Barnacle describes the musical properties of Achatinella in the following terms:[120] “When up the mountains of Oahu I heard the grandest but wildest music, as from hundreds of Aeolian harps, wafted to me on the breezes, and my companion (a native) told me it came from, as he called them, the singing shells. It was sublime. I could not believe it, but a tree close at hand proved it. On it were many of the Achatinella, the animals drawing after them their shells, which grated against the wood and so caused a sound; the multitude of sounds produced the fanciful music. On this one tree I took 70 shells of all varieties.”

Habits of the Agnatha.—Not much is known of the habits and mode of life of the Agnatha, or carnivorous Land Mollusca. In this country we have only two, or at most three, of this group, belonging to the genus Testacella, and, in all probability, not indigenous to our shores. There seems little doubt, when all the circumstances of their discovery are taken into account, that both Testacella haliotidea and T. Maugei have been imported, perhaps from Spain or Portugal in the first instance, along with roots imbedded in foreign earth, for their earliest appearances can almost invariably be traced back to the neighbourhood of large nursery grounds, or else to gardens supplied directly from such establishments.

The underground life of Testacella makes observation of its habits difficult. It is believed to feed exclusively on earthworms, which it pursues in their burrows. Continued wet weather drives it to the surface, for though loving damp soil it is decidedly averse to too much moisture, and under such circumstances it has even been noticed[121] in considerable numbers crawling over a low wall. In the spring and autumn months, according to Lacaze-Duthiers,[122] it comes to the surface at night, hiding itself under stones and débris during the day. Earthworms are, at these periods, nearer the surface, and the Testacella has been seen creeping down into their burrows. The author has taken T. Maugei abundantly under clumps of the common white pink in very wet weather, lying in a sort of open nest in the moist earth. On the other hand, when the earth is baked dry by continued drought, they either bury themselves deeper, sometimes at a depth of 3 feet, in the ground, or else become encysted in a capsule of hardened mucus to prevent evaporation from the skin. When first taken from the earth and placed in a box, the Testacella invariably resents its capture by spitting up the contents of its stomach in the shape of long fragments of half-digested worms.

Fig. 20.—Testacella haliotidea Drap., protruding its pharynx (ph) and radula (r); oe, oesophagus; p.o, pulmonary orifice; sh, shell; t, tentacles (after Lacaze-Duthiers).

It appears not to bite the worm up before swallowing it, but contrives, in the most remarkable manner, to take down whole worms apparently much too large for its stomach. Mr. Butterell relates[123] that, after teasing a specimen of T. Maugei, and making it emit a quantity of frothy mucus from the respiratory aperture, he procured a worm of about three inches long, and rubbed the worm gently across the head of the Testacella. The tongue was rapidly extended, and the victim seized. The odontophore was then withdrawn, carrying with it the struggling worm, which made every effort to escape, but in vain; in about five minutes all had disappeared except the head, which was rejected. This protrusion of the tongue (radula) and indeed of the whole pharynx, is a very remarkable feature in the habits of the animal. It appears, as it were, to harpoon its prey by a rapid thrust, and when the victim is once pierced by a few of the powerful sickle-shaped teeth (compare chap. viii.) it is slowly but surely drawn into the oesophagus (Fig. 20).

Most gardeners are entirely ignorant of the character of Testacella, and confuse it, if they happen to notice it at all, with the common enemies of their tender nurslings. Cases have been known, however, when an intelligent gardener has kept specimens on purpose to kill worms in ferneries or conservatories. In some districts these slugs are very numerous; Lacaze-Duthiers once dug 182 specimens from a good well-manured piece of ground whose surface measured only ten square yards.

Towards the end of September or beginning of October the period of hibernation begins. I infer this from the behaviour of specimens kept in captivity, which, for about a fortnight before this time, gorged themselves inordinately on as many worms as I chose to put into their box, and then suddenly refused food, buried themselves deeply in the earth, and appeared no more during the winter. The eggs are apparently much less numerous than is the case with Limax or Helix, and very large, measuring about ⅙ inch in diameter. They are enveloped in a remarkably tough and elastic membrane, and, if dropped upon any hard surface, rebound several inches, just like an india-rubber ball.

The animal creeps rather rapidly, and has the power of elongating its body to a remarkable extent. When placed on the surface of the ground, in the full light of day, it soon betrays uneasiness, and endeavours to creep into concealment. Its method of burying itself is very interesting to watch. It first elongates its neck and inserts its head into the soil; gradually the body begins to follow, while the tail tilts upwards into the air. No surface motion of the skin, no writhing or wriggling motion of any kind occurs; the creature simply works its way down in a stealthy and mysterious way, until at last it is lost to view.

The great Glandina, which attain their maximum development in Mexico and the southern United States, are a very noticeable family in this group. According to Mr. Binney,[124] Glandina truncata Gmel., one of the commonest species of the genus, is somewhat aquatic in its habits. It is found in the sea islands of Georgia and around the keys and everglades of Florida, where it attains a maximum length of 4 inches, while in less humid situations it scarcely measures more than 1 inch. It occurs most abundantly in the centre of clumps and tussocks of coarse grass in marshes close to the sea-coast. By the action of the sharp, sickle-shaped teeth of its radula the soft parts of its prey (which consists chiefly of living Helices) are rapidly rasped away; sometimes they are swallowed whole. It has been known to attack Limax when confined in the same box, rasping off large pieces of the integument. In one case an individual was noticed to devour one of its own species, thrusting its long neck into the interior of the shell, and removing all the viscera.

Fig. 21.—Glandina sowerbyana Pfr. (Strebel).

The Glandinae of southern Europe, although scarcely rivalling those of Central America in size or beauty, possess similar carnivorous propensities. Glandina Poireti has been observed,[125] on Veglia Island, attacking a living Cyclostoma elegans. By its powerful teeth it filed through two or three whorls of the shell of its victim, and then proceeded to devour it, exactly in the same manner as a Natica or Buccinum perforates the shell of a Tellina or Mactra in order to get at its contents.

Few observations appear to have been made on the habits or food of Streptaxis, Rhytida, Ennea, Daudebardia, Paryphanta, and other carnivorous Mollusca. A specimen of Ennea sulcata, enclosed in the same box as a Madagascar Helix (sepulchralis Fér.) many times its own size, completely emptied the shell of its inhabitant.[126] Mr. E. L. Layard informs me that certain Cape Rhytida, e.g. R. capsula Bens., R. dumeticola Bens., and R. vernicosa Kr., eat Cyclostoma affine, Helix capensis, H. cotyledonis, etc. To Mr. Layard I am also indebted for the—perhaps apocryphal—tradition that the best time to capture the great Aerope caffra Fér. in numbers was after an engagement between the Kaffirs and Zulus, when they might be observed streaming from all points of the compass towards the field of slaughter. The Cuban Oleacina are known to secrete a very bitter fluid which they emit; this perhaps produces a poisoning or benumbing effect upon their victims when seized. They devour operculates, e.g. Helicina regina and sagraiana.[127]

CHAPTER III ENEMIES OF THE MOLLUSCA—MEANS OF DEFENCE—MIMICRY AND PROTECTIVE COLORATION—PARASITIC MOLLUSCA—COMMENSALISM—VARIATION — Enemies of the Mollusca — Means of Defence — Mimicry and Protective Coloration. — Parasitic Mollusca — Commensalism — Variation

The juicy flesh and defenceless condition of many of the Mollusca make them the favourite food and often the easy prey of a host of enemies besides man. Gulls are especially partial to bivalves, and may be noticed, in our large sandy bays at the recess of the tide, busily devouring Tellina, Mactra, Mya, Syndosmya, and Solen. On the Irish coast near Drogheda a herring gull has been observed[128] to take a large mussel, fly up with it in the air over some shingly ground and let it fall. On alighting and finding that the shell was unbroken it again took it up and repeated the process a number of times, flying higher and higher with it until the shell was broken. Hooded crows, after many unavailing attempts to break open mussels with their beak, have been seen to behave in a similar way.[129] Crows, vultures, and aquatic birds carry thousands of mussels, etc., up to the top of the mountains above Cape Town, where their empty shells lie in enormous heaps about the cliffs.[130]

The common limpet is the favourite food of the oyster-catcher, whose strong bill, with its flattened end, is admirably calculated to dislodge the limpet from its seat on the rock. When the limpet is young, the bird swallows shell and all, and it has been calculated that a single flock of oyster-catchers, frequenting a small Scotch loch, must consume hundreds of thousands of limpets in the course of a single year. Rats are exceedingly fond of limpets, whose shells are frequently found in heaps at the mouth of rat holes, especially where a cliff shelves gradually towards a rocky shore. A rat jerks the limpet off with a sudden movement of his powerful jaw, and, judging from the size of the empty shells about the holes, has no difficulty in dislodging the largest specimens. ‘I once landed,’ relates a shepherd to Mr. W. Anderson Smith,[131] ‘on the I. of Dunstaffnage to cut grass, and it was so full of rats that I was afraid to go on; and the grass was so full of limpets that I could scarcely use the scythe, and had to keep sharpening it all the time.’ Sometimes, however, the limpet gets the better both of bird and beast. The same writer mentions the case of a rat being caught by the lip by a limpet shell, which it was trying to dislodge. A workman once observed[132] a bird on Plymouth breakwater fluttering in rather an extraordinary manner, and, on going to the spot, found that a ring dotterel had somehow got its toe under a limpet, which, in closing instantly to the rock, held it fast. Similar cases of the capture of ducks by powerful bivalves are not uncommon, and it is said that on some parts of the American coasts, where clams abound, it is impossible to keep ducks at all,[133] for they are sure to be caught by the molluscs and drowned by the rising tide.

The Weekly Bulletin of San Francisco, 17th May 1893, contains an account of the trapping of a coyote, or prairie wolf, at Punta Banda, San Diego Co., by a Haliotis Cracherodii. The coyote had evidently been hunting for a fish breakfast, and finding the Haliotis partially clinging to the rock, had inserted his muzzle underneath to detach it, when the Haliotis instantly closed down upon him and kept him fast prisoner.

Rats devour the ponderous Uniones of North America. When Unio moves, the foot projects half an inch or more beyond the valves. If, when in this condition, the valves are tightly pinched, the foot is caught, and if the pinching is continued the animal becomes paralysed and unable to make use of the adductor muscles, and consequently flies open even if the pressure is relaxed. The musk-rat (Fiber zibethicus) seizes the Unio in his jaws, and by the time he reaches his hole, the Unio is ready to gape.[134] Rats also eat Vivipara, and even Limnaea, in every part of the world.

Every kind of slug and snail is eaten greedily by blackbirds, thrushes, chaffinches, and in fact by many species of birds. A thrush will very often have a special sacrificial stone, on which he dashes the shells of Helix aspersa and nemoralis, holding them by the lip with his beak, until the upper whorls are broken; heaps of empty shells will be found lying about the place of slaughter. The bearded Titmouse (Parus biarmicus) consumes quantities of Succinea putris and small Pupa, which are swallowed whole and become triturated in the bird’s stomach by the aid of numerous angular fragments of quartz.[135]

Frogs and toads are very partial to land Mollusca. A garden attached to the Laboratory of Agricultural Chemistry at Rouen had been abandoned for three years to weeds and slugs. The director introduced 100 toads and 90 frogs, and in less than a month all the slugs were destroyed, and all kinds of vegetables and flowers, whose cultivation had until then been impossible, were enabled to flourish.[136]

Certain Coleoptera are known to prey upon Helices and other land Mollusca. Récluz noticed, near Agde, a beetle (Staphylinus olens) attack Helix ericetorum when crawling among herbage, sticking its sharp mandibles into its head. Every time the snail retreated into its shell the beetle waited patiently for its reappearance, until at last the snail succumbed to the repeated assaults. M. Lucas noticed, at Oran, the larva of a Drilus attacking a Cyclostoma. The Drilus stood sentinel at the mouth of a shell, which was closed by the operculum, until the animal began to issue forth. The Drilus then with its mandibles cut the muscle which attaches the operculum to the foot, disabling it sufficiently to prevent its being securely closed, upon which it entered and took possession of the body of its defenceless host, completing its metamorphosis inside the shell, after a period of six weeks.[137] The female glow-worm (Lampyris noctiluca) attacks and kills Helix nemoralis.

Among the Clavicornia, some species of Silpha carry on a determined warfare against small Helices. They seize the shell in their mandibles, and then, throwing their head backwards, break the shell by striking it against their prothorax.

The common water beetle, Dytiscus marginalis, from its strength and savage disposition, is a dangerous enemy to fresh-water Mollusca. One Dytiscus, kept in an aquarium, has been noticed to kill and devour seven Limnaea stagnalis in the course of one afternoon. The beetles also eat L. peregra, but apparently prefer stagnalis, for when equal quantities of both species were placed within their reach, they fixed on the latter species first.[138]

In East Africa a species of Ichneumon (Herpestes fasciatus) devours snails, lifting them up in its forepaws and dashing them down upon some hard substance.[139] In certain islands off the south coasts of Burmah, flat rocks covered with oysters are laid bare at low tide. A species of Monkey (Macacus cynomolgus) has been noticed to furnish himself with a stone, and knock the oysters open, always breaking the hinge-end first, and then pulling out the mollusc with his fingers.[140]

The walrus is said to support himself almost entirely on two species of Mya (truncata and arenaria), digging them out of the sand, in which they live buried at a depth of about 1½ feet, with his powerful tusks. Whales swallow enormous numbers of pelagic molluscs (Clio, Limacina), which are at times so abundant in the Arctic seas, as to colour the surface for miles. Many of the larger Cetacea subsist in great part on Cephalopoda; as many as 18 lbs. of beaks of Teuthidae have been taken from the stomach of a single Hyperoodon.

Fish are remarkably partial to Mollusca of various kinds. The cat-fish (Chimaera) devours Pectunculus and Cyprina, crushing the stout shells with its powerful jaws, while flounders and soles content themselves with the smaller Tellina and Syndosmya which they swallow whole. As many as from 30 to 40 specimens of Buccinum undatum have been taken from the stomach of a single cod, and the same ‘habitat’ has been recorded for some of the rarer whelks, e.g. Bucc. humphreysianum, Fusus fenestratus, the latter also occurring as the food of the haddock and the red gurnard. No less than 35,000 Turtonia minuta have been found in the stomach of a single mullet. Nudibranchs are no doubt dainty morsels for fish, and hence have developed, in many cases, special faculties for concealment, or, if distasteful, special means of remaining conspicuous (see pp. 71–74).

Fig. 22.—Two valves of Mytilus edulis L., representing diagrammatically the approximate position of the holes bored by Purpura in about 100 specimens of Mytilus, gathered at Newquay, Cornwall.

Besides the dangers to which they are exposed from other enemies, many of the weaker forms of Mollusca fall a prey to their own brethren. Nassa and Murex on this side of the Atlantic, and Urosalpinx on the other, are the determined foes of the oyster. Purpura lapillus prefers Mytilus edulis to any other food, piercing the shell in about two days’ time by its powerful radula, which it appears to employ somewhat in gimlet fashion. If Mytilus cannot be procured, it will eat Littorina or Trochus, but its attempts on the hard shell of Patella are generally failures. The statement which is sometimes made, that the Purpura makes its hole over the vital parts of the Mytilus, appears, according to the evidence embodied in the annexed figure, to be without foundation. The fact is that a hole in any part of its shell is fatal to the Mytilus, since the long proboscis of the Purpura, having once made an entrance, can reach from one end of the shell to the other. The branchiae are first attacked, the adductor muscles and edges of the mantle last. Natica and Nassa pierce in a similar way the shells of Mactra, Tellina, Donax, and Venus. Murex fortispina is furnished with a powerful tooth at the lower part of its outer lip. At Nouméa, in New Caledonia, its favourite food is Arca pilosa, which lives half buried in coral refuse. The Murex has been seen to drag the Arca from its place of concealment, and insert the tooth between the valves, so as to prevent their closing, upon which it was enabled to devour its prey at leisure.[141]

The carnivorous land Mollusca, with the exception of Testacella, appear to feed by preference upon other snails (pp. 54, 55).

Parasitic Worms, Mites, etc.—A considerable number of the Trematode worms pass one or more of the stages in the cycle of their development within the bodies of Mollusca, attaining to the more perfect or sexual form on reaching the interior of some vertebrate. Thus Distoma endolabum Duj. finds its first intermediate host in Limnaea stagnalis and L. ovata, its second in L. stagnalis, or in one of the fresh-water shrimps (Gammarus pulex), or in the larvae of one of the Phryganeidae (Limnophilus rhombicus), attaining to the sexual form in the common frog. Distoma ascidia v. Ben. passes firstly through Limnaea stagnalis or Planorbis corneus, secondly through certain flies and gnats (Ephemera, Perla, Chironomus), and finally arrives within certain species of bats. Distoma nodulosum Zed. inhabits firstly Paludina impura, secondly certain fishes (Cyprinus Acerina), and lastly the common perch. The sporocyst of Distoma macrostomum inhabits Succinea putris, pushing itself up into the tentacles, which become unnaturally distended (Fig. 23). While in this situation it is swallowed by various birds, such as the thrush, wagtail, and blackbird, which are partial to Succinea, and thus obtains lodgment in their bodies. Amphistoma subclavatum spends an early stage in Planorbis contortus, after which it becomes encysted on the skin of a frog. When the frog sheds its skin, it swallows it, and with it the Amphistoma, which thus becomes established in the frog’s stomach.[142]

Fig. 23.—A Trematode worm (Leucochloridium paradoxum Car.) parasitic in the tentacles of Succinea putris L. × 20 (after Baudon).

The common liver-fluke, which in the winter of 1879–1880 cost Great Britain the lives of no less than three million sheep, is perhaps the best known of these remarkable parasitic forms of life. Its history shows us, in one important particular, how essential it is for the creature to meet, at certain stages of its existence, with the exact host to which it is accustomed. Unless the newly-hatched embryo finds a Limnaea truncatula within about eight hours it becomes exhausted, sinks, and dies. It has been tried with all the other common pond and river Mollusca, with Limnaea peregra, palustris, auricularia, stagnalis, with Planorbis marginatus, carinatus, vortex, and spirorbis, with Physa fontinalis, Bithynia tentaculata, Paludina vivipara, as well as with Succinea putris, Limax agrestis and maximus, Arion ater and hortensis. Not one of them would it touch, except occasionally very young specimens of L. peregra, and in these its development was arrested at an early stage. But on touching a L. truncatula the embryo seems to know at once that it has got what it wants, and sets to work immediately to bore its way into the tissues of its involuntary host, making by preference for the branchial chamber; those which enter the foot or other outlying parts of the Limnaea proceed no farther.[143]

Many similar cases occur, in which littoral Mollusca, such as Littorina and Buccinum, form the intermediate host to a worm which eventually arrives within some sea-bird.

Certain Nematode worms (Rhabditis) are known to inhabit the intestine of Arion, and the salivary glands of Limax agrestis. Diptera habitually lay their eggs within the eggs of Helix and Limax. Many species of mite (Acarina) infest land Pulmonata. No adult Limax maximus is without at least one specimen of Philodromus (?) limacum, and the same, or an allied species, appears to occur on the larger of our Helices, retiring upon occasion into the pulmonary chamber.

Several of the Crustacea live associated with certain molluscs. Pinnotheres lives within the shell of Pinna, Ostrea, Astarte, Pectunculus, and others. Apparently the females alone reside within the shell of their host, while the males seize favourable opportunities to visit them there. A specimen of the great pearl-oyster (Meleagrina margaritifera) was recently observed which contained a male Pinnotheres encysted in nacre. It was suggested that he had intruded at an unfortunate time, when no female of his kind happened to be in, and that, having penetrated too far beneath the mantle in the ardour of his search, was made prisoner before he could escape.[144] Ostracotheres Tridacnae lives in the branchiae of the great Tridacna. A little brachyurous crustacean inhabits the raft of Ianthina, and assumes the brilliant blue colour of the mollusc.

As a rule, among the Mollusca, the shell forms a passive mode of resistance to the attacks of enemies. Bivalves are enabled, by closing their valves, to baffle the assault of their smaller foes, and the operculum of univalves, both marine and land, serves a similar purpose. Many land Mollusca, especially Helix and Pupa, as well as a number of Auriculidae, have the inside of the aperture beset with teeth, which are sometimes so numerous and so large that it is puzzling to understand how the animal can ever come out of its shell, or, having come out, can ever draw itself back again. Several striking cases of these toothed apertures are given in Fig. 24. Whatever may be the origin of these teeth, there can be little doubt that their extreme development must have a protective result in opposing a barrier to the entrance, predatory or simply inquisitive, of beetles and other insects. Sometimes, it will be noticed (G), the aperture itself is fairly simple, but a formidable array of obstacles is encountered a little way in. It is possible that the froth emitted by many land snails has a similar effect in involving an irritating intruder in a mass of sticky slime. The mucus of slugs and snails, on the other hand, is more probably, besides its use in facilitating locomotion, a contrivance for checking evaporation, by surrounding the exposed parts of their bodies with a viscid medium.

Fig. 24.—Illustrating the elaborate arrangement of teeth in the aperture of some land Pulmonata. A. Helix (Labyrinthus) bifurcata Desh., Equador. B. H. (Pleurodonta) picturata Ad., Jamaica. C. H. (Dentellaria) nux denticulata Chem., Demerara. D. Anostoma carinatum Pfr., Brazil; a, tube communicating with interior of shell. E. H. (Stenotrema) stenotrema Fér., Tennessee, × ³/₂. F. H. (Polygyra) auriculata Say, Florida, × ³/₂. G. H. (Plectopylis) refuga Gld., Tenasserim (a and b × 2).

Some species of Lima shelter themselves in a nest constructed of all kinds of marine refuse, held together by byssiferous threads. Modiola adriatica, M. barbata, and sometimes M. modiolus conceal themselves in a similar way. Gastrochaena frequently encloses itself in a sort of half cocoon of cement-like material. The singular genus Xenophora protects itself from observation by gluing stones, shells, and various débris to the upper side of its whorls (Fig. 25). Sometimes the selection is made with remarkable care; the Challenger, for instance, obtained a specimen which had decorated its body whorl exclusively with long and pointed shells (Fig. 26).

Fig. 25.—Xenophora (Phorus) conchyliophora Born., concealed by the stones which it glues to the upper surface of its shell. (From a British Museum specimen.)

Fig. 26.—Xenophora (Phorus) pallidula Reeve. A mollusc which escapes detection by covering itself with dead shells of other species. (From a Challenger specimen in the British Museum, × ½.)

The formidable spines with which the shells, e.g. of the Murex family, are furnished must contribute greatly to their protection against fishes, and other predatory animals. Murex tenuispina, for instance (see chap. ix.), would prove as dangerous a morsel in the mouth of a fish as a hedgehog in that of a dog. Whether the singular tooth in the outer lip of Leucozonia (see chap. xiv.), a feature which is repeated, to a less marked extent, in Monoceros and several of the West Coast muricoids, is developed for defensive purposes, cannot at present be decided.

The Strombidae possess the power of executing long leaps, which they doubtless employ to escape from their foes. In their case alone this power is combined with singular quickness of vision. On one occasion Mr. Cuming, the celebrated collector, lost a beautiful specimen of Terebellum, by the animal suddenly leaping into the water, as he was holding and admiring it in his hand. Miss Saul has informed me that the first living specimen of Trigonia that was ever obtained was lost in a similar way. It was dredged by Mr. Stutchbury in Sydney Harbour, and placed on the thwart of a small boat. He had just remarked to a companion that it must be a Trigonia, and his companion had laughed at the idea, reminding him that all known Trigonia were fossil, when the shell in question baffled their efforts to discover its generic position by suddenly leaping into the sea, and it was three months before Mr. Stutchbury succeeded in obtaining another.

Some genera possess more than merely passive means of defence. Many Cephalopoda emit a cloud of inky fluid, which is of a somewhat viscous nature, and perhaps, besides being a means of covering retreat, serves to entangle or impede the pursuer. The formidable suckers and hooks possessed by many genera in this Order are most dangerous weapons, both for offence and defence. Aplysia, when irritated, ejects a purple fluid which used to be considered dangerously venomous. Many of the Aeolididae, including our own common Aeolis papillosa, possess stinging cells at the end of their dorsal papillae, the effect of which is probably to render them exceedingly distasteful to fish.

The common Vitrina pellucida has a curious habit which in all probability serves for a defence against birds in the winter. When crawling on the edge of a stone or twig it has the power of suddenly jerking its ‘tail,’ so as to throw itself on the ground, where it is probably lost to sight among decaying leaves. At other times it rolls away a few inches and repeats the jump. It also possesses the power of attaching to itself bits of leaves or soil, which entirely cover and conceal both shell and animal.[145] The property of parting with the tail altogether, a remarkable form of self-defence, has already been noticed on p. 44.

The poisonous nature of the bite of certain species of Conus is well authenticated. Surgeon Hinde, R.N., saw[146] a native on the I. of Matupi, New Britain, who had been bitten by a Conus geographus, and who had at once cut small incisions with a sharp stone all over his arm and shoulder. The blood flowed freely, and the native explained that had he not taken these precautions he would have died. Instances have been recorded of poisonous wounds being inflicted by the bite of Conus aulicus, C. textile, and C. tulipa. According to Mr. J. Macgillivray[147] C. textile at Aneitum (S. Pacific) is called intrag, and the natives say it spits the poison upon them from several inches off! Two cases of bites from C. textile occurred to this gentleman’s notice, one of which terminated fatally by gangrene. Sir Edward Belcher, when in command of the Samarang, was bitten[148] by a Conus aulicus at a little island off Ternate in the Moluccas. As he took the creature out of the water, it suddenly exserted its proboscis and inflicted a wound, causing a sensation similar to that produced by the burning of phosphorus under the skin. The wound was a small, deep, triangular mark, succeeded by a watery vesicle. The natives of New Guinea have a wholesome dread of the bite of Cones. Mr. C. Hedley relates[149] that while collecting on a coral reef he once rolled over a boulder and exposed a living C. textile. Before he could pick it up, one of the natives hastily snatched it away, and explained, with vivid gesticulations, its hurtful qualities. On no account would he permit Mr. Hedley to touch it, but insisted on himself placing it in the bottle of spirits.

Fig. 27.—A tooth from the radula of Conus imperialis L., × 50, showing barb and poison duct.

Cases of Mimicry, or protective resemblance, when a species otherwise defenceless adopts the outward appearance of a better protected species, are rare among the Mollusca. Karl Semper[150] mentions an interesting case of the mimicry of Helicarion tigrinus by Xesta Cumingii, in the Philippines. It appears that all species of Helicarion possess the singular property of shaking off the ‘tail’ or hinder part of the foot, when seized or irritated. Specimens captured by collectors, Hel. tigrinus amongst them, have succeeded in escaping from the hand, and concealing themselves, by a sort of convulsive leap, among the dry leaves on the ground. This power of self-amputation must be of great value to Helicarion, not only as enabling it to escape from the clutch of its enemies, but also as tending to discourage them from attempting to capture it at all. Now the genus Xesta is, in anatomy, very far removed from Helicarion, and the majority of the species are also, as far as the shell is concerned, equally distinct. Xesta Cumingii, however, has, according to Semper, assumed the appearance of a Helicarion, the thin shell, the long tail, and the mantle lobes reflected over the shell; but it has not the power of parting with its tail at short notice. It lives associated with Helicarion, and so close is the resemblance between them that, until Semper pointed out its true position, it had always been classified as a member of that group.

In the same passage Semper draws attention to two other cases of apparent mimicry. The first is another species of Xesta (mindanaensis) which closely resembles a species of Rhysota (Antonii), a genus not indeed so far removed from Xesta as Helicarion, but, as far as the shell is concerned, well distinguished from it. In this case, however, there is no obvious advantage gained by the resemblance, since Rhysota as compared with Xesta is not known to possess any definite point of superiority which it would be worth while to counterfeit. A second case of resemblance between certain species of the genus Chloraea and the characteristic Philippine group Cochlostyla will not hold good as affording evidence of mimicry, for Chloraea is now recognised as a sub-genus of Cochlostyla.

The Mollusca are not much mimicked by creatures of different organisation. This appears at first sight strange, since it might have been thought that the strong defensive house of a snail was worth imitating. Still it is probably not easy for creatures bilaterally symmetrical to curl themselves up into an elevated spiral for any length of time. One or two instances, however, may be mentioned. The larva of a moth belonging to the Psychidae, and occurring in France, Germany, the Tyrol, and Syria, coils itself up into a sinistral spiral of three whorls, and is aptly named Psyche helix, a kindred species from Italy being known as Ps. planorbis.

An insect larva (Cochlophora valvata) from E. Africa is said to resemble a Valvata or young Cyclostoma. In this case the spiral is indifferently dextral or sinistral, the ‘shell’ being formed of masticated vegetable matter, united together by threads spun by the larva. Certain larvae of the Phryganeidae (“Caddis-worms”) enclose themselves in houses which more or less resemble a spiral shell, and have in some cases actually been described as molluscan; such species, some of which belong to Helicopsyche, have been noticed in S. Europe, Ceylon, Further India, China, Tasmania, New Zealand, Tennessee, Mexico, Central America, Venezuela, Brazil, and Argentina, and all[151] possess a dextral ‘shell.’ In all these cases ‘mimicry’ is probably not so much to be thought of as the practical advantages which accrue to the animal in question from the spiral form, which gives it greater strength to resist external blows, and enables it to occupy, during a very defenceless portion of its existence, a very small amount of space.

The larva of some species of the Syrphidae (Diptera) fixes itself on the under side of stones in the Tyrol, and closely resembles a small slug. The naturalist Von Spix, in 1825, described to the Bavarian Academy as a new genus of land Mollusca a somewhat similar larval form found in decaying wood on the banks of a German lake.[152] Simroth mentions[153] a curious case as occurring near Grimma. The caterpillars of certain Microlepidoptera occur on slabs of porphyry, associated with a species of Clausilia. Besides being of the same colour as the Clausiliae, the caterpillars have actually developed cross lines on the back, i.e. on the side turned away from the rock, in imitation of the suture of the mollusc.

It has been suggested[154] that there is mimicry between Aeolis papillosa (a common British nudibranch) and Sagartia troglodytes (an Actinian), and also between another species of Sagartia and Aeolidiella Alderi. The facts observed are not sufficient to warrant a decided opinion, but it seems more probable that the Actinian mimics the nudibranch than vice versâ, since Aeolis is known to be unpalatable to fishes.

Fig. 28.—A, Strombus mauritianus Lam., which mimics Conus in shape. B, Conus janus Hwass, Mauritius.

Certain species of Strombus (mauritianus L., luhuanus L.) show a remarkable similarity in the shape of the shell to that of Conus, so much so, that a tiro would be sure to mistake them, at first sight, for Cones. In the case of S. luhuanus at least, this similarity is increased by the possession of a remarkably stout brown epidermis. Now Conus is a flesh-eating genus, armed with very powerful teeth which are capable of inflicting even on man a poisonous and sometimes fatal wound (see p. 66). Strombus, on the other hand, is probably frugivorous, and is furnished with weak and inoffensive teeth. It is possible that this resemblance is a case of ‘mimicry.’ It is quite conceivable that powerful fishes which would swallow a Strombus whole and not suffer for it, might acquire a distaste for a Cone, which was capable of lacerating their insides after being swallowed. And therefore the more like a Cone the Strombus became, the better chance it would have of being passed over as an ineligible article of food.

Protective coloration is not uncommon among the Mollusca. Littorina obtusata is habitually found, on our own coasts, on Fucus vesiculosus, the air-bladders of which it closely resembles in colour and shape. Littorina pagodus, a large and showy species, resembles so closely the spongy crumbling rocks of Timor, on which it lives, that it can hardly be discerned a pace off. Helcion pellucidum, the common British ‘blue limpet,’ lives, when young, almost exclusively on the iridescent leaves of the great Laminariae, with the hues of which its own conspicuous blue lines harmonise exactly. In mature life, when the Helcion invariably transfers its place of abode to the lower parts of the stalk and finally to the root of the Laminaria, which are quite destitute of iridescence, these blue lines disappear or become much less marked.

The specimens of Purpura lapillus which occur at Newquay in Cornwall are banded with rings of colour, especially with black and white, in a more varied and striking way than any other specimens that have ever occurred to my notice. I am inclined to refer this peculiarity to a tendency towards protective coloration, since the rocks on which the Purpura occurs are often banded with veins of white and colour, and variegated to a very marked extent.

Ovula varies the colour of its shell from yellow to red, to match the colour of the Gorgonia on which it lives. The same is the case with Pedicularia, which occurs on red and yellow coral.

Helix desertorum, by its gray-brown colour, harmonises well with the prevailing tint of the desert sands, among which it finds a home. Benson observes that the gaudy H. haemastoma, which lives on the trunks of palm-trees in Ceylon, daubs its shell with its excrement. Our own Buliminus obscurus, which lives principally on the trunks of smooth-barked trees, daubs its shell with mud, and must often escape the observation of its enemies by its striking resemblance to the little knots on the bark, especially of beech trees, its favourite haunt. Some species of Microphysa, from the West Indies, habitually encrust their shells with dirt, and the same peculiarity in Vitrina has already been mentioned. Ariophanta Dohertyi Aldr., a recent discovery from Sumatra, is of a green colour, with a singularly delicate epidermis; it is arboreal in its habits, and is almost invisible amongst the foliage.[155] Many of our own slugs, according to Scharff, are coloured protectively according to their surroundings. A claret-coloured variety of Arion ater occurred to this observer only in pine woods, where it harmonised with the general colouring of the ground and the pine-needles, while young winter forms of the same species choose for hiding-places the yellow fallen leaves, whose colour they closely resemble. Limax marginatus (= arborum Bouch.) haunts tree trunks, and may easily be mistaken for a piece of bark; Amalia carinata lives on and under the ground, and in colour resembles the mould; Arion intermedius feeds almost exclusively on fungi, to which its colour, which is white, gray, or light yellow, tends to approximate it closely; Geomalacus maculosus conceals itself by its striking resemblance to the lichens which grow on the surface of rocks, and actually presumes on this resemblance so much as to expose itself, contrary to the usual custom of its congeners, to the full light of the afternoon sun.[156]

Several views have been advanced with regard to the dorsal papillae, or cerata, in the Nudibranchs. Professor W. A. Herdman, who has examined a considerable number of our own British species, in which these processes occur, is of opinion[157] that they are of two quite distinct kinds. In the first place, they may contain large offshoots, or diverticula, of the liver, and thus be directly concerned in the work of digestion. This is the case with Aeolis and Doto. In the second place, they may be simply lobes on the skin, with no connexion with the liver, and no special function to perform. This is the case with Tritonia, Ancula, and Dendronotus.

Professor Herdman is of opinion that although the cerata may in all cases aid in respiration to a certain extent, yet that extent is so small as to be left out of consideration altogether. He regards the cerata in both the two classes mentioned above as “of primary importance in giving to the animals, by their varied shapes and colours, appearances which are in some cases protective, and in others conspicuous and warning.”

Thus, for instance, Tritonia plebeia, which is fairly abundant at Puffin and Hilbre Is., appears always to be found creeping on the colonies of a particular polyp, Alcyonium digitatum, and nowhere else. The specimens in each colony of the polyp differ noticeably both in the matter of colour, and of size, and of varied degrees of expansion. The Tritonia differs also, being marked in varied tints of yellow, brown, blue, gray, black, and opaque white, in such a way as to harmonise with the varied colours of the Alcyonium upon which it lives. The cerata on the back of the Tritonia contribute to this general resemblance. They are placed just at the right distance apart, and are just the right size and colour, to resemble the crown of tentacles on the half-expanded polyp.

Similarly, Doto coronata, which, when examined by itself, is a very conspicuous animal, with showy, bright-coloured cerata, is found by Professor Herdman to haunt no other situations but the under side of stones and overhanging ledges of rock which are colonised by a hydroid, known as Clava multicornis. The Doto is masked by the tentacles and clusters of sporosacs on the zoophyte, with whose colouring and size its own cerata singularly correspond. A similar and even more deceptive correspondence with environment was noticed in the case of the very conspicuous Dendronotus arborescens.

In these cases, the colouring and general shape of the cerata are protective, i.e. they match their surroundings in such a way as to enable the animal, in all probability, to escape the observation of its enemies. According to Professor Herdman, however, the brilliant and showy coloration of the cerata of Aeolis is not protective but ‘warning.’ Aeolis does not hide itself away as if shunning observation, like Doto, Tritonia, and Dendronotus; on the contrary, it seems perfectly fearless and indifferent to being noticed. Its cerata are provided with sting-cells, like those of Coelenterata, at their tips, and its very conspicuousness is a warning to its enemies that they had better not try to attack it, just as the showy white tail of the skunk acts as a sort of danger-signal to its own particular foes. It is important for the Aeolis, not merely to be an unpalatable nettle in animal shape, but also to be conspicuous enough to prevent its being experimented upon as an article of food, in mistake for something less nasty.

Professor Herdman subsequently conducted some experiments[158] with fishes, with the view of testing his theory that the shapes and colours of Nudibranchs serve the purpose either of protection or warning, and bear direct relation to the creature’s edibility. These experiments, on the whole, distinctly tended to confirm the theory. Aeolis was evidently very nasty, and probably stung the mouths of the fishes who tried it. For the complete success of the theory, they ought to have let it severely alone, but the fish were evidently accustomed to make a dash at anything that was dropped into their tank. Another conspicuous mollusc, Ancula cristata, was introduced, Professor Herdman and his collaborator each commencing operations by eating a live specimen themselves. They found the taste pleasant, distinctly like that of an oyster. The fish, however, when the experiments were conducted under conditions which made the scene as much like ‘real life’ as possible, did not agree with Professor Herdman. The Ancula crawled over various parts of the tank for several days untouched by the fish, who sometimes went close to them and looked at them, but never attempted to taste them. Experiments with species whose colours were protective, such as Dendronotus, were also conducted, and the decided edibility of these species was established, the fish competing eagerly for them, and tearing them rapidly to pieces.

Mr. W. Garstang, of the Plymouth Laboratory of the Marine Biological Association, confirms[159] Professor Herdman’s views as to the shape and colour of Opisthobranchs. Pleurobranchus membranaceus is known to secrete, on the surface of the body, an acid which reddens blue litmus paper. It is, therefore, no doubt distasteful to fish, which all abominate the taste of acids, and is conspicuously marked with red-brown and yellowish ‘warning’ colours. Haminea and Philine, on the other hand, are good to eat, and consequently possess ‘protective’ coloration. Runcina Hancocki, which is of a brown colour, crawls over brown mud and weeds, but avoids green weeds, on whose surface it would appear conspicuous. Elysia viridis varies its colour according to its habitat, being green when on green weeds, and dark olive, brown, or reddish brown, on pools among tufts of littoral algae. Green specimens of Hermaea dendritica were kept in captivity, and placed in a dish with green and red sea-weeds. They were never observed crawling upon the red weed, upon which they would have been very conspicuous. Archidoris flammea occurred on bright red sponges, to which its colour was so closely assimilated that Mr. Garstang at first quite overlooked it. Goniodoris castanea was found under stones, feeding on compound Ascidians (Botryllus), which it sufficiently resembled to be very inconspicuous in that position.

Again, Jorunna Johnstoni lives[160] upon stones on our southern coast, associated with a certain sponge (Halichondria sp.), which it resembles so closely in outline, in colour, in character of surface, and in its projecting plumes, as to make it very difficult even for the careful observer to distinguish the one from the other. And, since fishes, are known to be distinctly averse to sponges of any kind as an article of food, this resemblance must be decidedly to the advantage of the Jorunna. Another Nudibranch (Calma glaucoides A. and H.) imitates the ova of certain fishes, on which it feeds. Its elongated and depressed form of body, transparent integuments, and silvery gray papillae combine to give it a strong resemblance to the spawn of the fish, which is deposited on stones, the roots of Laminaria, etc.[161]

The common Lamellaria perspicua appears to possess the power of protectively assimilating its colour, markings, etc., to the Ascidians on which it lives. A recent case, occurring off the Isle of Man, is thus described by Professor Herdman.[162] “The mollusc was on a colony of Leptoclinum maculatum, in which it had eaten a large hole. It lay in this cavity so as to be flush with the general surface; and its dorsal integument was not only whitish with small darker marks which exactly reproduced the appearance of the Leptoclinum surface with the ascidiozooids scattered over it, but there were also two larger elliptical clear marks which looked like the large common cloacal apertures of the Ascidian colony.... Presumably the Lamellaria escapes the observation of its enemies through being mistaken for part of the Leptoclinum colony; and the Leptoclinum, being crowded like a sponge with minute sharp-pointed spicules, is, I suppose, avoided as inedible (if not actually noxious through some peculiar smell or taste) by carnivorous animals which might devour such things as the soft unprotected mollusc.”

Various grades of parasitism occur among the Mollusca, from the true parasite, living and nourishing itself on the tissues and secretions of its host, to simple cases of commensalism. Some authors have divided these forms into endo- and ectoparasites, according as they live inside or outside of their host. Such a division, however, cannot be rigidly carried out, for certain forms are indifferently endo- and ecto-parasitical, while others are ecto-parasitic in the young form, and become endo-parasitic in the adult. It will be convenient, therefore, simply to group the different forms according to the home on which they find a lodgment.

Fig. 29.—Magilus antiquus L.: A, the adult, imbedded in coral, which has been broken away to show the tube; B, the young (free) form.

On Sponges.—Vulsella and Crenatula almost invariably occur in large masses of irregular shape, boring into sponges. They are especially abundant on Porifera from the Red Sea. Corals form a favourite home of many species, amongst which are several forms of Coralliophila, Rhizochilus, Leptoconchus, and Sistrum. Rhizochilus is a very singular creature, inhabiting branching corals. When adult, it forms irregular shelly extensions of both the inner and outer lips, which adhere to the shafts of the coral, or to the surface of neighbouring shells; at length the aperture becomes completely closed with the exception of the siphonal tube, which becomes long, and consists of the same shelly material. The common Magilus (Fig. 29), from the Red Sea and Indian Ocean, in the young form is shaped like a small Buccinum. As the coral (Meandrina) to which it attaches itself grows, the Magilus develops at the mouth a long calcareous tube, the aperture of which keeps pace with the growth of the coral, and prevents the mollusc from being entombed. The animal lives at the free, or outer, end of the tube, and is thus continually shifting its position, while the space it abandons becomes completely closed by a mass of solid calcareous matter. Certain species of Ovula inhabit Gorgonia, assuming the colour, yellow or red, of their host, and, in certain cases, developing, probably for prehensile purposes, a pointed extension of the two extremities of the shell. Pedicularia, a form akin to Cypraea, but with a more patulous mouth, inhabits the common Corallium rubrum of the Mediterranean, and another species has been noticed by Graeffe[163] on Melithaea ochracea in Fiji.

On Echinodermata.—(a) Crinoidea. Stylina comatulicola lives on Comatula mediterranea, fixed to the outer skin, which it penetrates by a very long proboscis; the shell is quite transparent.[164] A curious case of a fossil parasite has been noticed by Roberts.[165] A Calyptrea-shaped shell named Platyceras always occurred on the ventral side of a crinoid, encompassed by the arms. For some time this was thought to afford conclusive proof of the rapacity and carnivorous habits of the echinoderm, which had died in the act of seizing its prey. Subsequent investigations, however, showed that in all the cases noticed (about 150) the Platyceras covered the anal opening of the crinoid in such a way that the mouth of the mollusc must have been directly over the orifice of the anus. (b) Asteroidea. The comparatively soft texture of the skin of the starfishes renders them a favourite home of various parasites. The brothers Sarasin noticed[166] a species of Stilifer encysted on the rays of Linckia multiformis. Each shell was enveloped up to the apex, which just projected from a hole at the top of the cyst. The proboscis was long, and at its base was a kind of false mantle, which appeared to possess a pumping action. On the under side of the rays of the same starfish occurred a capuliform mollusc (Thyca ectoconcha), furnished with a muscular plate, whose cuticular surface was indented in such a way as to grip the skin of the Linckia. This plate was furnished with a hole, through which the pharynx projected into the texture of the starfish, acting as a proboscis and apparently furnished with a kind of pumping or sucking action. Adams and Reeve[167] describe Pileopsis astericola as living ‘on the tubercle of a starfish,’ and Stilifer astericola, from the coast of Borneo, as ‘living in the body of a starfish.’ In the British Museum there is a specimen of Pileopsis crystallina ‘in situ’ on the ray of a starfish, (c) On the brittle starfishes (Ophiuroidea) occur several species of Stiliferina. (d) Echinoidea. Various species of Stilifer occur on the ventral spines of echinoids, where they probably subsist on the excreta, and are sometimes found imbedded in the spines themselves. St. Turtoni occurs on the British coasts on several species of Echinus, and Montacuta substriata frequents Spatangus purpureus and certain species of Echinocardium, Cidaris, and Brissus. Lepton parasiticum has been described from Kerguelen I. on a Hemiaster, and a new genus, Robillardia, has recently been established[168] for a Hyalinia-shaped shell, parasitic on an Echinus from Mauritius. (e) Holothurioidea. The ‘sea-cucumbers’ afford lodgment to a variety of curious forms, some of which have experienced such modifications that their generic position is by no means established. Entoconcha occurs fixed by its buccal end to the blood-vessels of certain Synapta in the Mediterranean and the Philippines. Entocolax has been dredged from 180 fath. in Behring’s Straits, attached by its head to certain anterior muscles of a Myriotrochus.[169] A curious case of parasitism is described by Voeltzkow[170] as occurring on a Synapta found between tide-marks on the I. of Zanzibar. In the oesophagus of the Synapta was found a small bivalve (Entovalva), the animal of which was very large for its shell, and almost entirely enveloped the valves by its mantle. As many as five specimens occurred on a single Synapta. In the gut of the same Holothurian lived a small univalve, not creeping freely, but fixed to a portion of the stomach wall by a very long proboscis which pierced through it into the body cavity. This proboscis was nearly three times as long as the animal, and the forward portion of it was set with sharp thorns, no doubt in order to enable it to retain its hold and resist evacuation. Various species of Eulima have been noticed in every part of the world, from Norway to the Philippines, both inside and outside Holothurians.[171] Stilifer also occurs on this section of Echinoderms.[172]

On Annelida.—Cochliolepis parasiticus has been noticed under the scales of Acoetes lupina (a kind of ‘sea-mouse’) in Charleston Harbour.[173]

On Crustacea.—A mussel, ⅜ in. long, has been found[174] living under the carapace of the common shore-crab (Carcinus maenas), and one case has been noticed[175] where two mussels, one of several months’ growth, the other smaller, well secured by their byssi, were found under the abdomen of the same species, in such a position as to force the appendages apart and askew. These, however, are not so much cases of parasitism as of involuntary habitat, the mussel no doubt having become involved in the branchiae and the abdomen of the crab in the larval form.

On Mollusca.—A species of Odostomia (pallida Mont.) is found on our own coasts on the ‘ears’ of Pecten maximus, and also[176] on the operculum of Turritella communis. Another species (O. rissoides) frequently occurs in hiding under beds of mussels, but it is not clear whether the habitat is due to parasitism, or simply to the fact that the mass of mussels, knitted together and to the rock by the byssi, affords the Odostomia a safe lurking-place. At Panama the present writer found Crepidula (2 sp.) plentiful on the opercula of the great Strombus galea and of Cerithium irroratum. In each case the parasite exactly fitted the size of the operculum, and had assumed its colour, dark brown or chestnut. Amalthea is very commonly found on Conus, Turbo, and other large shells from the South Pacific, but this is probably not a case of parasitism, but simply of convenience of habitat, just as young oysters are frequently seen on the carapace and even on the legs of large crabs.

Fig. 30.—Crepidula onyx Sowb., parasitic on the operculum of Strombus galeatus Swains., Panama.

On Tunicata.—Lamellaria deposits its eggs and lives on an Ascidian (Leptoclinum), and the common Modiolaria marmorata lives in colonies imbedded in the test of Ascidia mentula and other simple Ascidians.

Fig. 31.—Two species of Eulima: A is sessile on the skin of a Holothurian, through which it plunges its sucking proboscis (Pr); B creeps freely in the stomach of a Holothurian. (After K. Semper.)

Special points of interest with regard to parasitic Mollusca relate to (1) Colour. This is in most cases absent, the shell being of a uniform hyaline or milky white. This may be due, in the case of the endo-parasitic forms, to absence of light, and possibly, in those living outside their host, to some deficiency in the nutritive material. A colourless shell is not necessarily protective, for though a transparent shell might evade detection, a milk-white hue would probably be conspicuous. (2) Modifications of structure. These are in many cases considerable. Entoconcha and Entocolax have no respiratory or circulatory organs, and no known nervous system; Thyca and certain Stilifer possess a curious suctorial apparatus; the foot in many cases has aborted, since the necessity for locomotion is reduced to a minimum, and its place is supplied by an enormous development of the proboscis, which enables the creature to provide itself with nutriment without shifting its position. K. Semper notices a case where a Eulima, whose habitat is the stomach of a Holothurian, retains the foot unmodified, while a species occurring on the outer skin, but provided with a long proboscis, has lost its foot altogether.[177] Special provision for holding on is noticed in certain cases, reminding us of similar provision in human parasites. Eyes are frequently, but not always wanting, even in endo-parasitic forms. A specially interesting modification of structure occurs in (3) the Radula or ribbon-shaped arrangement of the teeth. In most cases of parasitism (Eulima, Stilifer, Odostomia, Entoconcha, Entocolax, Magilus, Coralliophila, Leptoconcha) it is absent altogether. In Ovula and Pedicularia, genera which are in all other respects closely allied to Cypraea, the radula exhibits marked differences from the typical radula of the Cypraeidae. The formula (3·1·3) remains the same, but the laterals are greatly produced and become fimbriated, sometimes at the extremity only, sometimes along the whole length. A very similar modification occurs in the radula of Sistrum spectrum Reeve, a species which is known to live parasitically on one of the branching corals. Here the laterals differ from those of the typical Purpuridae in being very long and curved at the extremity. The general effect of these modifications appears to be the production of a radula rather of the type of the vegetable-feeding Trochidae, which may perhaps be regarded as a link in the chain of gradually-degraded forms which eventually terminate in the absence of the organ altogether. The softer the food, the less necessity there is for strong teeth to tear it; the teeth either become smaller and more numerous, or else longer and more slender, and eventually pass away altogether. It is curious, however, that the same modified form of radula should appear in species of Ovula (e.g. ovum) and that the same absence of radula should occur in species of Eulima (e.g. polita) known to be not parasitic. This fact perhaps points back to a time when the ancestral forms of each group are parasitic and whose radulae were modified or wanting, the modification or absence of that organ being continued in some of their non-parasitical descendants.

Mollusca are concerned in several interesting cases of commensalism, or the habitual association of two organisms, as distinguished from parasitism, where one form preys more or less upon the other.

Mr. J. T. Marshall has given[178] an interesting account of the association of Montacuta ferruginosa with Echinocardium cordatum. The Echinoderm lives in muddy sand in Torbay, at a depth of about 6 inches, and the Montacuta lives in a burrow leading from its ventral end and running irregularly in a sloping direction for 3 or 4 inches, the burrow, which is made by a current from the Echinoderm, being almost exactly the width of the Montacuta. The Montacuta were always arranged in the burrows in order of size, the largest being close to the Echinoderm, and the smallest of a string of about six at the other end of the burrow. In another part of S. Devon, where the sand was soft and sloppy, the Echinocardia rise to the surface and travel along the sand; in this case the Montacuta were attached to their host by means of a byssus, and were dragged along as it travelled.

The Rev. Dr. Norman has noted[179] a somewhat similar habitat for Lepton squamosum. This rare little British species was found at Salcombe, living in the burrows of Gebia stellata, in all probability feeding upon the secretions from the body of the crustacean. Dr. Norman suggests that the extreme flatness of the shell of the Lepton is of great advantage in enabling it not to get in the way of the Gebia as he scuttles up and down his burrow. Another species of Lepton is found on the coast of Florida in a precisely similar locality,[180] while a third species, occurring on the Oregon and California coasts, actually attaches itself to the inner surface of the abdomen of a Gebia.[181]

Fig. 32.—Ephippodonta Macdougalli Tate, S. Australia. A, Burrow of prawn, the X indicating the position of the mollusc; sp, sponge. B, Ventral view of Ephippodonta; by, byssus; f, foot; m, mantle; mm, fused mantle borders. C, View of interior of shells; h, hinge; m´m´, adductor muscles. (A × ½; B and C × 2.)

A very singular case of commensalism has been recently discovered with regard to a genus of Australian bivalve shells, Ephippodonta. This genus is never found except in the burrow of a species of prawn (Axius plectorhynchus Str.). For some reason at present unexplained, the burrow of this particular prawn appears to be exceedingly popular as a habitat for certain bivalves, for, besides two species of Ephippodonta, a Kellia and three Mylitta are found there, and there alone. Sometimes the prawn, when the rock is hard, builds a tunnel of mud upon it, at other times it excavates the soft calciferous sandstone. “This burrow is lined with a tenacious brown mud, composed of excrementitious matter; and, in addition to the mud lining, there is always more or less present an orange-coloured sponge which I have never found elsewhere. Upon the mud or sponge, and adhering very closely, are found the Ephippodonta. They quickly form a pit-like depression by means of their foot, and appear almost covered by the mud.” During the winter months (March-July) the prawn appears to fill his burrow, possibly as a provision against stormy weather, with large quantities of minced seaweed, underneath which immense numbers of very young Ephippodonta are found living.[182] The extreme flatness of the Ephippodonta must be due to the same cause as the flatness of the Lepton noticed above, namely, the necessity of not impeding or interfering with the lively motions of the prawn. In the case of Lepton the two valves close completely and the shell is still very flat; in Ephippodonta, on the other hand, the same result is produced by the valves being opened to their widest possible extent. As in Entovalva, a continuation of the mantle covers the outer surface of the shell.

It is a familiar experience to the student, not only of the Mollusca, but of every branch of animal or vegetable life, to come across examples which exhibit certain slight deviations from the type form as usually understood. These deviations may be more or less pronounced, but, as a rule, a series of forms can be discovered, gradually leading up to or down from the type. The definition of what constitutes a species,—and, still more, the rigid application of such definition—will always remain a difficult task, so long as the personal element persists in him who defines.[183] What seems to one authority ample ground for distinction of species, another may regard as of comparatively trivial importance. The practical outcome of these divergent views is sufficiently illustrated by the attitude of Mr. F. P. Marrat on the one hand, and of what may be called the modern French school of conchologists on the other. Mr. Marrat holds, or held, that the great genus Nassa, of which more than 150 species are generally recognised, is one shell (species) in an endless variety of forms. The modern French school go to the other extreme, and apparently proceed upon the view that almost any difference in form, however slight, is sufficient to constitute a separate species.

It will be generally admitted, however, that some structural difference in the organisation of the animal (as distinct from that of the shell alone) is necessary for the permanent constitution of specific rank.[184] What amount of structural difference is required, what particular organ or organs must exhibit this difference, will depend largely upon the idiosyncrasy of the observer. But if this, or something like this definition of a species be accepted, it will follow that a so-called ‘variety’ will be a form which exhibits differences from the type which do not amount to permanent structural differences in the organisation of the animal. The final court of appeal as to what affords sufficient evidence for ‘permanent structural differences’ will have to be, as with Aristotle of old, the judgment of the educated man.

It is, however, more to our present purpose to discuss the causes of variation than to lay down definitions of what variation is. One of the most obvious causes of variation lies in a change or changes in the environment. If we may assume, for the moment, that the type form of a species is the form which is the mean of all the extremes, and that this form is the resultant of all the varied forces brought to bear upon it, whether of food, climate, temperature, competition of numbers, soil, light, amount of water, etc., it will follow that any change in one or more of these forces, if continuous and considerable, any change, in other words, of the environment, will produce its effect upon the organism in question. And this effect will be for the better or for the worse, according to the particular nature of the change itself as tending towards, or away from, the optimum of environment for the species concerned. Hence may be produced varieties, more or less marked according to the gravity of the change, although it must be noted that at times a change apparently unimportant from our point of view, will produce very marked results upon the species. It is indeed scarcely possible to predict with any certainty, in the present state of our knowledge (beyond certain broad results) what will be the particular effect upon a species of any given change in its surroundings.

Effects of Change in the Environment as tending to produce Variation.

(a) Changes in Climate, Temperature, Elevation, etc.—In the eastern basin of the Baltic the marine Mollusca are much more stunted than in the western.[185] For instance, Mytilus edulis near Kiel is 8–9 cm. long, while near Gothland it only attains a length of 3–4 cm. Mollusca living at only a shallow depth (e.g. Tellina balthica, Mya arenaria, Cardium edule) do not differ much in size in different parts of the Baltic, but in the far eastern basin the calcareous layers of the shells of Mya arenaria and Tellina balthica are extraordinarily thin, and disappear very rapidly after death, leaving only the cuticular membrane, still united by the ligament, in a perfect state of preservation. These remarkable variations are no doubt to a large extent due to the violent changes of temperature which are experienced in the Baltic, and by which the steady development of the animals in question is interrupted and thrown out of gear. The same species occur on the coasts of Greenland and Iceland, where they attain a considerably larger size than in the Baltic, in spite of the lower mean temperature, probably because their development is not interrupted by any sudden change from cold to heat or vice versâ.

Karl Semper has shown that Limnaea stagnalis is developed, lives and feeds best in a mean temperature of about 20° C. (= 68° F.). This mean, however, must not be the mean of two distant extremes, for the Limnaea cannot digest its food and grow in a temperature which is less than 14° or 15° C. (= 57° or 59° F.), or more than 30° to 32° C. (= 86° to 90° F.). In certain localities, therefore, the interruption to the growth of this species must be serious and prolonged, and may tend towards the production of more or less dwarfed varieties. Thus specimens from Malham Tarn, a lake in Yorkshire 1250 feet above the sea, are permanently dwarfed, and have a very thin and fragile shell. Limnaea peregra in the Pyrenees, Alps, and Himalayas is generally of a very delicate form and dwarfed habit, while the small variety known as lacustris occurs, according to Jeffreys, only in mountain lakes in Zetland, Scotland, Ireland, and N. England. Specimens brought by Mr. Bateson from lakes near the Sea of Aral, which are salt for some months and comparatively fresh for others, exhibit clearly the effect of changes in the environment (Figs. 33 and 34). Excess of heat produces similar results to excess of cold. L. peregra var. thermalis, found in the warm springs of the Pyrenees and the Vosges, and the var. geisericola, from the hot water of the Iceland geysers, are alike thin and dwarfed forms.

Many instances may be given of ‘varieties due to locality.’ In some of these, the cause which predisposes towards variation can be inferred with some approach to certainty, in others we must be content to note the fact, without at present being able to perceive its explanation.

Fig. 33.—Four examples of Limnaea peregra Müll., from salt marshes near the Sea of Aral, showing different effects produced by abnormal conditions of life.

Fig. 34.—Four examples of Limnaea stagnalis L., from marshes in the Aral district which are salt for several months in the year, illustrating variation produced by changes in the environment. × ½.

Desert specimens of widely distributed species, e.g. Helix pomatia, H. niciensis, H. pisana, Leucochroa candidissima are much thicker than the type, and tend to lose all trace of coloured bands. These modifications are clearly the means of preventing evaporation of moisture, the dull white or grayish brown colour being calculated to absorb the smallest possible amount of heat. Desert shells in all parts of the world (e.g. N. Africa, Arabia, Central Asia, S. Africa, W. America) have been noticed to exhibit these peculiarities.

A very singular case of the reverse process, i.e. the production of darkened forms of shell through cold, has been noticed by Fischer as characteristic of the marine shells of the west coast of South America.[186] This melanism is especially noticeable in Trochus, Turbo, Chiton, Mitra, and Pleurotoma, and is attested by the specific names, not merely expressive of actual blackness (e.g. nigerrimus, ater, atramentarius, maurus), but also of a generally lugubrious tone (e.g. moestus, funebralis, tristis, lugubris, luctuosus). It is highly probable that this concurrence of specific melanism (which stands quite alone in the world) is due to the cold polar current which impinges on the Chilian coasts, for the same genera occur on the opposite shores of the continent without exhibiting any trace whatever of this mournful characteristic.

It is a well-known fact, attested by many observers, that our common Limax agrestis as well as the young of Arion ater become decidedly darker in summer than in winter. If these slugs were accustomed to disport themselves in the sun, it might have been suggested that this increased darkness of colour tended to absorb more of the heat rays. But since this is not the case, the result is probably due to some unexplained effect of higher temperature. According to Lessona and Pollonera, the length of the keel in Limax arborum varies greatly in different parts of Italy, being shorter in specimens from low ground, but much longer in those inhabiting more elevated regions. The longer the keel, the more obscure the colouring becomes, so that in the Upper Alps of Piedmont individuals are practically black. Roebuck has observed that Scottish specimens of this same slug are much darker and less translucent than English forms. According to Simroth, our common black slug, Arion ater, is a northern type, which in more southern latitudes assumes the form known as A. rufus. Similarly Limax maximus “in its northern form cinereo-niger is almost wholly black, but in the more genial climate of Italy develops a series of brilliantly coloured and strikingly marked variations which have received numerous distinctive names from Italian limacologists.”[187] According to Scharff, however[188] (who regards the colours of slugs as in the main protective), these dark forms are by no means exclusively northern, being found equally on the parched plains of Spain and Portugal, and in the bleak climate of Norway. The same authority observes that similar forms occur both in the dry regions of E. Germany, and in the very humid district of western Ireland.

It appears unquestionable that marine genera from high northern latitudes are provided with shells of uniform colour, or whitish with a pale brown epidermis; spots, bands, or stripes seldom occur. The arctic forms of Buccinum, Trophon, Chrysodomus, Margarita, Crenella, Leda, Yoldia, Astarte illustrate this fact. In the more temperate seas of Europe, colours tend on the whole to increase, although there are certain genera (e.g. Pecten) which are not more brightly coloured in Mediterranean than in Icelandic waters.

Land Mollusca inhabiting the mainland of a continent not unfrequently become smaller when they have spread to adjacent islands where perhaps the rainfall is less abundant or the soil and food-supply less nicely adjusted to their wants. Orthalicus undatus is decidedly larger on the mainland of S. America than on the adjacent islands of Trinidad and Grenada. Specimens of Bulimulus exilis from Barbados are invariably broader and more obese than those from S. Thomas, while those from the volcanic island of S. Lucia, where lime is deficient, are small and very slender. Streptaxis deformis, as occurring at Trinidad, is only half the size of specimens from Georgetown, Demerara.[189]

Certain localities appear, for some unexplained reason, to be particularly favourable to the production of albino varieties. The neighbourhood of Lewes, in Sussex, has produced no fewer than fourteen of these forms of land Mollusca and five of fresh-water.[190]

Our common Helix aspersa, as found near Bristol, is said to be ‘dark coloured’; about Western-super-mare ‘brown, with black markings’; near Bath ‘very pale and much mottled’; at Cheddar ‘very solid and large.’[191] Sometimes the same kind of variation is exhibited by different species in the same locality. Thus specimens of H. aspersa, H. nemoralis, and H. hortensis, taken from the same bank at Torquay, presented a straw-coloured tinge of ground colour, with red-brown bands or markings. Trochiform H. nemoralis and H. arbustorum, sinistral H. hortensis and H. aspersa, sinistral H. aspersa and H. virgata, and similarly banded forms of H. caperata and H. virgata, have been taken together.[192]

The immediate neighbourhood of the sea appears frequently to have the effect of dwarfing land Mollusca. Thus the var. conoidea of Helix aspersa, which is small, conical, with a compressed mouth, occurs ‘on sandhills and cliffs at the seaside.’ The varieties conica and nana of Helix hispida are found ‘near the sea.’ Helix virgata is exceedingly small in similar localities, and tends to become unicoloured. H. caperata var. Gigaxii, a small depressed form, occurs at ‘Sandwich and Falmouth.’[193] Sometimes, however, the exact opposite is the case, for H. nemoralis var. major, which is ‘much larger’ than the type, occurs on ‘sandhills and downs’ and is ‘remarkably large in the I. of Arran, Co. Galway.’ The dwarf form of Limnaea peregra known as maritima appears to be confined to the neighbourhood of the sea.

Dwarfing of the shell seems frequently to be the result of an elevated locality, not perhaps so much as the direct consequence of purer air and less barometric pressure, as of changes in the character of the food supply and in the humidity of the air. Several species of Helix have a variety minor which is characteristic of an Alpine habitat. Helix arbustorum var. alpestris, which is scarcely two-thirds the size of the type, occurs on the Swiss Alps in the region of perpetual snow. Sometimes a very slight elevation is sufficient to produce the dwarfed form. At Tenby the type form of Helix pisana is scattered in countless numbers over the sandhills just above high-water mark. At the extreme western end of these sandhills rises abruptly to a height of over 100 feet the promontory known as Giltar Head, the vegetation of which is entirely distinct from that of the burrows below. There is a colony of H. pisana at the end of Giltar, all of which are devoid of the characteristic markings of the typical form, and most are dwarfed and stunted in growth.

Occasionally the same variety will be found to be produced by surroundings of very different nature. Thus the var. alpestris, of H. arbustorum mentioned above, besides being characteristic of high Alpine localities, also occurs abundantly in low marshes at Hoddesdon on the River Lea. Helix pulchella var. costata, according to Jeffreys, is found in dry and sandy places, often under loose stones and bricks on walls, while other authorities have noticed it in wet and dry localities quite indifferently.

Sometimes the production of a variety may be traced to the intrusion of some other organism. According to Brot, nine-tenths of the Limnaea peregra inhabiting a certain pond near Geneva, were, during one season, afflicted with a malformation of the base of the columella. This deformity coincided with the appearance, in the same waters, of extraordinary numbers of Hydra viridis. The next season, when the Hydra disappeared, the next generation of Limnaea was found to have resumed its normal form.

It has been noticed that a form of Helix caperata with a flattened spire and wide umbilicus is restricted to tilled fields, especially the borders of clover fields, while a form with a more elevated spire and more compact whorls occurs exclusively in open downs and uncultivated places. The Rev. S. S. Pearce accounts[194] for this divergence by the explanation that the flatter spire enables the shell of the fields to creep about more easily under the leaves or matted weeds, seldom requiring to crawl up a stalk or stem, while on the short turf of the downs and pastures the smaller and more rounded shell enables the animal to manoeuvre in and out of the blades of grass, and even to crawl up them with considerable activity. The same writer endeavours to explain the causes which regulate the distribution of H. caperata var. ornata. He found that this variety (dark bands on a white ground) occurred almost exclusively on downs which were fed upon by sheep, associated with the common or mottled form, while the latter form alone occurred in localities where sheep were not accustomed to feed. Assuming then, as is probably the case, that sheep, in the course of their close pasturing, devour many small snails, he believes that individuals of the more conspicuous form ornata were more likely to be noticed, and therefore avoided, by the sheep, than the mottled form, which would more easily escape their observation. Hence the var. ornata is due to the advantage which strikingly coloured individuals obtained owing to their conspicuous habit, as compared with the typical form, which would be less readily detected.

(b) Changes in Soil, Station, Character of Water, etc.—A deficiency of lime in the composition of the soil of any particular locality produces very marked effects upon the shells of the Mollusca which inhabit it; they become small and very thin, occasionally almost transparent. The well-known var. tenuis of Helix aspersa occurs on downs in the Channel Islands where calcareous material is scarce. For similar reasons, H. arbustorum develops a var. fusca, which is depressed, very thin, and transparent, at Scilly, and also at Lunna I., E. Zetland.

Fig. 35.—19 specimens of Purpura lapillus L., Great Britain, illustrating variation.

(1) Felixstowe, sheltered coast; (2), (3) Newquay, on veined and coloured rock; (4) Herm, rather exposed; (5) Solent, very sheltered; (6) Land’s End, exposed rocks, small food supply; (7) Scilly, exposed rocks, fair food supply; (8) St. Leonards, flat mussel beds at extreme low water; (9) Robin Hood’s Bay, sheltered under boulders, good food supply; (10) Rhoscolyn, on oyster bed, 4–7 fath. (Macandrew); (11) Guernsey, rather exposed rocks; (12) Estuary of Conway, very sheltered, abundant food supply; (13), (14) Robin Hood’s Bay, very exposed rocks, poor food supply; (14) slightly monstrous; (15), (16), (17) Morthoe, rather exposed rocks, but abundant food supply; (18) St. Bride’s Bay; (19) L. Swilly, sheltered, but small food supply. All from the author’s collection, except (10).

The common dog-whelk (Purpura lapillus) of our own coasts is an exceedingly variable species, and in many cases the variations may be shown to bear a direct relation to the manner of life (Fig. 35). Forms occurring in very exposed situations, e.g. Land’s End, outer rocks of the Scilly Is., coasts of N. Devon and Yorkshire, are stunted, with a short spire and relatively large mouth, the latter being developed in order to increase the power of adherence to the rock and consequently of resistance to wave force. On the other hand, shells occurring in sheltered situations, estuaries, narrow straits, or even on open coasts where there is plenty of shelter from the waves, are comparatively of great size, with a well-developed, sometimes produced spire, and a mouth small in proportion to the area of shell surface. In the accompanying figure, the specimens from the Conway estuary and the Solent (12, 5) well illustrate this latter form of shell, while that from exposed rocks is illustrated by the specimens from Robin Hood’s Bay (13, 14). Had these specimens occurred alone, or had they been brought from some distant and unexplored region, they must inevitably have been described as two distinct species.

Fig. 36.—Valves of Cardium edule from the four upper terraces of Shumish Kul, a dry salt lake adjacent to the Aral Sea. (After Bateson.)

Mr. W. Bateson has made[195] some observations on the shells of Cardium edule taken from a series of terraces on the border of certain salt lakes which once formed a portion of the Sea of Aral. As these lakes gradually became dry, the water they contained became salter, and thus the successive layers of dead shells deposited on their borders form an interesting record of the progressive variation of this species under conditions which, in one respect at least, can be clearly appreciated. At the same time the diminishing volume of water, and the increasing average temperature, would not be without their effect. It was found that the principal changes were as follows: the thickness, and consequently the weight, of the shells became diminished, the size of the beaks was reduced, the shell became highly coloured, and diminished considerably in size, and the breadth of the shells increased in proportion to their length (Fig. 36). Shells of the same species of Cardium, occurring in Lake Mareotis, were found to exhibit very similar variations as regards colour, size, shape, and thickness.

Unio pictorum var. compressa occurs near Norwich at two similar localities six or seven miles distant from one another, under circumstances which tend to show that similar conditions have produced similar results. The form occurs where the river, by bending sharply in horse-shoe shape, causes the current to rush across to the opposite side and form an eddy near the bank on the outside of the bend. Just at the edge of the sharp current next the eddy the shells are found, the peculiar form being probably due to the current continually washing away the soft particles of mud and compelling the shell to elongate itself in order to keep partly buried at the bottom.[196]

The rivers Ouse and Foss, which unite just below York, are rivers of strikingly different character, the Ouse being deep, rapid, with a bare, stony bottom, and little vegetable growth, and receiving a good deal of drainage, while the Foss is shallow, slow, muddy, full of weeds and with very little drainage. In the Foss, fine specimens of Anodonta anatina occur, lustrous, with beautifully rayed shells. A few yards off, in the Ouse, the same species of Anodonta is dull brown in colour, its interior clouded, the beaks and epidermis often deeply eroded. Precisely the same contrast is shown in specimens of Unio tumidus, taken from the same rivers, Ouse specimens being also slightly curved in form. Just above Yearsley Lock in the Foss, Unio tumidus occurs, but always dwarfed and malformed, a result probably due to the effect of rapidly running water upon a species accustomed to live in still water.[197] Simroth records the occurrence of remarkably distorted varieties in two species of Aetheria which lived in swift falls of the River Congo.[198]

A variety of Limnaea peregra with a short spire and rather strong, stoutly built shell occurs in Lakes Windermere, Derwentwater, and Llyn-y-van-fach. It lives adhering to stones in places where there are very few weeds, its shape enabling it to withstand the surf of these large lakes, to which the ordinary form would probably succumb.[199]

Scalariform specimens of Planorbis are said to occur most commonly in waters which are choked by vegetation, and it has been shown that this form of shell is able to make its way through masses of dense weed much more readily than specimens of normal shape.

Continental authorities have long considered Limnaea peregra and L. ovata as two distinct species. Hazay, however, has succeeded in rearing specimens of so-called peregra from the ova of ovata, and so-called ovata from the ova of peregra, simply by placing one species in running water, and the other in still water.

According to Mr. J. S. Gibbons[200] certain species of Littorina, in tropical and sub-tropical regions, are confined to water more or less brackish, being incapable of living in pure salt water. “I have met,” says Mr. Gibbons, “with three of these species, and in each case they have been distinguished from the truly marine species by the extreme (comparative) thinness of their shells, and by their colouring being richer and more varied; they are also usually more elaborately marked. They are to be met with under three different conditions—(1) in harbours and bays where the water is salt with but a slight admixture of fresh water; (2) in mangrove swamps where salt and fresh water mix in pretty equal volume; (3) on dry land, but near a marsh or the dry bed of one.

“L. intermedia Reeve, a widely diffused E. African shell, attaches itself by a thin pellicle of dried mucus to grass growing by the margin of slightly brackish marshes near the coast, resembling in its mode of suspension the Old World Cyclostoma. I have found it in vast numbers in situations where, during the greater part of the year, it is exposed to the full glare of an almost vertical sun, its only source of moisture being a slight dew at night-time. The W. Indian L. angulifera Lam., and a beautifully coloured E. African species (? L. carinifera), are found in mangrove swamps; they are, however, less independent of salt water than the last.”

Mr. Gibbons goes on to note that brackish water species (although not so solid as truly marine species) tend to become more solid as the water they inhabit becomes less salt. This is a curious fact, and the reverse of what one would expect. Specimens of L. intermedia on stakes at the mouth of the Lorenço Marques River, Delagoa Bay, are much smaller, darker, and more fragile, than those living on grass a few hundred yards away. L. angulifera is unusually solid and heavy at Puerto Plata (S. Domingo) among mangroves, where the water is in a great measure fresh; at Havana and at Colon, where it lives on stakes in water but slightly brackish, it is thinner and smaller and also darker coloured.

(c) Changes in the Volume of Water.—It has long been known that the largest specimens, e.g. of Limnaea stagnalis and Anodonta anatina, only occurred in pieces of water of considerable size. Recent observation, however, has shown conclusively that the volume of water in which certain species live has a very close relation to the actual size of their shells, besides producing other effects. Lymnaea megasoma, when kept in an aquarium of limited size, deposited eggs which hatched out; this process was continued in the same aquarium for four generations in all, the form of the shell of the last generation having become such that an experienced conchologist gave it as his opinion that the first and last terms of the series could have no possible specific relation to one another. The size of the shell became greatly diminished, and in particular the spire became very slender.[201]

The same species being again kept in an aquarium under similar conditions, it was found that the third generation had a shell only four-sevenths the length of their great grandparents. It was noticed also that the sexual capacities of the animals changed as well. The liver was greatly reduced, and the male organs were entirely lost.[202]

K. Semper conducted some well-known experiments bearing on this point. He separated[203] specimens of Limnaea stagnalis from the same mass of eggs as soon as they were hatched, and placed them simultaneously in bodies of water varying in volume from 100 to 2000 cubic centimetres. All the other conditions of life, and especially the food supply, were kept at the known optimum. He found, in the result, that the size of the shell varied directly in proportion to the volume of the water in which it lived, and that this was the case, whether an individual specimen was kept alone in a given quantity of water, or shared it with several others. At the close of 65 days the specimens raised in 100 cubic cm. of water were only 6 mm. long, those in 250 cubic cm. were 9 mm. long, those in 600 cubic cm. were 12 mm. long, while those kept in 2000 cubic cm. attained a length of 18 mm. (Fig. 37).

An interesting effect of a sudden fall of temperature was noticed by Semper in connection with the above experiments. Vessels of unequal size, containing specimens of the Limnaea, happened to stand before a window at a time when the temperature suddenly fell to about 55° F. The sun, which shone through the window, warmed the water in the smaller vessels, but had no effect upon the temperature of the larger. The result was, that the Limnaea in 2000 cubic cm., which ought to have been 10 mm. long when 25 days old, were scarcely longer, at the end of that period, than those which had lived in the smaller vessels, but whose water had been sufficiently warm.

Fig. 37.—Four equally old shells of Limnaea stagnalis, hatched from the same mass of ova, but reared in different volumes of water: A in 100, B in 250, C in 600, and D in 2000 cubic centimetres. (After K. Semper.)

CHAPTER IV USES OF SHELLS FOR MONEY, ORNAMENT, AND FOOD—CULTIVATION OF THE OYSTER, MUSSEL, AND SNAIL—SNAILS AS MEDICINE—PRICES GIVEN FOR SHELLS

The employment of shells as a medium of exchange was exceedingly common amongst uncivilised tribes in all parts of the world, and has by no means yet become obsolete. One of the commonest species thus employed is the ‘money cowry’ (Cypraea moneta, L.), which stands almost alone in being used entire, while nearly all the other forms of shell money are made out of portions of shells, thus requiring a certain amount of labour in the process of formation.

One of the earliest mentions of the cowry as money occurs in an ancient Hindoo treatise on mathematics, written in the seventh century A.D. A question is propounded thus: ‘the ¼ of ¹/₁₆ of ⅕ of ¾ of ⅔ of ½ a dramma was given to a beggar by one from whom he asked an alms; tell me how many cowry shells the miser gave.’ In British India about 4000 are said to have passed for a shilling, but the value appears to differ according to their condition, poor specimens being comparatively worthless. According to Reeve[204] a gentleman residing at Cuttack is said to have paid for the erection of his bungalow entirely in cowries. The building cost him 4000 Rs. sicca (about £400), and as 64 cowries = 1 pice, and 64 pice = 1 rupee sicca, he paid over 16,000,000 cowries in all.

Cowries are imported to England from India and other places for the purposes of exportation to West Africa, to be exchanged for native products. The trade, however, appears to be greatly on the decrease. At the port of Lagos, in 1870, 50,000 cwts. of cowries were imported.[205]

A banded form of Nerita polita was used as money in certain parts of the South Pacific. The sandal-wood imported into the China market is largely obtained from the New Hebrides, being purchased of the natives in exchange for Ovulum angulosum, which they especially esteem as an ornament. Sometimes, as in the Duke of York group, the use of shell money is specially restricted to certain kinds of purchase, being employed there only in the buying of swine.

Among the tribes of the North-West coasts of America the common Dentalium indianorum used to form the standard of value, until it was superseded, under the auspices of the Hudson’s Bay Company, by blankets. A slave was valued at a fathom of from 25 to 40 of these shells, strung lengthwise. Inferior or broken specimens were strung together in a similar way, but were less highly esteemed; they corresponded more to our silver and copper coins, while the strings of the best shells represented gold.

The wampum of the eastern coast of North America differed from all these forms of shell money, in that it required a laborious process for its manufacture. Wampum consisted of strings of cylindrical beads, each about a quarter of an inch in length and half that breadth. The beads were of two colours, white and purple, the latter being the more valuable. Both were formed from the common clam, Venus mercenaria, the valves of which are often stained with purple at the lower margins, while the rest of the shell is white. Cut small, ground down, and pierced, these shells were converted into money, which appears to have been current along the whole sea-board of North America from Maine to Florida, and on the Gulf Coast as far as Central America, as well as among the inland tribes east of the Mississippi. Another kind of wampum was made from the shells of Busycon carica and B. perversum. By staining the wampum with various colours, and disposing these colours in belts in various forms of arrangement, the Indians were able to preserve records, send messages, and keep account of any kind of event, treaty, or transaction.

Another common form of money in California was Olivella biplicata, strung together by rubbing down the apex. Button-shaped disks cut from Saxidomus arata and Pachydesma crassatelloides, as well as oblong pieces of Haliotis, were employed for the same purpose, when strung together in lengths of several yards.

“There is a curious old custom,” writes Mr. W. Anderson Smith,[206] “that used formerly to be in use in this locality [the western coast of Scotland], and no doubt was generally employed along the sea-board, as the most simple and ready means of arrangement of bargains by a non-writing population. That was, when a bargain was made, each party to the transaction got one half of a bivalve shell—such as mussel, cockle, or oyster—and when the bargain was implemented, the half that fitted exactly was delivered up as a receipt! Thus a man who had a box full of unfitted shells might be either a creditor or a debtor; but the box filled with fitted shells represented receipted accounts. Those who know the difficulty of fitting the valves of some classes of bivalves will readily acknowledge the value of this arrangement.”

Shells are employed for use and for ornament by savage—and even by civilised—tribes in all parts of the world. The natives of Fiji thread the large Turbo argyrostoma and crenulatus as weights at the edge of their nets, and also employ them as sinkers. A Cypraea tigris cut into two halves and placed round a stone, with two or three showy Oliva at the sides, is used as a bait for cuttles. Avicula margaritifera is cut into scrapers and knives by this and several other tribes. Breast ornaments of Chama, grouped with Solarium perspectivum and Terebra duplicata are common among the Fijians, who also mount the Avicula on a backing of whales’ teeth sawn in two, for the same purpose. The great Orange Cowry (Cypraea aurantiaca) is used as a badge of high rank among the chieftains. One of the most remarkable Fijian industries is the working of whales’ teeth to represent this cowry, as well as the commoner C. talpa, which is more easily imitated.

Among the Solomon islanders, cowries are used to ornament their shields on great field days, and split cowries are worn as a necklace, to represent human teeth. Small bunches of Terebellum subulatum are worn as earrings, and a large valve of Avicula is employed as a head ornament in the centre of a fillet. The same islanders ornament the raised prows of their canoes, as well as the inside of the stern-post, with a long row of single Natica.

The native Papuans employ shells for an immense variety of purposes. Circlets for the head are formed of rows of Nassa gibbosula, rubbed down till little but the mouth remains. Necklaces are worn which consist of strings of Oliva, young Avicula, Natica melanostoma, opercula of Turbo, and valves of a rich brown species of Cardium, pendent at the end of strings of the seeds known as Job’s tears. Struthiolaria is rubbed down until nothing but the mouth is left, and worn in strings round the neck. This is remarkable, since Struthiolaria is not a native Papuan shell, and indeed occurs no nearer than New Zealand. Sections of Melo are also worn as a breast ornament, dependent from a necklace of cornelian stones. Cypraea erosa is used to ornament drinking bowls, and Ovulum ovum is attached to the native drums, at the base of a bunch of cassowary feathers, as well as being fastened to the handle of a sago-beater.

In the same island, the great Turbo and Conus millepunctatus are ground down to form bracelets, which are worn on the biceps. The crimson lip of Strombus luhuanus is cut into beads and perforated for necklaces. Village elders are distinguished by a single Ovulum verrucosum, worn in the centre of the forehead. The thick lip of Cassis cornuta is ground down to form nose pieces, 4½ inches long. Fragments of a shell called Kaïma (probably valves of a large Spondylus) are worn suspended from the ears, with little wisps of hair twisted up and thrust through a hole in the centre. For trumpets, Cassis cornuta, Triton tritonis, and Ranella lampas are used, with a hole drilled as a mouthpiece in one of the upper whorls. Valves of Batissa, Unio, and Mytilus are used as knives for peeling yams. Spoons for scooping the white from the cocoa-nut are made from Avicula margaritifera. Melo diadema is used as a baler in the canoes.[207]

In the Sandwich Islands Melampus luteus is worn as a necklace, as well as in the Navigator Islands. A very striking necklace, in the latter group, is formed of the apices of a Nautilus, rubbed down to show the nacre. The New Zealanders use the green opercula of a Turbo, a small species of Venus, and Cypraea asellus to form the eyes of their idols. Fish-hooks are made throughout the Pacific of the shells of Avicula and Haliotis, and are sometimes strengthened by a backing made of the columella of Cypraea arabica. Small axe-heads are made from Terebra crenulata ground down (Woodlark I.), and larger forms are fashioned from the giant Tridacna (Fiji).

Shells are used to ornament the elaborate cloaks worn by the women of rank in the Indian tribes of South America. Specimens of Ampullaria, Orthalicus, Labyrinthus, and Bulimulus depend from the bottom and back of these garments, while great Bulimi, 6 inches long, are worn as a breast ornament, and at the end of a string of beads and teeth.[208]

The chank-shell (Turbinella rapa) is of especial interest from its connexion with the religion of the Hindoos. The god Vishnu is represented as holding this shell in his hand, and the sinistral form of it, which is excessively rare, is regarded with extraordinary veneration. The chank appears as a symbol on the coins of some of the ancient Indian Empires, and is still retained on the coinage of the Rajah of Travancore.

The chief fishery of the chank-shell is at Tuticorin, on the Gulf of Manaar, and is conducted during the N. E. monsoon, October-May. In 1885–86 as many as 332,000 specimens were obtained, the net amount realised being nearly Rs.24,000. In former days the trade was much more lucrative, 4 or 5 millions of specimens being frequently shipped. The government of Ceylon used to receive £4000 a year for licenses to fish, but now the trade is free. The shells are brought up by divers from 2 or 3 fathoms of water. In 1887 a sinistral specimen was found at Jaffna, which sold for Rs.700.[209] Nearly all the shells are sent to Dacca, where they are sliced into bangles and anklets to be worn by the Hindoo women.

Perhaps the most important industry which deals only with the shells of Mollusca is that connected with the ‘pearl-oyster.’ The history of the trade forms a small literature in itself. It must be sufficient here to note that the species in question is not an ‘oyster,’ properly so called, but an Avicula (margaritifera Lam.). The ‘mother-of-pearl,’ which is extensively employed for the manufacture of buttons, studs, knife-handles, fans, card-cases, brooches, boxes, and every kind of inlaid work, is the internal nacreous laminae of the shell of this species. The most important fisheries are those of the Am Islands, the Soo-loo Archipelago, the Persian Gulf, the Red Sea, Queensland, and the Pearl Islands in the Bay of Panama. The shell also occurs in several of the groups of the South Pacific—the Paumotu, Gambier and Navigator Islands, Tahiti being the centre of the trade—and also on the coasts of Lower California.[210]

Pearls are the result of a disease in the animal of this species of Avicula and probably in all other species within which they occur. When the Avicula is large, well formed, and with ample space for individual development, pearls scarcely occur at all, but when the shells are crowded together, and become humped and distorted, as well as affording cover for all kinds of marine worms and parasitic creatures, then pearls are sure to be found. Pearls of inferior value and size are also produced by Placuna placenta, many species of Pinna, the great Tridacna, the common Ostrea edulis, and several other marine bivalves. They are not uncommon in Unio and Anodonta, and the common Margaritana margaritifera of our rapid streams is still said to be collected, in some parts of Wales, for the purpose of extracting its small ‘seed-pearls.’ Pink pearls are obtained from the giant conch-shell of the West Indies (Strombus gigas), as well as from certain Turbinella.

In Canton, many houses are illuminated almost entirely by skylights and windows made of shells, probably the semitransparent valves of Placuna placenta. In China lime is commonly made of ground cockle-shells, and, when mixed with oil, forms an excellent putty, used for cementing coffins, and in forming a surface for the frescoes with which the gables of temples and private houses are adorned. Those who suffer from cutaneous diseases, and convalescents from small-pox, are washed in Canton with the water in which cockles have been boiled.[211]

A recent issue of the Peking Gazette contains a report from the outgoing Viceroy of Fukhien, stating that he had handed over the insignia of office to his successor, including inter alia the conch-shell bestowed by the Throne. A conch-shell with a whorl turning to the right, i.e. a sinistral specimen, is supposed when blown to have the effect of stilling the waves, and hence is bestowed by the Emperor upon high officers whose duties oblige them to take voyages by sea. The Viceroy of Fukhien probably possesses one of these shells in virtue of his jurisdiction over Formosa, to which island periodical visits are supposed to be made.[212]

Shells appear to be used occasionally by other species besides man. Oyster-catchers at breeding time prepare a number of imitation nests in the gravel on the spit of land where they build, putting bits of white shell in them to represent eggs.[213] This looks like a trick in order to conceal the position of the true nest. According to Nordenskjöld, when the eider duck of Spitzbergen has only one or two eggs in its nest, it places a shell of Buccinum glaciale beside them. The appropriation of old shells by hermit-crabs is a familiar sight all over the world. Perhaps it is most striking in the tropics, where it is really startling, at first experience, to meet—as I have done—a large Cassis or Turbo, walking about in a wood or on a hill side at considerable distances from the sea. A Gephyrean (Phascolion strombi) habitually establishes itself in the discarded shells of marine Mollusca. Certain Hymenoptera make use of dead shells of Helix hortensis in which they build their cells.[214] Magnus believes that in times when heavy rains prevail, and the usual insects do not venture out, certain flowers are fertilised by snails and slugs crawling over them, e.g. Leucanthemum vulgare by Limax laevis.[215]

Mollusca as Food for Man.—Probably there are few countries in the world in which less use is made of the Mollusca as a form of food than in our own. There are scarcely ten native species which can be said to be at all commonly employed for this purpose. Neighbouring countries show us an example in this respect. The French, Italians, and Spanish eat Natica, Turbo, Triton, and Murex, and, among bivalves, Donax, Venus, Lithodomus, Pholas, Tapes, and Cardita, as well as the smaller Cephalopoda. Under the general designation of clam the Americans eat Venus mercenaria, Mya arenaria, and Mactra solidissima. In the Suez markets are exposed for sale Strombus and Melongena, Avicula and Cytherea. At Panama Donax and Solen are delicacies, while the natives also eat the great Murex and Pyrula, and even the huge Arca grandis, which lives embedded in the liquid river mud.

The common littoral bivalves seem to be eaten in nearly all countries except our own, and it is therefore needless to enumerate them. The Gasteropoda, whose habits are scarcely so cleanly, seem to require a bolder spirit and less delicate palate to venture on their consumption.

The Malays of the East Indian islands eat Telescopium fuscum and Pyrazus palustris, which abound in the mangrove swamps. They throw them on their wood fires, and when they are sufficiently cooked, break off the top of the spire and suck the animal out through the opening. Haliotis they take out of the shell, string together, and dry in the sun. The lower classes in the Philippines eat Arca inaequivalvis, boiling them as we do mussels.[216] In the Corean islands a species of Monodonta and another of Mytilus are quite peppery, and bite the tongue; our own Helix revelata, as I can vouch from personal experience, has a similar flavour. Fusus colosseus, Rapana bezoar, and Purpura luteostoma are eaten on the southern coasts of China; Strombus luhuanus, Turbo chrysostomus, Trochus niloticus, and Patella testudinaria, by the natives of New Caledonia; Strombus gigas and Livona pica in the West Indies; Turbo niger and Concholepas peruvianus on the Chilian coasts; four species of Strombus and Nerita, one each of Purpura and Turbo, besides two Tridacna and one Hippopus, by the natives of British New Guinea. West Indian negroes eat the large Chitons which are abundant on their rocky coasts, cutting off and swallowing raw the fleshy foot, which they call ‘beef,’ and rejecting the viscera. Dried cephalopods are a favourite Chinese dish, and are regularly exported to San Francisco, where the Chinamen make them into soup. The ‘Challenger’ obtained two species of Sepia and two of Loligo from the market at Yokohama.

The insipidity of fresh-water Mollusca renders them much less desirable as a form of food. Some species of Unionidae, however, are said to be eaten in France. Anodonta edulis is specially cultivated for food in certain districts of China, and the African Aetheriae are eaten by negroes. Navicella and Neritina are eaten in Mauritius, Ampullaria and Neritina in Guadeloupe, and Paludina in Cambodia.

The vast heaps of empty shells known as ‘kitchen-middens,’ occur in almost every part of the world. They are found in Scotland, Denmark, the east and west coasts of North America, Brazil, Tierra del Fuego, Australia and New Zealand, and are sometimes several hundred yards in length. They are invariably composed of the edible shells of the adjacent coast, mixed with bones of Mammals, birds, and fish. From their great size, it is believed that many of them must have taken centuries to form.

Pre-eminent among existing shell-fish industries stands the cultivation of the oyster and the mussel, a more detailed account of which may prove interesting.

The cultivation of the oyster[217] as a luxury of food dates at least from the gastronomic age of Rome. Every one has heard of the epicure whose taste was so educated that

The first artificial oyster-cultivator on a large scale appears to have been a certain Roman named Sergius Orata, who lived about a century B.C. His object, according to Pliny the elder,[219] was not to please his own appetite so much as to make money by ministering to the appetites of others. His vivaria were situated on the Lucrine Lake, near Baiae, and the Lucrine oysters obtained under his cultivation a notoriety which they never entirely lost, although British oysters eventually came to be more highly esteemed. He must have been a great enthusiast in his trade, for on one occasion when he became involved in a law-suit with one of the riparian proprietors, his counsel declared that Orata’s opponent made a great mistake if he expected to damp his ardour by expelling him from the lake, for, sooner than not grow oysters at all, he would grow them upon the roof of his house.[220] Orata’s successors in the business seem to have understood the secret of planting young oysters in new beds, for we are told that specimens brought from Brundisium and even from Britain were placed for a while in the Lucrine Lake, to fatten after their long journey, and also to acquire the esteemed “Lucrine flavour.”

Oysters are ‘in season’ whenever there is an ‘r’ in the month, in other words, from September to April. ‘Mensibus erratis,’ as the poet has it, ‘vos ostrea manducatis!’ It has been computed that the quantity annually produced in Great Britain amounts to no less than sixteen hundred million, while in America the number is estimated at five thousand five hundred million, the value being over thirteen million dollars, and the number of persons employed fifty thousand. Arcachon, one of the principal French oyster-parks, has nearly 10,000 acres of oyster beds, the annual value being from eight to ten million francs; in 1884–85, 178,359,000 oysters were exported from this place alone. In the season 1889–90, 50,000 tons of oysters were consumed in London.

Few will now be found to echo the poet Gay’s opinion:

There were halcyon days in England once, when oysters were to be procured at 8d. the bushel. Now it costs exactly that amount before a bushel, brought up the Thames, can even be exposed for sale at Billingsgate (4d. porterage, 4d. market toll), and prime Whitstable natives average from 3½d. to 4d. each. The principal causes of this rise in prices, apart from the increased demand, are (1) over-dredging; (2) ignorant cultivation, and to these may be added (3) the effect of bad seasons in destroying young oysters, or preventing the spat from maturing. Our own principal beds are those at Whitstable, Rochester, Colchester, Milton (famous for its ‘melting’ natives), Faversham, Queenborough, Burnham, Poole, and Carlingford in Co. Down, and Newhaven, near Edinburgh.

The oyster-farms at Whitstable, public and private, extend over an area of more than 27 square miles. The principal of these is a kind of joint-stock company, with no other privilege of entrance except birth as a free dredgeman of the town. When a holder dies, his interest dies with him. Twelve directors, known as “the Jury,” manage the affairs of the company, which finds employment for several thousand people, and sometimes turns over as much as £200,000 a year. The term ‘Natives,’ as applied to these Whitstable or to other English oysters, requires a word of explanation. A ‘Native’ oyster is simply an oyster which has been bred on or near the Thames estuary, but very probably it may be developed from a brood which came from Scotland or some other place at a distance. For some unexplained reason, oysters bred on the London clay acquire a greater delicacy of flavour than elsewhere. The company pay large sums for brood to stock their own grounds, since there can be no certainty that the spat from their own oysters will fall favourably, or even within their own domains at all. Besides purchases from other beds, the parks are largely stocked with small oysters picked up along the coast or dredged from grounds public to all, sometimes as much as 50s. a bushel being paid for the best brood. It is probably this system of transplanting, combined with systematic working of the beds, which has made the Whitstable oyster so excellent both as to quality and quantity of flesh. The whole surface of the ‘layings’ is explored every year by the dredge, successive portions of the ground being gone over in regular rotation, and every provision being made for the well-being of the crop, and the destruction of their enemies. For three days of every week the men dredge for ‘planting,’ i.e. for the transference of suitable specimens from one place to another, the separation of adhering shells, the removal of odd valves and of every kind of refuse, and the killing off of dangerous foes. On the other three days they dredge for the market, taking care only to lift such a number as will match the demand.

The Colne beds are natural beds, as opposed to the majority of the great working beds, which are artificial. They are the property of the town of Colchester, which appoints a water-bailiff to manage the concern. Under his direction is a jury of twelve, who regulate the times of dredging, the price at which sales are to be made, and are generally responsible for the practical working of the trade. Here, and at Faversham, Queenborough, Rochester, and other places, ‘natives’ are grown which rival those of Whitstable.

There can be no question, however, that the cultivation of oysters by the French is far more complete and efficient than our own, and has reached a higher degree of scientific perfection combined with economy and solid profits. And yet, between 40 and 50 years ago, the French beds were utterly exhausted and unproductive, and showed every sign of failure and decay. It was in 1858 that the celebrated beds on the Ile de Ré, near Rochelle, were first started. Their originator was a certain shrewd stone-mason, by name Boeuf. He determined to try, entirely on his own account, whether oysters could not be made to grow on the long muddy fore-shore which is left by the ebb of the tide. Accordingly, he constructed with his own hands a small basin enclosed by a low wall, and placed at the bottom a number of stones picked out of the surrounding mud, stocking his ‘parc’ with a few bushels of healthy young brood. The experiment was entirely successful, in spite of the jeers of his neighbours, and Boeuf’s profits, which soon began to mount up at an astonishing rate, induced others to start similar or more extensive farms for themselves. The movement spread rapidly, and in a few years a stretch of miles of unproductive mud banks was converted into the seat of a most prosperous industry. The general interests of the trade appear to be regulated in a similar manner to that at Whitstable; delegates are appointed by the various communities to watch over the business as a whole, while questions affecting the well-being of oyster-culture are discussed in a sort of representative assembly.

At the same time as Boeuf was planting his first oysters on the shores of the Ile de Ré, M. Coste had been reporting to the French government in favour of such a system of ostreiculture as was then practised by the Italians in the old classic Lakes Avernus and Lucrinus. The principle there adopted was to prevent, as far as possible, the escape of the spat from the ground at the time when it is first emitted by the breeding oyster. Stakes and fascines of wood were placed in such a position as to catch the spat and give it a chance of obtaining a hold before it perished or was carried away into the open sea. The old oyster beds in the Bay of St. Brieuc were renewed on this principle, banks being constructed and overlaid with bundles of wood to prevent the escape of the new spat. The attempt was entirely successful, and led to the establishment or re-establishment of those numerous parcs, with which the French coast is studded from Brest to the Gironde. The principal centres of the industry are Arcachon, Auray, Cancale, and la Teste.

It is at Marennes, in Normandy, that the production of the celebrated ‘green oyster’ is carried out, that especial luxury of the French epicure. Green oysters are a peculiarly French taste, and, though they sometimes occur on the Essex marshes, there is no market for them in England. The preference for them, on the continent, may be traced back as early as 1713, when we find a record of their having been served up at a supper given by an ambassador at the Hague. Green oysters are not always green, it is only after they are placed in the ‘claires,’ or fattening ponds, that they acquire the hue; they never occur in the open sea. The green colour does not extend over the whole animal, but is found only in the branchiae and labial tentacles, which are of a deep blue-green. Various theories have been started to explain the ‘greening’ of the mollusc; the presence of copper in the tanks, the chlorophyll of marine algae, an overgrowth of some parasite, a disease akin to liver complaint, have all found their advocates. Prof. Lankester seems to have established[221] the fact,—which indeed had been observed 70 years before by a M. Gaillon,—that the greening is due to the growth of a certain diatom (Navicula ostrearia) in the water of the tanks. This diatom, which is of a deep blue-green colour, appears from April to June, and in September. The oyster swallows quantities of the Navicula; the pigment enters the blood in a condition of chemical modification, which makes it colourless in all the other parts of the body, but when the blood reaches the gills the action of the secretion cells causes the blue tint to be restored. The fact that the colour is rather green than blue in the gills, which are yellowish brown, is due to certain optical conditions.

Not till the young white oyster has been steeped for several years in the muddy waters of the ‘claires’ does it acquire the proper tint to qualify it for the Parisian restaurant. The ‘claires’ are each about 100 feet square, surrounded by low broad banks of earth, about 3 feet high and 6 feet thick at the base. Before the oysters are laid down, the gates which admit the tide are carefully opened and shut a great many times, in order to collect a sufficient amount of the Navicula. When this is done, the beds are formed, and are not again overflowed by the sea, except at very high tides. The oysters are shifted from one ‘claire’ to another, in order to perfect the ‘greening’ process. About fifty million of these ‘huitres de Marennes’ are produced annually, yielding a revenue of 2,500,000 francs.

It appears, from the experience of one of the most enthusiastic of French oyster-growers (Dr. Kemmerer), that oysters grow best in muddy water, and breed best in clear water. Thus the open sea is the place where the spat should fall and be secured, and, as soon as it is of a suitable size, it should be transferred to the closed tank or reservoir, where it will find the quiet and the food (confervae, infusoria, minute algae) which are so requisite for its proper growth. In muddy ground the animal and phosphorous matter increases, and the flesh becomes fatter and more oily. A sudden change from the clear sea-water to the muddy tank is inadvisable, and thus a series of shiftings through tanks with water of graduated degrees of nourishment is the secret of proper oyster cultivation.

The American oyster trade is larger even than the French. The Baltimore oyster beds in the Chesapeake River and its tributaries cover 3000 acres, and produce an annual crop of 25 million bushels, as many as 100,000 bushels being sometimes taken from Chesapeake Bay in a single day. Baltimore is the centre of the tinned oyster trade, while that in raw oysters centres in New York. Most of the beds whose produce is carried to New York are situated in New Jersey, Connecticut, Delaware, or Virginia. The laws of these states do not allow the beds to be owned by any but resident owners, and the New York dealers have consequently to form fictitious partnerships with residents near the various oyster beds, supply them with money to buy the beds and plant the oysters, and then give them a share in the profits. It has been estimated that from the Virginia beds 4,000,000 bushels of oysters are carried every year to Fair Haven in New England, 4,000,000 to New York, 3,000,000 to Providence, and 2,000,000 each to Boston, Philadelphia, and Baltimore. The American ‘native’ (O. virginica) is a distinct species from our own, being much larger and longer in proportion to its breadth; it is said to be also much more prolific.

According to Milne-Edwards,[222] in the great oyster parks on the coasts of Calvados, the oysters are educated to keep their shells closed when out of water, and so retain water enough inside to keep their gills moist, and arrive at their destination in good condition. As soon as an oyster is taken out of the sea, it closes its shells, and keeps them closed until the shock of removal has passed away, or perhaps until the desirability of a fresh supply of water suggests itself. The men take advantage of this to exercise the oysters, removing them from the sea for longer and longer periods. In time this has the desired effect; the well-educated mollusc learns that it is hopeless to ‘open’ when out of the water, and so keeps his shell closed and his gills moist, and his general economy in good condition.

Oysters have been known to live entirely out of water for a considerable time. Prof. Verrill once noticed[223] a large cluster of oysters attached to an old boot, hanging outside a fish-shop in Washington. They had been taken out of the water on about 10th December, and on 25th February following some of the largest were still alive. It was noticed that all those which survived had the hinge upward and the ventral edge downward, this being the most favourable position possible for the retention of water within the gill-cavity, since the edge of the mantle would pack against the margins of the shell, and prevent the water from leaking away.

Such a succulent creature as the oyster has naturally many enemies. One of the worst of these is the ravenous Starfish, or Five-finger. His omnivorous capacities are well described by a clever writer and shrewd observer of nature: “Here is one doubled up like a sea-urchin, brilliant of hue, and when spread out quite 16 inches in diameter; where, and oh where, can you obtain a prey? The hoe we carry is thrust out and the mass dragged shorewards, when the rascal disgorges two large dog-whelks he has been in the process of devouring. We feel a comfortable glow of satisfaction to think that this enemy of our oyster-beds is also the enemy of our other enemy, this carnivorous borer. Here, quite close alongside, is another, only inferior in size, and we drag him ashore likewise, to find that the fellow has actually had the courage and audacity to suck the contents out of a large horse-mussel (Modiola), the strong muscle alone remaining undevoured. We proceed along but a short way when we meet with still another in the curled-up condition in which they gorge themselves, and as we drag it shorewards the shell of a Tapes pullastra drops from the relaxing grasp of the ogre. Slowly the extended stomach returns to its place, and the monster settles back to an uncomfortable after-dinner siesta on an exposed boulder; for the starfish wraps its turned-out stomach around the prey it has secured, in place of attempting to devour the limey covering in which most of its game is protected. Once the mouth of the shell is enclosed in the stomach of the starfish, the creature soon sickens, the hinge-spring relaxes its hold, and the shell opening permits the starfish to suck out the gelatinous contents, and cast free the calcareous skeleton.”[224]

According to other observers the starfish seizes the oyster with two of his fingers, while with the other three he files away the edge of the flat or upper valve until the points of contact with the round valve are reduced almost to nothing; then he can introduce an arm, and the rest is easy work. Others suggest that the starfish suffocates the oyster by applying two of its fingers so closely to the edge of the valves that the oyster is unable to open them; after a while the vital powers relax and the shell gapes. The Rev. J. G. Wood holds[225] that the starfish pours a secretion from its mouth which “paralyses the hinge muscle and causes the shell to open.” Sometimes in a single night a whole bed of oysters will be totally destroyed by an invasion of starfish. Another dreaded enemy is the ‘whelk,’ a term which includes Purpura lapillus, Murex erinaceus, Buccinum undatum, and probably also Nassa reticulata. All these species perforate the shell with the end of their radula, and then suck out the contents through the neatly drilled hole. Skate fish are the cause of terrible destruction in the open beds, and a scarcely less dangerous visitant is the octopus. Crabs crush the young shells with their claws, and are said to gather in bands and scratch sand or mud over the larger specimens, which makes them open their shells. Yet another, and perhaps unconscious, foe is found in the common mussel, which takes up room meant for the young oysters, grows over the larger individuals, and harbours all sorts of refuse between and under its closely packed ranks. Cliona, a parasitic sponge, bores in between the layers of the oyster’s shell, pitting them with tiny holes (corresponding to its oscula), and disturbing the inmate, who has constantly to construct new layers of shell from the inside. Weed, annelids, ‘blubber,’ shifting sand or mud, sewage or any poisoning of the water, are seriously harmful to the oyster’s best interests. A very severe winter is often the cause of wholesale destruction in the beds. According to the Daily News of 26th March 1891, the Whitstable oyster companies lost property to the value of £30,000 in the exceptionally cold winter of 1890–91, when, on the coast of Kent, the surface temperature of the sea sank below 32°, and the advancing tide pushed a small ice-floe before it. Two million oysters were laid down in one week of the following spring, to make up for the loss. During the severe winter of 1892–93 extraordinary efforts were made at Hayling I. to protect the oysters from the frost. Twenty million oysters were placed in ponds for the winter, and a steam-engine was for days employed to keep the ponds thawed and supplied with water, while large coal and coke fires were kept burning at the edge of the ponds.[226] On the other hand, the unusually warm and sunny summer of 1893 is said to have resulted in the finest fall of spat known in Whitstable for fifty years.[227]

The reproductive activity of the oyster is supposed to commence about the third year. Careful research has shown[228] that the sexes in the English oyster are not separate, but that each individual is male as well as female, producing spermatozoa as well as ova in the same gland. Here, however, two divergent views appear. Some authorities hold that the oyster does not fecundate its own eggs, but that this operation is performed by spermatozoa emitted by other specimens. It is believed that, in each individual, the spermatozoa arrive at maturity first, and that the ova are not produced until after the spermatozoa have been emitted; thus the oyster is first male and then female, morphologically hermaphrodite, but physiologically unisexual. Others are of opinion that the oyster does fecundate its own eggs, ova being first produced, and passed into the infrabranchial chamber—the ‘white-sick’ stage—and then, after an interval, spermatozoa being formed and fecundating these ova—the ‘black-sick’ stage. In this latter view the oyster is first female and then male, and is, both morphologically and physiologically, hermaphrodite. The old view, that ‘black-sick’ oysters are the male, and ‘white-sick’ the female, is therefore quite incorrect.

The ova, in their earliest stage, consist of minute oval clusters of globules floating in a transparent mucus. They pass from the ovary into the gills and folds of the mantle, and are probably fecundated within the excretory ducts of the ovary, before arriving in the mantle chamber. In this stage the oyster is termed ‘white-sick.’ In about a fortnight, as the course of development proceeds, the fertilised ova become ciliated at one end (the so-called veliger stage, p. 131), and soon pigment appears in various parts of the embryos, giving them a darker colour, which varies from grayish to blue, and thus the white-sick oyster becomes ‘black-sick.’ When the black spat emerge, they are still furnished with cilia for their free-swimming life. This is of very short duration, for unless the embryo finds some suitable ground on which to affix itself within forty-eight hours, it perishes. As the spat escapes from the parent oyster, which slightly opens its valves and blows the spat out in jets, it resembles a thick cloud in the water, and is carried about at the mercy of wind and tide. April to August are the usual spawning months, warm weather being apparently an absolute necessity to secure the adhering of the spat. A temperature of 65° to 72° F. seems requisite for their proper deposit. Thus on a fine, warm day, with little wind or tide running, the spat will fall near the parents and be safely secured, while in cold, blustering weather it will certainly be carried off to a distance, and probably be altogether lost. The number of young produced by each individual has been variously estimated at from 300,000 to 60,000,000. Either extreme seems enormous, but it must be remembered that besides climatal dangers, hosts of enemies—other Mollusca, fish, and Crustacea—beset the opening career of the young oyster.

As soon as the spat has safely ‘fallen,’ it adheres to some solid object, and loses the cilia which were necessary for its swimming life. It begins to grow rapidly, increasing from about ¹/₂₀ inch in diameter to about the size of a threepenny piece in five or six months, and in a year to one inch in diameter. Roughly speaking, the best guide to an oyster’s age is its size; it is as many years old as it measures inches across.

The oyster is at its prime at the age of five; its natural life is supposed to be about ten years. The rings, or ‘shoots’ on a shell are not—as is frequently supposed—marks of annual growth; cases have been noticed where as many as three ‘shoots’ were made during the year.

An oyster is furnished, on the protruding edges of the mantle, with pigmented spots which may be termed ‘visual organs,’ though they hardly rise to the capacities and organisation of real ‘eyes.’ But there is no doubt that they are sufficiently sensitive to the action of light to enable the oyster to apprehend the approach of danger, and close his doors accordingly. ‘How sensitive,’ notes Mr. W. Anderson Smith,[229] ‘the creatures are to the light above them; the shadow of the iron as it passes overhead is instantaneously noted, and snap! the lips are firmly closed.’

The geographical distribution of Ostrea edulis extends from Tränen, in Norway, close to the Arctic circle, to Gibraltar and certain parts of the Mediterranean, Holland, and N. Germany to Heligoland, and the western shores of Sleswick and Jutland. It occurs in Iceland, but does not enter the Baltic, where attempts to colonise it have always failed. Some authorities regard the Mediterranean form as a distinct species.

The literature of oyster-cookery may be passed over in silence. The curious may care to refer to M. S. Lovell’s Edible British Mollusks, where no less than thirty-nine different ways of dressing oysters are enumerated. It may, however, be worth while to add a word on the subject of poisonous oysters. Cases have been known where a particular batch of oysters has, for some reason, been fatal to those who have partaken of them. It is possible that this may have been due, in certain instances, to the presence of a superabundance of copper in the oysters, and there is no doubt that the symptoms detailed have often closely resembled those of copper poisoning. Cases of poisoning have occurred at Rochefort through’ the importation of ‘green oysters’ from Falmouth. It would no doubt be dangerous ever to eat oysters which had grown on the copper bottom of a ship. But copper is present, in more or less minute quantities, in very many Mollusca, and it is more probable that a certain form of slow decomposition in some shell-fish develops an alkaloid poison which is more harmful to some people than to others, just as some people can never digest any kind of shell-fish.[230] These alkaloid developments from putrescence are called ptomaines. In confirmation of this view, reference may be made to a case, taken from an Indian Scientific Journal, in which an officer, his wife, and household ate safely of a basket of oysters for three days at almost every meal. The basket then passed out of their hands, not yet exhausted of its contents, and a man who had already eaten of these oysters at the officer’s table was afterwards poisoned by some from the same basketful.

The cultivation of the common mussel (Mytilus edulis L.) is not practised in this country, although it is used as food in the natural state of growth all round our coasts. The French appear to be the only nation who go in for extensive mussel farming. The principal of these establishments is at a little town called Esnaudes, not far from La Rochelle, and within sight of the Ile de Ré and its celebrated oyster parks. The secret of the cultivation consists in the employment of ‘bouchots,’ or tall hurdles, which are planted in the mud of the fore-shore, and upon which the mussel (la moule, as the French call it) grows. The method is said[231] to have been invented as long ago as 1235 by a shipwrecked Irishman named Walton. He used to hang a purse net to stakes, in the hope of capturing sea birds. He found, however, that the mussels which attached themselves to his stakes were a much more easily attainable source of food, and he accordingly multiplied his stakes, out of which the present ‘bouchot’ system has developed. The shore is simply a stretch of liquid mud, and the bouchots are arranged in shape like a single or double V, with the opening looking towards the sea. The fishermen, in visiting the bouchots, glide about over the mud in piroques or light, flat-bottomed boats, propelling them by shoving the mud with their feet. Each bouchot is now about 450 yards long, standing 6 feet out of the mud, making a strong wall of solid basket-work, and as there are altogether at least 500 bouchots, the total mussel-bearing length of wall is nearly 130 miles.

The mussel-spat affixes itself naturally to the bouchots nearest the sea, in January and February. Towards May the planting begins. The young mussels are scraped off these outermost bouchots, and placed in small bags made of old canvas or netting, each bag holding a good handful of the mussels. The bags are then fastened to some of the inner bouchots, and the mussels soon attach themselves by their byssus, the bag rotting and falling away. They hang in clusters, increasing rapidly in size, and at the proper time are transplanted to bouchots farther and farther up the tide level, the object being to bring the matured animal as near as possible to the land when it is time for it to be gathered. This process, which aims at keeping the mussel out of the mud, while at the same time giving it all the nutrition that comes from such a habitat, extends over about a year in the case of each individual. Quality, rather than quantity, is the aim of the Esnaudes boucholiers. The element of quantity, however, seems to come in when we are told that each yard of the bouchots is calculated to yield a cartload of mussels, value 6 francs, and that the whole annual revenue is at least £52,000.

In this country, and especially in Scotland, mussels are largely used as bait for long-line fishing. Of late years other substances have rather tended to take the place of mussels, but within the last twenty years, at Newhaven on the Firth of Forth, three and a half million mussels were required annually to supply bait for four deep-sea craft and sixteen smaller vessels. According to Ad. Meyer,[232] boughs of trees are laid down in Kiel Bay, and taken up again, after three, four, or five years, between December and March, when they are found covered with fine mussels. The boughs are then sold, just as they are, by weight, and the shell-fish sent into the interior of Germany.

Mussels are very sensitive to cold weather. In 1874, during an easterly gale, 195 acres of mussels at Boston, in Lincolnshire, were killed in a single night. They soon affix themselves to the bottom of vessels that have lain for any length of time in harbour or near the coast. The bottom of the Great Eastern steamship was at one time so thickly coated with mussels that it was estimated that a vessel of 200 tons could have been laden from her. In some of our low-lying coast districts mussels are a valuable protection against inundation. “An action for trespass was brought some time ago for the purpose of establishing the right of the lord of the manor to prevent the inhabitants of Heacham from taking mussels from the sea-shore. The locality is the fore-shore of the sea, running from Lynn in a north-westerly direction towards Hunstanton in Norfolk; and the nature of the shore is such that it requires constant attention, and no little expenditure of money, to maintain its integrity, and guard against the serious danger of inundations of the sea. Beds of mussels extend for miles along the shore, attaching themselves to artificial jetties running into the sea, thereby rendering them firm, and thus acting as barriers against the sea [and as traps to catch the silt, and thus constantly raise the level of the shore]. Therefore, while it is important for the inhabitants, who claim a right by custom, to take mussels and other shell-fish from the shore, it is equally important for the lord of the manor to do his utmost to prevent these natural friends of his embankments and jetties from being removed in large quantities.”[233]

The fable that Bideford Bridge is held together by the byssi of Mytilus, which prevent the fabric from being carried away by the tide, has so often been repeated that it is perhaps worth while to give the exact state of the case, as ascertained from a Town Councillor. The mussels are supposed to be of some advantage to the bridge, consequently there is a by-law forbidding their removal, but the corporation have not, and never had, any boat or men employed in any way with regard to them.

Poisoning by mussels is much more frequent than by oysters. At Wilhelmshaven,[234] in Germany, in 1885, large numbers of persons were poisoned, and some died, from eating mussels taken from the harbour. It was found that when transferred to open water these mussels became innocuous, while, on the other hand, mussels from outside, placed in the harbour, became poisonous. The cause obviously lay in the stagnant and corrupted waters of the harbour, which were rarely freshened by tides. It was proved to demonstration that the poison was not due to decomposition; the liver of the mussels was the poisonous part. In the persons affected, the symptoms were of three kinds, exanthematous (skin eruptions), choleraic, and paralytic. Cases of similar poisoning are not unfrequent in our own country, and the circumstances tend to show that, besides the danger from mussels bred in stagnant water, there is also risk in eating them when ‘out of season’ in the spawning time.

Whelks are very largely employed for bait, especially in the cod fishery. The whelk fishery in Whitstable Bay, both for bait and for human food, yields £12,000 a year. Dr. Johnston, of Berwick, estimated that about 12 million limpets were annually consumed for bait in that district alone. The cockle fishery in Carmarthen Bay employs from 500 to 600 families, and is worth £15,000 a year, that in Morecambe Bay is worth £20,000.

Cultivation of Snails for Food; Use as Medicine.—It was a certain Fulvius Hirpinus who, according to Pliny the elder,[235] first instituted snail preserves at Tarquinium, about 50 B.C. He appears to have bred several species in his ‘cochlearia,’ keeping them separate from one another. In one division were the albulae, which came from Reate; in another the ‘very big snails’ (probably H. lucorum), from Illyria; in a third the African snails, whose characteristic was their fecundity; in a fourth those from Soletum, noted for their ‘nobility.’ To increase the size of his snails, Hirpinus fed them on a fattening mixture of meal and new wine, and, says the author in a burst of enthusiasm, ‘the glory of this art was carried to such an extent that a single snail-shell was capable of holding eighty sixpenny pieces.’ Varro[236] recommends that the snaileries be surrounded by a ditch, to save the expense of a special slave to catch the runaways. Snails were not regarded by the Romans as a particular luxury. Pliny the younger reproaches[237] his friend Septicius Clarus for breaking a dinner engagement with him, at which the menu was to have been a lettuce, three snails and two eggs apiece, barley water, mead and snow, olives, beetroot, gourds and truffles, and going off somewhere else where he got oysters, scallops, and sea-urchins. In Horace’s time they were used as a gentle stimulant to the appetite, for

Escargotières, or snail-gardens, still exist in many parts of Europe, e.g. at Dijon, at Troyes and many other places in central and southern France, at Brunswick, Copenhagen, and Ulm. The markets at Paris, Marseilles, Bordeaux, Toulouse, Nantes, etc., are chiefly supplied by snails gathered from the open country, and particularly from the vineyards, in some of which Helix pomatia abounds. In the Morning Post of 8th May 1868 there is an account of the operation of clearing the celebrated Clos de Vougeot vineyard of these creatures. No less than 240 gallons were captured, at a cost in labour of over 100 francs, it being estimated that these snails would have damaged the vines to an extent represented by the value of 15 to 20 pipes of wine, against which may be set the price fetched by the snails when sold in the market.

It is generally considered dangerous to eat snails at once which have been gathered in the open country. Cases have occurred in which death by poisoning has resulted from a neglect of this precaution, since snails feed on all manner of noxious herbs. Before being sent to table at the restaurants in the great towns, they are fattened by being fed with bran in the same way as oysters.

The Roman Catholic Church permits the consumption of snails during Lent. Very large numbers are eaten in France and Austria at this time. At the village of Cauderon, near Bordeaux, it is the proper thing to end Carnival with especial gaiety, but to temper the gaiety with a dish of snails, as a foretaste of Lenten mortification.

The following species appear to be eaten in France at the present day: H. pomatia, aspersa, nemoralis, hortensis, aperta, pisana, vermiculata, lactea. According to Dr. Gray, the glassmen at Newcastle used to indulge in a snail feast once a year, and a recent writer informs us that H. aspersa is still eaten by working people in the vicinity of Pontefract and Knottingley.[239] But in this country snails appear to be seldom consciously used as an article of food; the limitation is necessary, for Lovell tells us that they are much employed in the manufacture of cream, and that a retired (!) milkman pronounced it the most successful imitation known.

Preparations made from snails used to be highly esteemed as a cure for various kinds of diseases and injuries. Pliny the elder recommends them for a cough and for a stomach-ache, but it is necessary “to take an uneven number of them.”[240] Five African slugs, roasted and beaten to a powder, with half a drachm of acacia, and taken with myrtle wine, is an excellent remedy for dysentery. Treated in various ways, snails have been considered, in modern times, a cure for ague, corns, web in the eye, scorbutic affections, hectic fevers, pleurisy, asthma, obstructions, dropsy, swelling of the joints, headache, an impostume (whitlow), and burns. One of Pliny’s remedies for headache, which competes with the bones of a vulture’s head or the brain of a crow or an owl, is a plaister made of slugs with their heads cut off, which is to be applied to the forehead. He regards slugs as immature snails, whose growth is not yet complete (nondum perfectae). Lovell states that “a large trade in snails is carried on for Covent Garden market in the Lincolnshire fens, and that they are sold at 6d. per quart, being much used for consumptive patients and weakly children.”

The custom still seems to linger on in some parts of the country. Mr. E. Rundle, of the Royal Cornwall Infirmary, gives his experience in the following terms: “I well remember, some twelve years since, an individual living in an adjoining parish [near Truro] being pointed out to me as ‘a snail or slug eater.’ He was a delicate looking man, and said to be suffering from consumption. Last summer I saw this man, and asked him whether the statement that he was a ‘snail eater’ was true: he answered, ‘Yes, that he was ordered small white slugs—not snails—and that up till recently he had consumed a dozen or more every morning, and he believed they had done him good.’ There is also another use to which the country people here put snails, and that is as an eye application. I met with an instance a few weeks since, and much good seemed to have followed the use.”[241]

A reverend Canon of the Church of England, whose name I am not permitted to disclose, informs me that there was a belief among the youth of his native town (Pontypool, in Monmouthshire) that young slugs were ‘good for consumption,’ and that they were so recommended by a doctor who practised in the town. The slugs selected were about ¾ inch long, “such as may be seen crawling on the turf of a hedge-bank after a shower of rain.” They were “placed upon the tongue without any previous preparation, and swallowed alive.” My informant himself indulged in this practice for some time, “not on account of any gustatory pleasure it afforded, but from some vague notion that it might do him good.”

A colleague of mine at King’s College tells me that the country people at Ponteland, near Morpeth, habitually collect Limax agrestis and boil it in milk as a prophylactic against consumption. He has himself frequently devoured them alive, but they must be swallowed, not scrunched with the teeth, or they taste somewhat bitter.

Snails have occasionally fallen, with other noxious creatures, under the ban of the Church. In a prayer of the holy martyr Trypho of Lampsacus (about 10th cent. A.D.) there is a form of exorcism given which may be used as occasion requires. It runs as follows: “O ye Caterpillars, Worms, Beetles, Locusts, Grasshoppers, Woolly-Bears, Wireworms, Longlegs, Ants, Lice, Bugs, Skippers, Cankerworms, Palmerworms, Snails, Earwigs, and all other creatures that cling to and wither the fruit of the grape and all other herbs, I charge you by the many-eyed Cherubim, and by the six-winged Seraphim, which fly round the throne, and by the holy Angels and all the Powers, etc. etc., hurt not the vines nor the land nor the fruit of the trees nor the vegetables of —— the servant of the Lord, but depart into the wild mountains, into the unfruitful woods, in which God hath given you your daily food.”

Prices given for Shells.—Very high prices have occasionally been given for individual specimens, particularly about thirty or forty years ago, when the mania for collecting was at its height. In those days certain families, such as the Volutidae, Conidae, and Cypraeidae, were the especial objects of a collector’s ardour, and he spared no expense to make his set of the favourite genus as complete as possible. Thus at Stevens’ auction-rooms in Covent Garden, on 21st July 1854, one specimen of Conus cedo nulli fetched £9: 10s., and another £16, a C. omaicus 16 guineas, C. victor £10, and C. gloria maris, the greatest prize of all, £43: 1s. At the Vernède sale, on 14th Dec. 1859 two Conus omaicus fetched £15 and £22, and a C. gloria maris £34. At the great Dennison sale, in April 1865, the Conidae fetched extravagant prices, six specimens averaging over £20 apiece. Conus cedo nulli went for £18 and £22, C. omaicus for £12, C. malaccanus for 10 guineas (this and one of the cedo nulli being the actual specimens figured in Reeve’s Conchologia Iconica), C. cervus for £19 and C. gloria maris for £42. On 9th May 1866 a Cypraea Broderipii was sold at Stevens’ auction-rooms for £13, and at the Dennison sale a Cypraea princeps fetched £40, and C. guttata £42. The Volutidae, although not quite touching these prices, have yet done fairly well. Mr. Dennison’s Voluta fusiformis sold for £6: 15s., V. papillaris for £5, V. cymbiola for £5: 15s., V. reticulata for eight guineas, and two specimens of the rarest of all Volutas, V. festiva, for £14 and £16, both being figured in the Conchologia. At the same sale, two unique specimens of Oniscia Dennisoni fetched £17 and £18 respectively, and, at the Vernède sale, Ancillaria Vernèdei was bought for £6: 10s., and Voluta piperata for £7: 10s.

A unique specimen of a recent Pleurotomaria (quoyana F. and B.) was purchased by Miss de Burgh in 1873 for 25 guineas, and another species of the same genus (adansoniana Cr. and F.), of extraordinary size and beauty, is now offered for sale for about £100.

Bivalves have never fetched quite such high prices as univalves, but some of the favourite and showy genera have gone near to rival them. On 22nd June 1869, at Stevens’, Pecten solaris fetched £4: 5s., P. Reevii £4: 8s., and Cardita varia 5 guineas. Mr. Dennison’s specimens of Pecten subnodosus sold for £7, of Corbula Sowerbyi for £10, of Pholadomya candida for £8 and £13, while at the Vernède sale a Chama damicornis fetched £7.

CHAPTER V REPRODUCTION—DEPOSITION OF EGGS—DEVELOPMENT OF THE FERTILISED OVUM—DIFFERENCES OF SEX—DIOECIOUS AND HERMAPHRODITE MOLLUSCA—DEVELOPMENT OF FRESH-WATER BIVALVES

Reproduction in the Mollusca invariably takes place by means of eggs, which, after being developed in the ovary of the female, are fertilised by the spermatozoa of the male. As a rule, the eggs are ‘laid,’ and undergo their subsequent development apart from the parent. This rule, however, has its exceptions, both among univalve and bivalve Mollusca, a certain number of which hatch their young from the egg before expelling them. Such ovoviviparous genera are Melania, Paludina, Balea, and Coeliaxis among land and fresh-water Mollusca, and Cymba and many Littorina amongst marine. The young of Melania tuberculata, in Algeria, have been noticed to return, as if for shelter, to the branchial cavity of the mother, some days after first quitting it. Isolated species among Pulmonata are known to be ovoviviparous, e.g. Patula Cooperi, P. Hemphilli, and P. rupestris, Acanthinula harpa, Microphysa vortex, Pupa cylindracea and muscorum, Clausilia ventricosa, Opeas dominicensis, Rhytida inaequalis, etc. All fresh-water Pelecypoda yet examined, except Dreissensia, are ovoviviparous.

The number of eggs varies greatly, being highest in the Pelecypoda. In Ostrea edulis it has been estimated at from 300,000 to 60,000,000; in Anodonta from 14,000 to 20,000; in Unio pictorum 200,000. The eggs of Doris are reckoned at from 80,000 to 600,000, of Loligo and Sepia at about 30,000 to 40,000. Pulmonata lay comparatively few eggs. Arion ater has been observed to lay 477 in forty-eight days (p. 42). Nests of Helix aspersa have been noticed, in which the number of eggs varied from about 40 to 100. They are laid in little cup-shaped hollows at the roots of grass, with a little loose earth spread over them. The eggs of Testacella are rather large, and very elastic; if dropped on a stone floor they will rebound sharply several inches. The Cochlostyla of the Philippines lay their eggs at the tops of the great forest trees, folding a leaf together to serve as a protection.

The eggs of the great tropical Bulimus and Achatina, together with those of the Macroön group of Helix (Helicophanta, Acavus, Panda) are exceedingly large, and the number laid must be decidedly less than in the smaller Pulmonata. Bulimus oblongus, for instance, from Barbados, lays an egg about the size of a sparrow’s (Fig. 38), Achatina sinistrorsa as large as a pigeon’s. The Cingalese Helix Waltoni when first hatched is about the size of a full-grown H. hortensis. There is, in the British Museum, a specimen of the egg of a Bulimus from S. America (probably maximus or popelairanus) which measures exactly 1¾ inch in length.

The Limnaeidae deposit their eggs in irregular gelatinous masses on the under side of the leaves of water-plants, and on all kinds of débris.

Fig. 38.—Newly-hatched young and egg of Bulimus oblongus Müll., Barbados. Natural size.

The Rachiglossa or marine carnivorous families lay their eggs in tough leathery or bladdery capsules, which are frequently joined together in shapes which differ with the genus. Each capsule contains a varying number of ova. The cluster of egg-capsules of Buccinum undatum is a familiar object on all our sandy coasts. The capsules of Purpura lapillus are like delicate pink grains of rice, set on tiny stalks. They are not attached to one another, but are set closely together in groups in sheltered nooks of the rocks. A single Purpura has been observed to produce 245 capsules! Busycon lays disc-shaped capsules which are all attached at a point in the edge to a cartilaginous band nearly 3 feet in length, looking like a number of coins tied to a string at equal distances from one another. In Murex erinaceus the egg-capsules are triangular, with a short stalk. They are deposited separately in clusters of from 15 to 150, there being about 20 ova in each capsule. It appears that all the species of the same genus have by no means the same method of depositing their eggs, nor do they always produce eggs of at all similar size or shape. Thus, of two British species of Nassa, N. reticulata lays egg-capsules in shape like flattened pouches with a short stalk, and fastens them in rows to the leaves of Zostera; M. incrassata, on the other hand, deposits solitary capsules, which are shaped like rounded oil-flasks. Neptunea antiqua lays its eggs in bunched capsules, like Bucc. undatum (Fig. 40), but the capsules of N. gracilis are solitary.

Fig. 39.—Various forms of spawn in Prosobranchiata: A and D, Pyrula or Busycon; B, Conus; C, Voluta musica; E, Ampullaria (from specimens in the British Museum); all × ⅔.

In Natica the eggs are deposited in what looks like a thick piece of sand-paper, curled in a spiral form (Fig. 41). The sand is agglutinated by copious mucus into a sort of sheet, and the eggs are let into this, sometimes (N. heros) in regular quincunx form. Ianthina attaches its eggs to the under side of its float (Fig. 42). The Trochidae deposit their eggs on the under side of stones and sea-weeds, each ovum being contained in a separate capsule, and all the capsules glued together into an irregular mass of varying size. The female of Galerus chinensis hatches her eggs by keeping them between her foot and the stone she adheres to. They are laid in from 6 to 10 capsules, connected by a pedicle and arranged like the petals of a rose, with 10 to 12 eggs in each capsule. Those Littorina which are not ovoviviparous deposit their spawn on sea-weeds, rocks, and stones. The eggs are enveloped in a glairy mass which is just firm enough to retain its shape in the water; each egg has its own globule of jelly and is separated from the others by a very thin transparent membrane.[242]

Fig. 40.—Egg-capsules of, A, Nassa reticulata L. × ⅔; B, Buccinum undatum L. × ⅔; C, Neptunea antiqua L. × ⅓.

Fig. 41.—Spawn of a species of Natica (from a specimen in the British Museum) × ½.

Fig. 42.—Ianthina fragilis Lam. FL, float; O, ova; Pr, proboscis; Br, branchiae; F, foot. (Quoy and Gaimard.)

Chiton marginatus, when kept in captivity, has been noticed[243] to elevate the posterior part of the girdle, and to pour out a continuous stream of flaky white matter like a fleecy cloud, which proved to be of a glutinous nature. It then discharged ova, at the rate of one or two every second, for at least fifteen minutes, making a total of 1300 to 1500, each being about ¹/₁₀₀ inch diameter. The ova were shot into the glutinous cloud, which seemed to serve as a sort of nidus to entangle the ova and prevent them being carried away. The subsequent development was rapid, and in seven days the young Chiton was hatched, being then about ¹/₂₀ inch long. Lovén has described the same species as laying its eggs, loosely united in clusters of seven to sixteen, upon small stones. There is probably some mistake about the identification, but the observation illustrates the varying methods of oviposition among allied forms.

Fig. 43.—Egg-capsules of A, Sepia elegans Orb., and B, Octopus vulgaris Lam.

Not very much is known with regard to the ovipositing of the Cephalopoda, especially those which inhabit deep water. Masses of ova arranged in very various forms have occasionally been met with floating in the ocean, but it is next to impossible to determine to what species, or even genus, they belong.[244]

In Loligo punctata the ova are contained in small cylindrical cases measuring 3 to 4 in. by ½ in., to the number of about 250 ova in each case. Hundreds of these cases are attached together like a bundle of sausages or young carrots, and the movements of the embryos within can be distinctly noted. Sepia officinalis lays large black pear-shaped capsules, each of which is tied to some place of attachment by a kind of ribbon at the upper end of the capsule, the whole forming a large group like a bunch of grapes. Octopus vulgaris deposits thousands of small berry-shaped ova, attached to a string which runs along the centre of the mass (Fig. 43).

The so-called shell of the female Argonauta is nothing more than a form of protection for the ova, and is in no sense homologous to the ordinary molluscan shell. The ova consist of a large granulated mass, attached to a many branched stem; they are contained in the spire of the shell, in contact with the posterior part of the body of the mother, but sometimes project externally beyond the coil of the spire.

Certain species possess the curious property of laying their eggs on the outside of their own shells. Buccinopsis Dalei is not unfrequently found decorated with its own egg-capsules. Possibly this species, which lives on oozy ground, finds this the only secure place of attachment for its progeny. Neritina fluviatilis has a similar habit, and so have many other species of Neritina and Navicella. It is not quite clear, in the latter cases, whether the eggs are laid by the specimens on whose shell they are found, or whether they are deposited by others. In either case, perhaps the shell is the safest place for them in the rapid streams which both genera frequent. Specimens of Hydrobia ulvae taken on the wet sands at the mouth of the Dee, are found to have several little rounded excrescences scattered over the surface of the shell. These, on examination, are found to be little masses of small sand-grains, in the centre of which is a clear jelly containing segmenting ova or young embryos. Here again, in all probability, the shell is the only comparatively stable object, in the expanse of shifting sands, on which the eggs can be laid.[245]

The pulmonate genus Libera, which occurs on a few of the island groups in the Central Pacific, is remarkable for the habit of laying its eggs within its own cavernous umbilicus, which is narrowed at the lower part. The eggs number from four to six, or the same number of very young shells may be seen closely packed in the cavity, each being in shape exactly like a young Planorbis. This constriction of the umbilicus does not occur till the formation of the last two whorls, i.e. till the animal is sexually mature. Some species, but not all, provide for the safety of their eggs more completely by forming a very thin shelly plate, which nearly closes the umbilical region, and breaks away or is absorbed to facilitate the escape of the young shells.[246]

Union of Limax.—With regard to the act of union itself, the method in certain species of Limax deserves special notice. L. maximus has been observed at midnight to ascend a wall or some perpendicular surface. A pair then crawl round and round one another emitting a quantity of mucus which at length forms a patch, 2 to 2½ inches in diameter. When this acquires consistency the pair begin to twist round each other in corkscrew form, and detach themselves from the wall, hanging by a cord of the thickened mucus, about 8–15 inches long, and still twisting round each other. The external generative organs are then protruded and copulation takes place, after which the bodies untwist, separate, and crawl up the cord again to the wall.[247]

Periodicity in Breeding.—In the marine Mollusca, the winter months appear to be the usual time for the deposition of eggs. Careful observations have been made on the Mollusca occurring at Naples,[248] and the general result seems to be that for all Orders alike the six winter months from November to April, roughly speaking, are the breeding time. Scarcely any forms appear to breed habitually in August, September, or October. On our own coasts, Nudibranchiata come in shore to deposit their ova from January to April. Purpura lapillus may be observed depositing ova all the year round, but is most active from January to April. Buccinum undatum breeds from October to May; Littorina all the year round.

The land Mollusca exhibit rather more periodicity than the marine. In temperate climates they breed exclusively in the summer months. In the tropics their periods are determined by the dry and rainy seasons, where such occur, otherwise they cohabit all the year round. According to Karl Semper, the snails of the warm Mediterranean region arrive at sexual maturity when they are six months old, i.e. before they are fully grown. After a rest of about three months during the heat of summer, a second period of ovipositing occurs.[249] Helix hortensis and H. nemoralis ascend trees, sometimes to a height of forty feet, when pairing.[250]

Hybridism as the result of union between different species of Mollusca is exceedingly rare. Lecoq once noticed[251] on a wall at Anduze (Gard) as many as twenty specimens of Pupa cinerea united with Clausilia papillaris. No offspring seem to have resulted from what the professor calls ‘this innocent error,’ for the wall was carefully scrutinised for a long time, and no hybrid forms were ever detected.

The same observer noticed, in the Luxembourg garden at Paris, and M. Gassies has noticed[252] at various occasions, union between Helix aspersa and nemoralis, H. aspersa and vermiculata, between Stenogyra decollata and a Helix (sp. not mentioned), H. variabilis and pisana, H. nemoralis and hortensis. In the two latter cases a hybrid progeny was the result. It has been noticed that these unions generally took place when the air was in a very electric condition, and rain had fallen, or was about to fall, abundantly.

Of marine species Littorina rudis has been noticed[253] in union both with L. obtusata and with L. littorea, but no definite facts are known as to the result of such unions.

Self-impregnation (see p. 44).

Development of the Fertilised Ovum.—The first stages in the development of the Mollusca are identical with those which occur in other classes of animals. The fertilised ovum consists of a vitellus or yolk, which is surrounded with albumen, and is either contained in a separate capsule, or else several, sometimes many, ova are found in the same capsule, only a small proportion of which ultimately develop. The germinal vesicle, which is situated at one side of the vitellus, undergoes unequal segmentation, the result of which is usually the formation of a layer of small ectoderm cells overlying a few much larger cells which contain nearly the whole of the yolk. The large cells are then invaginated, or are simply covered by the growth of the ectoderm cells. The result in either case is the formation of an area, the blastopore, where the inner cells are not covered by the ectoderm. The blastopore gradually narrows to a circular opening, which, in the great majority of cases, eventually becomes the mouth. The usual differentiation of germinal layers takes place, the epiblast eventually giving rise to the epidermis, nervous system, and special sense organs, the hypoblast to the liver and to the middle region of the alimentary tract, the mesoblast to the muscles, the body cavity, the vascular, the excretory and reproductive systems. The next, or trochosphere (trochophora) stage, involves the formation of a circlet of praeoral cilia, dividing the still nearly spherical embryo into two unequal portions, the smaller of which consists simply of the prostomium, or part in front of the mouth, the larger bearing the mouth and anus.

So far the series of changes undergone by the embryo are not peculiar to the Mollusca; we now come to those which are definitely characteristic of that group. The stage next succeeding the development of the trochosphere is the definitive formation of the velum, a process especially characteristic of the Gasteropoda and Pelecypoda, but apparently not occurring in the great majority of land Pulmonata.

Fig. 44.—Veligers of Dentalium entalis L.: A, longitudinal section of a larva 14 hours old, × 285; B, larva of 37 hours, × 165; C, longitudinal section of larva of 34 hours, × 165; m, mouth; v, v, velum. (After Kowalewsky.)

The circlet of cilia becomes pushed more and more towards the anterior portion of the embryo, the cilia themselves become longer, while the portion of the body from which they spring becomes elevated into a ridge or ring, which, as a rule, develops on each side a more or less pronounced lobe. The name velum is applied to this entire process of ciliated ring and lobes, and to the area which they enclose.

Fig.45.—Veliger of Patella vulgata L., 130 hours old: f, rudimentary foot; op, operculum; sh, shell; v, v, velum. (After Patten, highly magnified.)

Fig. 46.—Developed larva of Cyclas cornea L.: br, rudimentary branchiae; by, byssus; f, foot; m.e, mantle edge; sh, shell. (After Ziegler, highly magnified.)

Fig. 47.—A, Advanced veliger of Dreissensia: f, foot; m, mouth; sh, shell; v, v, velum. (After Korschelt and Heider, much enlarged.) B, Veliger of a Pteropod (Tiedemannia): op, operculum; sh, shell; v, velum. (After Krohn, much enlarged.)

In this so-called veliger stage, the velum serves, in the first place, to cause rotation of the larva within the egg-capsules, and, after hatching, as an organ of locomotion. As a rule, the velum disappears entirely in the adult mollusc after the free-swimming stage is over, but in the common Limnaea stagnalis it persists, losing its cilia, as the very prominent circum-oral lobes. Simultaneously with the development of the velum, and in some cases earlier, appear the rudiments of the shell-gland and of the foot, the latter being situated on the ventral side, between the mouth and anus, the former on the dorsal side, behind the velum, and above the surface of the eventual visceral sac. Thus the prime characteristics of the veliger stage, subsequent to the appearance of the velum itself, are the development of the visceral sac and shell-gland on the upper, and of the foot on the under side. According to Lankester the primitive shell-gland does not, as a rule, directly give rise to the shell of the adult mollusc, but becomes filled up by a horny substance, and eventually disappears; the permanent shell then forms over the surface of the visceral hump from the original centre of the shell-gland. It is only in Chiton, and possibly in Limax, that the primitive shell-sac is retained and developed into the final shell-forming area, which is much wider, and extends to the edges of the mantle. Within the velar area first appear the rudiments of the tentacles and eyes; when these become developed the velum atrophies and disappears.

Several of these veligers when captured in the open sea have been mistaken for perfect forms, and have been described as such. Thus the larva of Dolium has been described as Macgillivrayia, that of a Purpura as Chelotropis and Sinusigera, that of Aporrhais pes pelecani as Chiropteron, that of Marsenia conspicua as Brownia, Echinospira, and Calcarella.

Cephalopoda.—The embryonic development of the Cephalopoda is entirely distinct from that of all other Mollusca. The segmentation of the vitellus is partial, and the embryo is furnished with a vitelline sac, which is very large in the majority of cases (Fig. 48). There is no free-swimming stage, but the embryo emerges from the egg fully developed.

Differences of Sex.—In the Mollusca there are two main types of sexual difference: (i) sexes separate (dioecious type), (ii) sexes united in the same individual (hermaphrodite type).

Fig. 48.—Two stages in the development of Loligo vulgaris Lam.: a₁, a₁, first, and a₂, a₂, second pairs of arms; br, branchiae, seen through m, mantle; e, e, eyes; fi, fins; fu, funnel; v.s, vitelline sac. (After Kowalewsky.)

In some cases—e.g. certain Pelecypoda—what is practically a third type occurs. The animal is hermaphrodite, but the male and female elements are not developed simultaneously, i.e. the same individual is at one time female, at another male.

1. The sexes are separate in

• All Cephalopoda.
• Gasteropoda Amphineura (except Neomeniidae).
• Gasteropoda Prosobranchiata (except Valvata and some species of Marsenia).
• Scaphopoda.
• Many Pelecypoda.

2. The sexes are united in

• Gasteropoda Opisthobranchiata.
• Gasteropoda Pulmonata.
• Certain Pelecypoda.[254]

In the dioecious Mollusca, sexual union is the rule, but is by no means universal. In some instances,—e.g. Vermetus, Magilus, Patella, Haliotis, Crepidula, Chiton, the Scaphopoda—the form and habits of the animal do not admit of it; in others (many Trochus) a male copulative organ is wanting. When this is the case, the male scatters the spermatozoa freely; the majority must perish, but some will be carried by currents in the direction of the female.

When the sexes are separate, the female is frequently larger than the male. This is markedly the case in Littorina, Buccinum, and all the Cephalopoda; in Argonauta the difference is extreme, the male not being more than ¼ the size of the female.

Those hermaphrodite Mollusca which are capable of sexual union (Gasteropoda, Pulmonata, and Opisthobranchiata) are conveniently divided into two sections, according as (1) there are separate orifices for the male and female organs, or (2) one orifice serves for both. To the former section (Digonopora[2]) belong the Limnaeidae, Vaginulidae, and Onchidiidae, and many Opisthobranchiata, including all the Pteropoda; to the latter (Monogonopora[255]) nearly all the Nudibranchiate Opisthobranchiata, and all the rest of the Pulmonata. In the latter case during union, mutual impregnation takes place, and each of the two individuals concerned has been observed (compare p. 42) to deposit eggs. In the former, however, no such reciprocal act can take place, but the same individual can play the part of male to one and female to another, and we sometimes find a string of Limnaea thus united, each being at once male and female to its two adjacent neighbours.

The Reproductive System.—Broadly speaking, the complicated arrangements which are found in Mollusca resolve themselves into modifications of three important factors:—

(a) The gonads or germ-glands, in which are developed the ova and the spermatozoa. These glands are generally known as the ovary in the female, the sperm-gland or testis in the male.

(b) The channels which provide for the passage of the seminal products; namely, the oviduct in the female, the vas deferens or sperm-duct in the male.

(c) The external generative organs.

Fig. 49.—Generative and other organs of Littorina obtusata L., female.

• A, anus.
• Br, branchia.
• Buc, buccal mass.
• H, heart.
• Hep, hepatic duct.
• I, continuation of oesophagus.
• Ki, kidney.
• Li, liver.
• M, muscle of attachment.
• O´, female orifice.
• Od, oviduct.
• Oes, oesophagus.
• Ov, ovary.
• Ra, radula.
• St, stomach.
• U, uterus.

(After Souleyet.)

Fig. 50.—Generative and other organs of Littorina obtusata L., male.

• A, anus.
• Br, branchia.
• H, heart.
• I, intestine.
• Li, liver.
• M, muscle of attachment.
• Pe, penis.
• Te, testis.
• VD, vas deferens.

(After Souleyet.)

Dioecious Mollusca.—The common Littorina obtusata will serve as a typical instance of a dioecious prosobranchiate, exhibiting the simplest form of organs. In the female the ovary, a lobe-shaped body, is embedded in the liver. An oviduct with many convolutions conveys the ova into the uterus, an oblong chamber which consists simply of a dilatation of the oviduct. The ova descend into the uterus, which is sometimes furnished with a seminal pouch. In this seminal pouch, or above it, in the oviduct, the ova come into contact with the spermatozoa. The lower part of the uterus secretes a gelatinous medium (or capsule, as the case may be) in which the fertilised ova become enclosed previous to exclusion. In position the oviduct abuts on the kidney, while the uterus is in close proximity to the rectum, and the female external orifice is found close to the anus, within the branchial cavity.

The male organs of Littorina are more simple. The testis is lodged, like the ovary, in the liver; the vas deferens is, like the oviduct, convoluted, and eventually traverses the right side of the neck, emerging near the right tentacle, and terminating in the penis or external copulative organ (Fig. 50).

This system prevails, with but slight modifications in detail, throughout the prosobranchiate Gasteropoda. The most important modification is the passage of the seminal products in certain cases (many of the Diotocardia) through the right kidney, with which the oviduct and vas deferens always stand in close relation. The same arrangement occurs in the Scaphoda and some Pelecypoda.

The penis varies greatly in form and size. In the Strombidae (see Fig. 99) and Buccinidae (Fig. 62) it is very large and prominent; in Littorina it is somewhat spinulose at one side; in Paludina a portion of it is lodged in the right tentacle, which becomes atrophied and much more obtuse than the tentacle on the left side.

Spermatozoa.—The shape of the spermatozoa and of the ova in Mollusca is of the usual type. In Paludina Ampullaria, and certain species of Murex two types of spermatozoa occur, one hair-like, the other worm-like, three times as long as the former, and not tapering at one end. The former type alone take part in fertilisation, and penetrate the ovum. It has been suggested that these worm-like spermatozoa are a kind of incipient ova, and indicate a possible stage in commencing hermaphroditism. And, since the nearest allies of the Prosobranchiata (in which these types occur) are hermaphrodite (i.e. the Opisthobranchiata and Pulmonata), it is not unreasonable to suppose that the Prosobranchiata should show some tendency towards hermaphroditism in their genital glands.[256]

Cephalopoda.—The special characteristic of the reproductive organs in female Cephalopoda is the development of various glands, some of considerable size, in connexion with the ovary and oviduct. Sepia, Loligo, and Sepiola are furnished with two large nidamental glands, which open into the mantle cavity independently of the oviduct. Their purpose is to produce a viscid mucus, which envelops the ova at the moment of their emission and eventually hardens into the egg-capsules. A pair of accessory nidamental glands occur in Sepia, as well as a pair of smaller glands situated on the oviduct itself.

In many of the male Cephalopoda the vas deferens is long and dilated at its outer end into a glandular reservoir, within which are formed the spermatophores, or narrow cylindrical packets which contain a very large number of spermatozoa. When charged, the spermatophores pass into what is known as Needham’s sac, where they remain until required for use. These spermatophores are a very characteristic part of the reproductive arrangements in the Cephalopoda. The male of Sepia has been noticed to deposit them, during union, upon the buccal membrane of the female. During the emission of the ova by the female, the spermatophores, apparently through the agency of a kind of spring contained at one end, burst, and scatter the spermatozoa over the ova.

The Hectocotylus Arm.—Perhaps the most remarkable feature in the sexual relations of all the Mollusca is the so-called hectocotylus of the Cephalopoda. In the great majority of the male Cephalopoda, one of the ‘arms,’ which is modified for the purpose in various ways and to a greater or less extent, becomes charged with spermatophores, and sometimes, during union, becomes detached and remains within the mantle of the female, preserving for some considerable time its power of movement.

The hectocotylus is confined to the dibranchiate Cephalopoda, and its typical form, i.e. when part of the arm becomes disengaged and left with the female, occurs only in three genera of the Octopodidae, viz.[Argonauta, Ocythoe (Philonexis), and Tremoctopus. In all of these, the male is many sizes smaller than the female. In Argonauta the third arm on the left side becomes hectocotylised. At first it is entirely enveloped in a kind of cyst, in such a way that only a small portion of the tip projects; subsequently the cyst parts asunder, and allows the arm to become expanded to its full length, which considerably exceeds that of the other arms. At a certain point the acetabula or suckers terminate, and the remainder of the arm consists of a very long, tapering, sometimes thread-like filament, which is pointed at the extreme tip. It is not yet known how the spermatophores find their way into the hectocotylus, or how the hectocotylus impregnates the ova of the female. The arm thus affected is not always the same. In Tremoctopus it is the third of the right side, in the Decapoda the modification usually affects the fourth of the left.

Fig. 51.—Male of Ocythoe tuberculata Raf. (= Philonexis catenulatus Fér.), Mediterranean, showing three stages, A, B, and C, in the development of the hectocotylus arm: h.cy, hectocotylus still in the cyst; c´y´, spoon-shaped cyst at the end of the arm when freed; th, thread-like organ freed by the rupture of c´y´. Natural size. From specimens in the British Museum.

This singular property of the male Cephalopoda has only recently been satisfactorily explained. It is true that Aristotle, more than twenty-two centuries ago, distinctly stated that certain of the arms were modified for sexual purposes. Speaking of what he calls the polypus (which appears to represent the Octopus vulgaris of the Mediterranean), he says: ‘It differs from the female in having what the fishermen call the white sexual organ on its arm;’ again, ‘Some say that the male has something of a sexual nature (αὶδοιῶδές τι) on one of its arms, that on which the largest suckers occur; that this is a kind of muscular appendage attached to the middle of the arm, and that it is entirely introduced within the funnel of the female’. Unfortunately the word translated by introduced is corrupt, and can only be restored conjecturally. He again remarks, ‘The last of the arms, which tapers to a fine point and is the only whitish arm, it uses in sexual union.’[257]

The typical hectocotylus seems to have entirely escaped notice until early in the present century, when both Delle Chiaje and Cuvier described it, as detected within the female, as a parasite, the latter under the name of Hectocotylus octopodis. Kölliker, in 1845–49, regarded the Hectocotylus of Tremoctopus as the entire male animal, and went so far as to discern in it an intestine, heart, and reproductive system. It was not until 1851 that the investigations of Vérany and Filippi confirmed a suggestion of Dujardin,[258] while H. Müller, in 1853, completed the discovery by describing the entire male of Argonauta.

In all genera of dibranchiate Cephalopoda except Argonauta, Ocythoe, and Tremoctopus, one of the arms is sexually modified in various ways, but never becomes so much prolonged, and is never detached and left with the female. In Loligo Forbesii Stp. the fourth arm on the left has 23 pairs of regularly developed acetabula, which then lessen in size and disappear, being replaced by long pedunculated papillae, of which there are about 40 pairs. In Loligo vulgaris Lam. and L. Pleii Orb. 18 or 19 pairs of acetabula are regularly formed, and then occur 40 pairs of papillae, as in Forbesii. In other species of Loligo (gahi Orb., brevis Bl., brasiliensis Orb.) only the outer row of suckers becomes modified into papillae after about the 20th to the 22nd pair. In Sepioteuthis sepioides the modification is the same as in the Loligo last mentioned, but the corresponding arm on the right side is so covered with acetabula towards its extreme end, that it is thought that it in some way co-operates with the hectocotylised left arm.

In Octopus, the third arm on the right side is subject to modification. This arm is always shorter than the corresponding arm on the other side, and carries fewer suckers, but is furnished at the extreme tip with a peculiar kind of plate, which connects with the membrane at the base of the arm by a channel of skin, which probably conveys the spermatophores up to the tip.

In Octopus vulgaris, the species referred to by Aristotle, the hectocotylised arm is short, thin in its outer half and pointed at the extremity, while the fold of skin is very white, and gives the arm an appearance of being divided by a cleft at the side. At the same time, an unusual development of one or two suckers on the arm is not uncommon.[259]

Fig. 52.—Octopus lentus Baird, N. Atlantic, showing the peculiar formation of the hectocotylus arm, h.a. (After Verrill, × ½.)

It is believed that in the Tetrabranchiate Cephalopoda (Nautilus) a union of the four inner ventral arms may correspond functionally to the hectocotylising of the arm in the Dibranchiates.

Hermaphrodite Mollusca.—(a) Monogonopora.—The reproductive system in the hermaphrodite Mollusca is far more complicated than in the dioecious, from the union of the male and female organs in the same individual. As a type of the Monogonopora, in which a single orifice serves for both male and female organs, may be taken the common garden snail (Helix aspersa), the accompanying figure of which is drawn from two specimens found in the act of union (Fig. 53).

Fig. 53.—Genitalia of Helix aspersa Müller, drawn from two individuals in the act of union, from a dissection by F. B. Stead.

• A.G, albumen gland.
• C, coecum.
• Cr, crop.
• D.S, dart sac.
• E, eye (retracted).
• Fl, flagellum.
• H.D, hermaphrodite duct.
• H.DF, ditto, female portion.
• H.DM, ditto, male portion.
• H.G, hermaphrodite gland.
• L, liver.
• M.G, M.G, mucous glands.
• Ov, oviduct.
• P.S, penis sac.
• R.M, retractor muscle of penis.
• Sp, spermatheca.
• V, vagina.
• V.D, vas deferens.

Beginning from the inside and proceeding outwards we have firstly the hermaphrodite gland or ovo-testis (H.G.), a yellowish white mass of irregular shape, embedded in the liver (L.) and forming part of its spiral but not reaching quite to the apex. Within this gland are developed the ova and spermatozoa. The former are rather large round cells, produced within the outer wall of the gland, while the spermatozoa, which are produced in the more central part, are thread-like bodies, generally aggregated in small bundles. From the hermaphrodite gland the ova and spermatozoa pass through the upper part of the hermaphrodite duct (H.D.), which is always more or less convoluted. Below the convoluted portion, the duct opens into the albumen gland (A.G.), a large linguiform mass of tissue which becomes dilated at the time of pairing, and secretes a thick viscid fluid which probably serves to envelop the ova. Up to this point both the male and female elements follow the same course, but on their exit from the albumen gland they diverge. The hermaphrodite duct becomes greatly enlarged, and is partially divided by a kind of septum into a male and female portion. These run parallel to one another, the larger or female portion (H.DF.), through which the ova pass (and which is sometimes termed the uterus) being dilated into a number of puckered folds, while the smaller or male portion (H.DM.) is comparatively narrow, and not dilated. At their anterior end, the two portions of the duct separate completely from one another, the female portion being then termed the oviduct (OV.) and the male portion the vas deferens (V.D.).

Following first the oviduct, we find that it soon widens into the vagina (V.), which is furnished with a pair of mucous glands (M.G.), one on each side. These are much branched, and resemble little bunches of whitish seaweed. A little above the mucous glands a long tube diverges from the vagina, which is furnished with a produced coecum (C.) and a pouch, the spermatheca (SP.) at the extreme end. In this pouch, and in the duct leading to it, is stored the spermatophore received in union with another snail. Just below the mucous glands the vagina is joined by the dart sac (D.S.), which is more fully described below. Finally, at its lower end the vagina unites with the penis sac at a point just posterior to the common orifice.

Returning now to the male organs, we find that the vas deferens is the continuation of the male portion of the hermaphrodite duct, after its final separation from the female portion. It passes under the retractor muscle of the upper right tentacle, which has been cut away in the specimen figured, to dissect it out. Just before the vas deferens widens into the penis sac, it branches off into a long and tapering tube, the flagellum, in which the spermatozoa are stored and become massed together in the long packet known as the spermatophore. The penis sac (P.S.) is the continuation of the vas deferens beyond the point at which the flagellum diverges. It joins the vagina at its extreme anterior end, uniting with it to form the common genital aperture, which cannot be exactly represented in the figure. The penis itself lies in the interior of the penis sac, and is a rather long muscular tube which is protruded during union, but at other times remains retracted within the sac.

In the Helicidae generally, the form of the generative organs varies with each separate species, sometimes merely as regards the size of the different parts, at others in the direction of greater simplicity or complication. The mucous glands may be absent, and the flagellum greatly reduced in size, or absent altogether.

The Dart Sac.—A remarkable part of the reproductive system in many of the true Helicidae is the so-called dart, Liebespfeil, or telum veneris. It consists of ‘a straight, or curved, sometimes slightly twisted tubular shaft of carbonate of lime, tapering to a fine point above, and enlarging gradually, more often somewhat abruptly, to the base.’ The sides of the shaft are sometimes furnished with two or more blades; these are apparently not for cutting purposes, but simply to brace the stem. The dart is contained in a dart sac, which is attached as a sort of pocket to the vagina, at no great distance from its orifice. There are four different forms of sac. It may be single or double, and each of these divisions may be bilobed, each lobe containing one dart at a time. In Helix aspersa the dart is about ⁵/₁₆ in. in length, and ⅛ in. in breadth at its base (see Fig. 54).

It appears most probable that the dart is employed as an adjunct to the sexual act. Besides the fact of the position of the dart sac anatomically, we find that the darts are extruded and become embedded in the flesh just before or during the act of copulation. It may be regarded, then, as an organ whose punctures induce excitement preparatory to sexual union. It only occurs in well-grown specimens. When once it begins to form, it grows very rapidly, perhaps not more than a week being required for its entire formation.

Fig. 54.—Darts of British land snails: A, Hyalinia excavata Bean; B, Helix hortensis Müll.; C, Helix aspersa Müll. (After Ashford.)

The dart is almost confined to Helicidae, a certain number of exceptions being known which border on Helix. Hyalinia nitida and excavata are the only British species, not Helices, which are known to possess it. It has not been noticed to occur in the slugs, except in the N. American genus Tebennophorus. About one-third of the British Helices are destitute of the dart.[260] H. rufescens possesses a double bilobed sac, but only two darts, which lie in the lower lobes. It does not use the darts, and could not do so, from the relative sizes of dart and sac; it has often been watched when uniting, but the use of the darts has never been observed. From this it has been inferred that the darts are degenerate weapons of defence, and that they were in fact at one time much stronger organs and more often used.[261] This theory, however, does not seem consistent with the whole circumstances of the occurrence, position, and present use of the darts.

Hermaphrodite Mollusca.—(b) Digonopora.—As an example of the Digonopora or hermaphrodite Mollusca with separate generative apertures for the male and female organs, we may take the common Limnaea stagnalis (Fig. 55). It will be seen from the figure that the relative positions of the hermaphrodite gland and duct, and of the albumen gland, are the same as in Helix. When the oviduct parts company from the vas deferens, it becomes furnished with several accessory glands, one of which (Gl.E.) probably serves as a reservoir for the ova, and answers more or less to a uterus. The tube leading to the spermatheca is short, and there is no divergent caecum. The female orifice lies near to the external opening of the branchial cavity. The vas deferens, which is very long, is furnished with a large prostate gland. The penis sac is greatly dilated, and there is no flagellum. The male orifice is behind the right tentacle, slightly in advance of the female orifice (compare Fig. 102).

Fig. 55.—Genitalia of Limnaea stagnalis L. (from a dissection by F. B. Stead), × 2.

• A.G, albumen gland.
• Ac.G, accessory gland.
• F.O, female orifice.
• Gl.E, glandular enlargement.
• H.D, hermaphrodite duct.
• H.G, hermaphrodite gland.
• Li, liver.
• M.O, male orifice.
• P, penis sac.
• Pr, prostate.
• R.M, retractor muscle of penis.
• Sp, spermatheca.
• V.D, vas deferens.]

Most of the Opisthobranchiata, but not all, have separate sexual orifices. Numerous variations from the type just described will be found to occur, particularly in the direction of the development of accessory glands, which are sometimes very large, and whose precise purpose has in many cases not been satisfactorily determined.

Pelecypoda.—In the dioecious Pelecypoda, which form the great majority, the reproductive system is simple, and closely parallel in both sexes. It consists of a pair of gonads, which are either ovaries or testes, and a pair of oviducts or sperm-ducts which lead to a genital aperture. The gonads are usually placed symmetrically at the sides or base of the visceral mass. The oviduct is short, and the genital aperture is usually within the branchial chamber, thus securing the fertilisation of the ova by the spermatozoa, which are carried into the branchial chamber with the water which passes through the afferent siphon.

Hermaphrodite Pelecypoda are rare, the sexes being usually separate. The following are assured instances: Pecten glaber, P. jacobaeus, P. maximus, Ostrea edulis, Cardium norvegicum, Pisidium pusillum, Cyclas cornea, Pandora rostrata, Aspergillum dichotomum, and perhaps Clavagella. The greater number of these have only a single genital gland (gonad) on each side, with a single efferent duct from each, but part of the gland is male and part female, e.g. in the Pectens above mentioned. Pandora and Aspergillum have two distinct glands, respectively male and female, on each side, each of the two glands possessing its separate duct, and the two ducts from each side eventually opening near one another. It appears probable that the Septibranchiata (Cuspidaria, Poromya, Lyonsiella, etc.) must also be added to the number of hermaphrodite Pelecypoda which have separate male and female glands.

It is worthy of remark that all the hermaphrodite Pelecypoda belong to forms decidedly specialised, while forms distinctly primitive, such as Nucula, Solenomya, Arca, and Trigonia are all dioecious. In Gasteropoda similarly, the least specialised forms (the Amphineura, with the exception of the Neomeniidae, and the Rhipidoglossa) are dioecious. It is possible therefore that in the ancestors of the Mollusca the separation of the sexes had already become the normal type of things, and that hermaphroditism in the group is, to a certain extent, a sign or accompaniment of specialisation.[262]

Development of Fresh-water Bivalves.—The vast majority of fresh-water bivalves either pass the larval stage entirely within the mother, and do not quit her except in a perfectly developed form (Cyclas, Pisidium), or assume a mode of development in which free larvae indeed occur, but are specially modified for adaptation to special circumstances (Unio). Cyclas and Pisidium, and no doubt all the kindred genera, preserve their ova in a sort of brood-pouch within the gills, in which the ova pass the earlier stages of their development. But, even so, the larva of these genera retains some traces of its original free-swimming habits, for a rudimentary velum, which is quite useless for its present form of development, has been detected in Cyclas.

The larva of Dreissensia (see Fig. 47, A), so far as is at present known, stands alone among fresh-water bivalves in being free-swimming, and to this property has been attributed, no doubt with perfect justice, the fact of the extraordinarily rapid spread of Dreissensia over the continent of Europe (chap. xvi.). In expelling the ova, the parent slightly opens the shells and then quickly closes them, shooting out a small point of white slime, which is in fact a little ball of eggs. The general course of development is precisely parallel to that of marine Pelecypoda, greatly resembling, so far as form is concerned, certain stages in the growth of the larvae of Modiolaria and Cardium, as figured by Lovén.[263]

In June and July the larvae appear in large numbers on the surface of the water, when in spite of their exceedingly small size, they can be captured with a fine hand-net. They pass about eight days on the surface, feeding apparently on minute floating algae. During this time, the principal change they undergo is in the formation of the foot, which first appears as a small prominence midway between the mouth and anus, and gradually increases in length and flexibility. When the larva sinks to the bottom, the velum soon disappears entirely, the foot becomes exceedingly long and narrow, while the shell is circular, strongly resembling a very young Cyclas.

Larvae of Unionidae.—The early stages of the development of Unio and Anodonta (so far as the species of North America, Europe, and Asia are concerned) is of extreme interest, from the remarkable fact that the young live for some time parasitically attached to certain species of fresh-water fishes. In order to secure this attachment, the larva, which is generally known as Glochidium, develops a long filament which perhaps renders it aware of the neighbourhood of a fish, and also a larval shell furnished with strong hooks by which it fastens itself to the body of its unconscious host (Fig. 56). According to some interesting observations made by Mr. O. H. Latter,[264] the ova pass into the external gill of the mother, in which is secreted a nutritive mucus on which they are sustained until they arrive at maturity and a suitable opportunity occurs for their ‘being born.’ If this opportunity is deferred, and the Glochidia mature, their so-called ‘byssus’ becomes developed, and by being entangled in the gill filaments of the parent, prevents their escaping. It is interesting to notice that, when the nutritive mucus of the parent is used up, it becomes, as it were, the turn of the children to provide for themselves a secondary mode of attachment.

Fig. 56.—A, Glochidium immediately after it is hatched: ad, adductor muscle; by, ‘byssus’ cord; s, sense organs; sh, shell. B, Glochidium after it has been on the fish for some weeks: a.ad, p.ad, anterior and posterior adductors; al, alimentary canal; au.v, auditory vesicle; br, branchiae; f, foot; mt, mantle. (Balfour.)

The mother Anodonta does not always retain the Glochidium until fish are in her neighbourhood. Gentle stirring of the water caused them to emit Glochidium in large masses, if the movement was not so violent as to cause alarm. The long slimy masses of Glochidium were observed to be drawn back again within the shell of the mother, even after they had been ejected to a distance of 2 or 3 inches.

It is a mistake to assert that the young Glochidium can swim. When they finally quit the mother, they sink to the bottom, and there remain resting on their dorsal side, with the valves gaping upwards and the so-called byssus streaming up into the water above them. There they remain, until a convenient ‘host’ comes within reach, and if no ‘host’ comes within a certain time, they perish. They are evidently peculiarly sensitive to the presence of fish, but whether they perceive them by smell or some other sense is unknown. “The tail of a recently killed stickleback thrust into a watch-glass containing Glochidium throws them all into the wildest agitation for a few seconds; the valves are violently closed and again opened with astonishing rapidity for 15–25 seconds, and then the animals appear exhausted and lie placid with widely gaping shells—unless they chance to have closed upon any object in the water (e.g. another Glochidium), in which case the valves remain firmly closed.”

In about four weeks after the Glochidium has quitted its host, and the permanent shell has made its appearance within the two valves of the Glochidium, the projecting teeth of the latter press upon the ventral edge of the permanent shell, at a point about half way in its lengthward measurement, retarding the growth of the shell at that particular point, and indenting its otherwise uninterrupted curve with an irregular notch or dent. As growth proceeds, this dent becomes less and less perceptible on the ventral margin of the shell itself, but its effects may be detected, in well-preserved specimens, by the wavy turn in the lines of growth, especially near the umbones of the young shell.

Mr. Latter found that all species of fish with which he experimented had a strong dislike to Glochidium as an article of food. Sometimes a fish would taste it “just to try,” but invariably spit it out again in a very decided manner. The cause of unpleasantness seemed not to be the irritation produced in the mouth of the fish by the attempt of the Glochidium to attach itself, but was more probably due to what the fish considered a nasty taste or odour in the object of his attentions.

The following works will be found useful for further study of this portion of the subject:—

CHAPTER VI RESPIRATION AND CIRCULATION—THE MANTLE — Circulatory System — The Mantle

The principle of respiration is the same in the Mollusca as in all other animals. The blood is purified by being brought, in successive instalments, into contact with pure air or pure water, the effect of which is to expel the carbonic acid produced by animal combustion, and to take up fresh supplies of oxygen. Whether the medium in which a mollusc lives be water or air, the effect of the respiratory action is practically the same.

Broadly speaking, Mollusca whose usual habitat is the water ‘breathe’ water, while those whose usual habitat is the land ‘breathe’ air. But this rule has its exceptions on both sides. The great majority of the fresh-water Mollusca which are not provided with an operculum (e.g. Limnaea, Physa, Planorbis), breathe air, in spite of living in the water. They make periodic visits to the surface, and take down a bubble of air, returning again for another when it is exhausted. On the other hand many marine Mollusca which live between tide-marks (e.g. Patella, Littorina, Purpura, many species of Cerithium, Planaxis, and Nerita) are left out of the water, through the bi-diurnal recess of the tide, for many hours together. Such species invariably retain several drops of water in their branchiae, and, aided by the moisture of the air, contrive to support life until the water returns to them. Some species of Littorina (e.g. our own L. rudis and many tropical species) live so near high-water mark that at neap-tides it must frequently happen that they are untouched by the sea for several weeks together, while they are frequently exposed to a burning sun, which beats upon the rocks to which they cling. In this case it appears that the respiratory organs will perform their functions if they can manage to retain an extremely small amount of moisture.[265]

The important part which the respiratory organs play in the economy of the Mollusca may be judged from the fact that the primary subdivision of the Cephalopoda into Dibranchiata and Tetrabranchiata is based upon the number of branchiae they possess. Further, the three great divisions of the Gasteropoda have been named from the position or character of the breathing apparatus, viz. Prosobranchiata, Opisthobranchiata and Pulmonata, while the name Pelecypoda has hardly yet dispossessed Lamellibranchiata, the more familiar name of the bivalves.

Respiration may be conducted by means of—(a) Branchiae or Gills, (b) a Lung or Lung-cavity, (c) the outer skin.

In the Pelecypoda, Cephalopoda, Scaphopoda, and the great majority of the Gasteropoda, respiration is by means of branchiae, also known as ctenidia[266], when they represent the primitive Molluscan gill and are not ‘secondary’ branchiae (pp. 156, 159).

In all non-operculate land and fresh-water Mollusca, in the Auriculidae, and in one aberrant operculate (Amphibola), respiration is conducted by means of a lung-cavity, or rarely by a true lung, whence the name Pulmonata. The land operculates (Cyclophoridae, Cyclostomatidae, Aciculidae, and Helicinidae) also breathe air, but are not classified as Pulmonata, since other points in their organisation relate them more closely to the marine Prosobranchiata. Both methods of respiration are united in Ampullaria, which breathes indifferently air through a long siphon which it can elevate above the surface of the water, and water through a branchia (see p. 158). Siphonaria (Fig. 57) is also furnished with a lung-cavity as well as a branchia. Both these genera may be regarded as in process of change from an aqueous to a terrestrial life, and in Siphonaria the branchia is to a great extent atrophied, since the animal is out of the water, on the average, twenty-two hours out of the twenty-four. In the allied genus Gadinia, where there is no trace of a branchia, but only a lung-cavity, and in Cerithidea obtusa, which has a pulmonary organisation exactly analogous to that of Cyclophorus,[267] this process may be regarded as practically completed.

Fig. 57.—A, Siphonaria gigas Sowb., Panama, the animal contracted in spirit: gr, siphonal groove on right side. B, Gadinia peruviana, Sowb., Chili, shell only: gr, mark of siphonal groove to right of head.

Respiration by means of the skin, without the development of any special organ, is the simplest method of breathing which occurs in the Mollusca. In certain cases, e.g. Elysia, Limapontia, and Cenia among the Nudibranchs, and the parasitic Entoconcha and Entocolax, none of which possess breathing organs of any kind, the whole outer surface of the body appears to perform respiratory functions. In others, the dorsal surface is covered with papillae of varied size and number, which communicate with the heart by an elaborate system of veins. This is the case with the greater number of the Aeolididae (Fig. 58, compare Fig. 5, C), but it is curious that when the animal is entirely deprived of these papillae, respiration appears to be carried on without interruption through the skin.

Fig. 58.—Aeolis despecta Johnst., British coasts. (After Alder and Hancock.)

In the development of a distinct breathing organ, it would seem as if progress had been made along two definite lines, each resulting in the exposure of a larger length of veins, i.e. of a larger amount of blood, to the simultaneous operation of fresh air or fresh water. Either (a) the skin itself may have developed, at more or less regular intervals, elevations, or folds, which gradually took the form of papillae, or else (b) an inward folding, or ‘invagination,’ of the skin, or such a modification of the mantle-fold as is described below (p. 172) may have taken place, resulting in the formation of a cavity more or less surrounded by walls, within which the breathing organs were ultimately developed. Sometimes a combination of both processes seems to have occurred, and after a papilliform organ has been produced, an extension or prolongation of the skin has taken place, in order to afford a protection to it. Respiration by means of a lung-cavity is certainly subsequent, in point of time, to respiration by means of branchiae.

Fig. 59.—Chiton squamosus L., Bermuda: A, anus; Br, branchiae; M, mouth.

Fig. 60.—Fissurella virescens Sowb., Panama, showing position of the branchiae: Br, branchiae: E, E, eyes; F, foot; M, mantle; T, T, tentacles.

The branchiae seem to have been originally paired, and arranged symmetrically on opposite sides of the body. It is not easy to decide whether the multiple form of branchia which occurs in Chiton (Fig. 59), or the simple form as in Fissurella (Fig. 60), is the more primitive. Some authorities hold that the multiple branchia has gradually coalesced into the simple, others that the simple form has grown, by serial repetition, into the multiple. There appears to be no trace of any intermediate forms, and, as a matter of fact, the multiple branchia is found only in the Amphineura, while one or rarely two (never more) pairs of branchiae, occur, with various important modifications, in the vast majority of the Mollusca.

Amphineura.—In Chiton the branchiae are external, forming a long row of short plumes, placed symmetrically along each side of the foot. The number of plumes, at the base of each of which lies an osphradial patch, varies from about 70 to as few as 6 or 7. When the plumes are few, they are confined to the posterior end, and thus approximate to the form and position of the branchiae in the other Amphineura. In Chaetoderma, the branchiae consist of two small feather-shaped bodies, placed symmetrically on either side of the anus, which opens into a sort of cloaca within which the branchiae are situated. In Neomenia the branchiae are still further degraded, consisting of a single bunch of filaments lying within the cloaca, while in Proneomenia there is no more than a few irregular folds on the cloaca-wall (Fig. 61).

Fig. 61.—Terminal portions of the Amphineura, illustrating the gradual degradation of the branchiae, and their grouping round the anus in that class. A, Chiton (Hemiarthrum) setulosus Carp., Torres Str.; B, Chiton (Leptochiton) benthus Hadd., Torres Str.; C, Chaetoderma; D, Neomenia; a, anus; br, br, branchiae; k, k, kidneys; p, pericardium. (A and B after Haddon, C and D after Hubrecht.)

In the Prosobranchiata, symmetrically paired branchiae occur only in the Fissurellidae, Haliotidae, and Pleurotomariidae, in the former of which two perfectly equal branchiae are situated on either side of the back of the neck. These three families taken together form the group known as Zygobranchiata.[268] In all other families the asymmetry of the body has probably caused one of the branchiae, the right (originally left), to become aborted, and consequently there is only one branchia, the left, in the vast majority of marine Prosobranchiata, which have been accordingly grouped as Azygobranchiata. Even in Haliotis the right branchia is rather smaller than the left, while the great size of the attachment muscle causes the whole branchial cavity to become pushed over towards the left side. In those forms which in other respects most nearly approach the Zygobranchiata, namely, the Trochidae, Neritidae, and Turbinidae, the branchia has two rows of filaments, one on each side of the long axis, while in all other Prosobranchiata there is but one row (see Fig. 79, p. 169).

Fig. 62.—Bullia laevissima Gmel., showing branchial siphon S; F, F, F, foot; OP, operculum; P, penis; Pr, proboscis; T, T, tentacles. (After Quoy and Gaimard.)

In the great majority of marine Prosobranchiata the branchia is securely concealed within a chamber or pouch (the respiratory cavity), which is placed on the left dorsal side of the animal, generally near the back of the neck. For breathing purposes, water has to be conveyed into this chamber, and again expelled after it has passed over the branchia. In the majority of the vegetable-feeding molluscs (e.g. Littorina, Cerithium, Trochus) water is carried into the chamber by a simple prolongation of one of the lobes or lappets of the mantle, and makes its exit by the same way, the incoming and outgoing currents being separated by a valve-like fringe depending from the lobe. In the carnivorous molluscs, on the other hand, a regular tube, the branchial siphon, which is more or less closed, has been developed from a fold of the mantle surface, for the special purpose of conducting water to the branchia. After performing its purpose there, the spent water does not return through the siphon, but is conducted towards the anus by vibratile cilia situated on the branchiae themselves. In a large number of cases, this siphon is protected throughout its entire length by a special prolongation of the shell called the canal. Sometimes, as in Buccinum and Purpura, this canal is little more than a mere notch in the ‘mouth’ of the shell, but in many of the Muricidae (e.g. M. haustellum, tenuispina, tribulus) the canal becomes several inches long, and is set with formidable spines (see Fig. 164, p. 256). In Dolium and Cassis the canal is very short, but the siphon is very long, and is reflected back over the shell.

The presence or absence of this siphonal notch or canal forms a fairly accurate indication of the carnivorous or vegetarian tendencies of most marine Prosobranchiata, which have been, on this basis, subdivided into Siphonostomata and Holostomata. But this classification is of no particular value, and is seriously weakened by the fact that Natica, which is markedly ‘holostomatous,’ is very carnivorous, while Cerithium, which has a distinct siphonal notch, is of vegetarian tendencies.

In the Zygobranchiata the water, after having aerated the blood in the branchiae, usually escapes by a special hole or holes in the shell, situated either at the apex (Fissurella) or along the side of the last whorl (Haliotis). In Pleurotomaria the slit answers a similar purpose, serving as a sluice for the ejection of the spent water, and thus preventing the inward current from becoming polluted before it reaches the branchiae (see Fig. 179, p. 266).

In Patella the breathing arrangements are very remarkable. In spite of their apparent external similarity, this genus possesses no such symmetrically paired plume-shaped branchiae as Fissurella, but we notice a circlet of gill-lamellae, which extends completely round the edge of the mantle. It has been shown by various authorities that these lamellae are in no sense morphologically related to the paired branchiae in other Mollusca, but only correspond to them functionally. The typical paired branchiae, as has been shown by Spengel, exist in Patella in a most rudimentary form, being reduced to a pair of minute yellow bodies on the right and left sides of the back of the ‘neck.’ A precisely similar abortion of the true branchiae, and special development of a new organ to perform their work, is shown in Phyllidia and Pleurophyllidia (see below under Opisthobranchiata). This circlet of functional gills in Patella has therefore little systematic value, being only developed in an unusual position, like the eyes on the mantle in certain Pelecypoda, to supply the place of the true organs which have fallen into disuse. Accordingly Cuvier’s class of Cyclobranchiata, which included Patella and Chiton, has no value, and has indeed long been discarded. In Chiton the gills never extend completely round the animal, but are always more or less interrupted at the head and anus. They are the true gills, the plumes being serially repeated in the same way as the shell plates.

Fig. 63.—Patella vulgata L., seen from the ventral side: f, foot; g.l, circlet of gill lamellae; m.e, edge of the mantle; mu, attachment muscle; sl, slits in the same; sh, shell; v, vessel carrying aerated blood to the heart; v´, vessel carrying blood from the heart; ve, small accessory vessels.

Fig. 64.—Patella vulgata L., seen from the dorsal side after the removal of the shell and the black pigment covering the integument; the anterior portion of the mantle is cut away or turned back: a, anus; br, br, remains of the true branchiae (ctenidia); i, intestine; k, k´, kidneys; k.ap, their apertures on each side of the anus; l, liver; m, m, mantle; mu, attachment muscles, severed in removal of shell; t, t, tentacles.

In the land Prosobranchiata (Cyclostomatidae, Cyclophoridae, Aciculidae, Helicinidae) which, having exchanged a marine for an aerial life, breathe air instead of water, the branchia has completely disappeared, and breathing is conducted, as in the Pulmonata, by a lung-cavity. In certain genera of land operculates, e.g. Pupina, Cataulus, Pterocyclus, a slight fissure or tube in the last whorl (see Fig. 180, p. 266) serves to introduce air into the shell, which is perhaps otherwise closed to air by the operculum. In Aulopoma, which has no tube, the operculum admits free circulation of air. In certain other Cyclostomatidae the apex is truncated, and air can enter there. De Folin closed with wax the aperture of Cycl. elegans, and found that on placing it in a pneumatic machine, the shell gave off air through its whole surface. On the other hand, Cylindrella and Stenogyra decollata, on being submitted to the same test, showed that the truncated part alone was permeable by air.

Fischer and Bouvier have made some interesting observations on the breathing of a species of Ampullaria (insularum Orb.). The species has, in common with all Ampullaria, two siphons, but while the right siphon is but slightly developed, the left is very long, almost twice as long as the shell (see Fig. 65). The animal, when under the water, lengthens its siphon, brings the orifice to the surface, and by alternately raising and depressing its head produces in the pulmonary sac movements of ex- and inspiration; these are repeated about ten or fifteen times at regular intervals of from six to eight seconds, a method of respiration strongly resembling that of the Cetacea. At the same time, branchial respiration takes place. If powdered carmine is added to water, the particles are seen to enter the branchial cavity by the siphon and pass out by the short right siphon. Sometimes the animal remains under water for hours without rising to the surface to inspire air. In Valvata (Fig. 66) the branchia is very large, and projects like a leaf or fan above the shell on the left side; on the corresponding position on the right side is a long filiform appendage, which some have regarded as representing the other branchia.

Fig. 65.—Ampullaria insularum Orb.: A, breathing water; B, breathing air; Si, siphon; T, upper; t, lower tentacles; X, pallial expansion, performing the part of excurrent siphon. (After Fischer and Bouvier, x ⅓.)

Opisthobranchiata.—A true branchia occurs only in the Tectibranchiata and the Ascoglossa. It lies on the right side, and is usually more or less external, being partly covered sometimes by the shell (as in Umbrella, Fig. 5), sometimes by a fold of the mantle. In the Pteropoda (which are probably derived from the Tectibranchiata), all the Thecosomata, with the exception of Cavolinia, have no specialised branchia, but probably respire through portions or the whole of the integument. In the Gymnosomata an accessory branchia has in many cases been developed at the posterior end of the body. Pneumodermon alone has both lateral and posterior branchiae well developed, Clione and Halopsyche are destitute of either, while the four remaining families have one branchia, sometimes lateral, sometimes posterior.[269]

Fig. 66.—Valvata piscinalis Müll.: br, branchia; fi, filament; f.l, foot lobes. (After Boutan.)

Fig. 67.—Doris (Archidoris) tuberculata L., Britain: a, anus; br, branchiae, surrounding the anus; m, male organ; rh, rh, rhinophores. × ⅔.

Fig. 68.—Pleurophyllidia lineata Otto, Mediterranean: a, anus; br, secondary branchiae; m, mouth; s.o, sexual orifice.

Certain of the Nudibranchiata possess no special breathing organs, and probably respire through the skin (Elysia, Limapontia, Cenia, Phyllirrhoë). The majority, however, have developed secondary branchiae, in the form of prominent lobes or leaf-like processes (the cerata), which are carried upon the back, without any means of protection. These cerata are, as a rule, of extreme beauty and variety of form, consisting sometimes of long whip-like tentaculae, in other cases of arborescent plumes of fern-like leafage, in others of curious bead-like appendages of every imaginable shape and colour. In Doris they lie at the posterior end of the body, in a sort of rosette, which is generally capable of retraction into a chamber. In Phyllidia and Pleurophyllidia these secondary branchiae lie, as in Patella, on the lateral portions of the mantle.

The Scaphopoda in all probability possess neither true nor secondary branchiae.

Pulmonata.—When we use the term ‘lung,’ it must be remembered that this organ in the Mollusca does not correspond, morphologically, with the spongy, cellular lung of vertebrates; it simply performs the same functions. The ‘lung,’ in the Mollusca, is a pouch or cavity, lined with blood-vessels which are disposed over its vaulted surface in various patterns of network. The pulmonary sac or cavity is therefore a better name by which to denote this organ.

Fig. 69.—Geomalacus maculosus Allm., S. Ireland: P.O, pulmonary orifice.

It seems probable, as has been already shown (pp. 18–22), that all Pulmonata are ultimately derived from marine forms which breathed water by means of branchiae. Thus we find intermediate forms, such as Siphonaria, possessed of both a branchia and a pulmonary sac, the former being evanescent, while in Gadinia and Amphibola it has quite disappeared. In the vast majority of Pulmonata no trace of a branchia remains; its function is performed by a chamber, always situated at the right side of the animal, and generally more or less anterior, admitting air by a narrow aperture which is rhythmically opened and closed. In Arion and Geomalacus (Fig. 69) this aperture is in the front of the right side of the ‘shield,’ in Limax (Fig. 71) in the hinder part, in Testacella (Fig. 20) it is near the extremity of the tail, under the spire of the shell; in Janella it is on the middle of the right edge of the shield (Fig. 70). If a specimen of Helix aspersa, or better, of H. pomatia, is held up to the light, the beautiful arborescent vessels, with which the upper part of the pulmonary chamber is furnished, can be clearly seen by looking through the aperture as it dilates. It is only in the Auriculidae that an actual spongy mass of lung material appears to exist. When in motion, a Helix inspires air much more frequently than when at rest. Temperature, too, seems to affect the number of inspirations; it appears doubtful whether, during hibernation, a snail breathes at all. In any case, the amount of air required to sustain life must be small.

Fig. 70.—Janella hirudo Fisch., N. Caledonia: G, generative orifice; P, pulmonary orifice; T, T, tentacles. (After Fischer.)

Fig. 71.—Limax maximus L.: PO, pulmonary orifice. × ⅔.

With regard to the respiration of fresh-water Pulmonata there appears to be some difference of opinion. It is held, on the one hand, that the Limnaeidae only respire air, making periodic visits to the surface to procure it, and that they perish, if prevented from doing so, by asphyxiation. If, we are told,[270] as a Limnaea is floating on the surface of the water in a glass jar, a morsel of common salt be dropped upon its outstretched foot, it will sink heavily to the bottom, emitting a stream of air from its pulmonary orifice. On recovering from the shock, it will anxiously endeavour to regain the surface, but will have some difficulty in doing so, owing to its now much greater specific gravity. When it succeeds, it creeps almost out of the water, and exposes its respiratory orifice freely to the air. If the experiment is repeated several times on the same individual, it becomes so much weakened that it has to be taken out of the water to save its life. Moquin-Tandon, on the other hand, is strongly of opinion[271] that there is no absolute necessity for Limnaea to obtain air by rising to the surface, and that, if prevented from emerging, it can obtain air from the water. When covered in by a roof of ice, Limnaea has not been observed to suffer any inconvenience. Moquin-Tandon kept L. glabra and Planorbis rotundatus in good health under 20 mm. of water for eighteen and nineteen days, and relates a case in which Physa was kept alive under water for four days, and Planorbis for twelve. Young specimens, both of Limnaea and Planorbis, do not rise to the surface for a supply of air; they are hatched with the pulmonary cavity full of water.

It is probable, therefore, that Limnaeidae are capable, on occasion, of respiration through the skin. Some authorities are of opinion that certain long and narrow lamellae, situated within the pulmonary sac, are employed for the purpose of aqueous respiration. Ancylus, which never makes periodic excursions to the surface, perhaps respires by receiving into its pulmonary chamber the minute quantities of oxygen given off by the vegetation on which it feeds.

Limnaeidae taken from a great depth of water, e.g. from 130 fathoms in the lake of Geneva, have been examined by Forel.[272] The pulmonary sac is full of water, but there is no transformation of organs, no appearance of a branchia, to meet the changed circumstances of their environment. Doubtless a good deal of respiration is done by the skin; being soft and vascular, it respires the air dissolved in the water. Forel cites cases of Limnaea living at much shallower depths, which come to the surface once, and then remain below for months. The oxygen of this supply must soon have become exhausted, and the animals, discontinuing for a time the use of the pulmonary chamber, must have respired through the skin. Shallow-water Limnaea, according to the same authority, remain beneath the surface during cold weather; when warm weather returns they rise to the surface to take in a supply of air. Since the water at great depths is always very cold, there is no need for the Limnaea living there to rise to the surface at all.

It is a curious fact that Limnaea, which have been respiring by the skin for the whole winter, should suddenly, on the first warm days of summer, take to rising to the surface and breathing air. But exactly the same phenomenon is shown in the case of Limnaea from great depths. Placed in an aquarium, they immediately begin rising to the surface and inspiring air; in other words, they experience instantaneously a complete transformation of their respiratory system.

In Onchidium, a land pulmonate which has retrogressed to an amphibious or quasi-marine mode of life, there is no organ which represents the pulmonary or branchial cavity, the so-called lung being only a cavity of the kidney. Respiration is, however, conducted by the skin as well, and by the dorsal papillae.[273]

Land Mollusca can sustain, for a considerable time, complete deprivation of atmospheric air. Helices placed in an exhausted receiver show no signs of being inconvenienced for about 20 hours, and are able to survive for about two or three days. If detained under water, they are very active for about 6 hours, then become motionless, the body swells, owing to the water absorbed, and death ensues in about 36 hours. Immersion for only 24 hours is generally followed by recovery. In the latter case, the cause of death is not so much deprivation of air as compulsory absorption of water by the skin. The amount of water thus taken up is surprising. Spallanzani found that a Helix which weighed 18 grammes increased in weight by 13½ grammes after a prolonged immersion. Even slugs enclosed in moist paper gained more than 2 grammes in the course of half an hour. Experiment has shown that the amount of carbonic acid gas produced by respiration stands in direct relation to the amount of food consumed. Four pairs of snails were taken which had recently awakened from their winter sleep and had eaten heartily, and an equal number, under the same circumstances, which had been prevented from eating. It was found that the first four pairs produced, in consuming a given amount of oxygen, 11, 9, 10, and 13 parts respectively of carbonic acid, while the second set produced, in consuming the same amount of oxygen, only 4, 8, 7, and 9 parts of carbonic acid.[274] Hibernating Helices, if weighed in December and again in April, will be found to have lost weight, due to the expiration of carbonic acid. Owing to the difficulty of experiment, opinions vary as to the absolute temperature of snails. It appears to be established that several snails, if placed together in a tube, raise the temperature one or two degrees C., but as a rule, the temperature of a solitary Helix differs very slightly from that of the surrounding air. Increased activity, whether in respiration or feeding, is found to raise the temperature.

Fig. 72.—Cardium edule L.: A, anal; B, branchial siphon; F, foot. (After Möbius.)

W. H. Dall, writing of the branchia in Pelecypoda, remarks[275] that there can be no doubt that its original form was a simple pinched-up lamella or fold of the skin or mantle. This, elongated, becomes a filament. Filaments united by suitable tissue, trussed, propped, and stayed by a chitinous skeleton, result in the forms, wonderful in number and complexity, which puzzle the student to describe, much more to classify.

Fig. 73.—Scrobicularia piperata Gmel., in its natural position in the sand: A, efferent or anal siphon; B, afferent or branchial siphon. (After Möbius.)

In Pelecypoda the branchiae are placed on each side of the body, between the mantle and the visceral mass. They lie in a chamber known as the branchial cavity. Leading into this cavity, and behind it, are, as a rule, two tubes or siphons, one of which conducts water to the branchiae, while the other carries it away after it has passed over them. The lower is known as the branchial or afferent siphon, the upper as the anal or efferent siphon (see Figs. 72 and 73). The action of these siphons can readily be observed by placing a little carmine in water, near to the siphonal apertures of an Anodonta or Unio. In many cases (e.g. Psammobia, Tellina, Mya, genera which burrow deeply in sand) both the siphons are exceedingly long, sometimes considerably longer than the whole shell. In some cases the two tubes are free throughout their entire length, in others they become fused together before their entrance within the shell (Fig. 74). In other genera, which do not burrow (e.g. Ostrea, Pecten, Arca, Mytilus), the siphons are rudimentary or altogether absent (Fig. 75).

Fig. 74.—Solecurtus strigillatus L., Naples: s.af, afferent siphon; s.ef, efferent siphon, the two uniting in SS externally to the shell, × ½.

Fig. 75.—Mytilus edulis L., attached by its byssus (By) to a piece of wood: F, foot; S, anal siphon, the branchial siphon being below it and not closed. (After Möbius.)

The number and arrangement of the branchiae varies considerably. It appears probable that the different degrees of complication of the gill indicate degrees of specialisation in the different groups of Pelecypoda, in other words, assuming that a simpler form of gill precedes, in point of development, a more complicated form, the nature of the gill may be taken as indicating different degrees of removal from the primitive form of bivalve.

1. The simplest form of gill (Nucula, Leda, Solenomya, etc.) is that which consists (Fig. 76, A, compare Fig. 100, p. 201) of two rows of very short, broad, not reflected filaments, the rows being placed in such a way that they incline at right angles to one another from a common longitudinal axis. The filaments are not connected with one another, nor are the two leaves of each gill united at any point. (Protobranchiata.)

Fig. 76.—Morphology of the branchiae of Pelecypoda, seen diagrammatically in section: A, Protobranchiata; B, Filibranchiata; C, Eulamellibranchiata; D, Septibranchiata; e, e, external row of filaments; i, i, internal row of filaments; e´, external row or plate folded back; i´, internal row folded back; f, foot; m, mantle; s, septum; v, visceral mass. (From A. Lang.)

Fig. 77.—Four gill filaments of Mytilus, highly magnified; cj, ciliary junctions; f, filament. (After Peck.)

2. In the Anomiidae, Arcadae, Trigoniidae, and Mytilidae each gill consists of two plates or rows of much longer filaments, which consequently occupy a much larger space in the mantle cavity (Fig. 76, B). Unable to extend beyond the limits of the mantle, filaments are reflected or doubled back upon one another, those of the external plate being reflected towards the outside, those of the internal plate towards the inside. Each separate filament is not connected with the filament next adjacent, except by surface cilia situated on small projections on the sides of the filaments, and interlocking with the cilia of the adjacent filament. The two superposed plates or leaves of the gill may or may not be united by cords running between the two parts of a filament. (Filibranchiata.)

3. In the Pectinidae, Aviculidae, and Ostreidae a further development takes place. The filaments of each gill are reflected in the same way as in the Filibranchiata, but the part thus reflected may become completely united or ‘concresce’ with the mantle on the exterior and with the base of the foot on the interior side. The leaves of each gill plate, which have thus become doubled (the gills being apparently two instead of one on each side), are folded or crumpled, and the filaments are modified at the re-entrant angles of the fold. (Pseudolamellibranchiata.)

4. In all the remaining Pelecypoda, except class 5, in other words, in the very large majority of families, the filaments are either reflected, as in (3), or simple; but the process of concrescence is so far advanced that the adjacent filaments are always intimately connected with one another in such a way as to admit the passage of the blood; and the leaves of each gill-plate (Fig. 76, C) are united by cross channels in a similar way. (Eulamellibranchiata.)

5. In certain of the Anatinacea alone (Cuspidaria, Lyonsiella, Poromya, Silenia) the gills are transformed into a more or less muscular partition, extending from one adductor muscle to the other (Fig. 76, D), and separating off the pallial chamber into two distinct divisions, which communicate by means of narrow slits in the partition. (Septibranchiata.)

Fig. 78.—Transverse section of portion of an outer gill plate of Anodonta, highly magnified: il, inner lamella; il´, outer lamella; ilj, interlamellar junctions; v, large vertical vessels. (After Peck.)

Thus the process of gill development in the Pelecypoda appears to lead up from a simple to a very complex type. In its original form, at all events in the most primitive form known to us, the gill is a series of short filaments, quite independent of one another, strung in two rows; then the filaments become longer and double back, while at the same time they begin to show signs of adhesion, as yet only superficial, to one another. In a further stage, the reflected portions become fused to the adjacent surfaces of the foot and mantle, while the interlamellar junctions serve to lock the two gill-plates together; finally, the mere ciliary junction of adjacent filaments is exchanged for intimate vascular connection, while the gill-plates as a whole become closely fused together in a similar manner.

This theory of origin is strengthened by closer observation of the phenomena of a single group. Taking the Septibranchiata as an instance, we find that in Lyonsiella the branchiae unite with the mantle in such a way as to form two large pallial chambers, the structure of the branchiae being preserved, and their lamellae covering the partition. A further stage is observed in Poromya. There, a similar partition exists, but it has become muscular, preserving, however, on each side two groups of branchial lamellae, separated one from the other by a series of slits, which form a communication between the two pallial chambers. A further stage still is seen in Silenia. There the same muscular partition exists, but the branchial lamellae on either side have disappeared, the slits between the two chambers, which occur in Poromya, still persisting, but separated into three groups. Cuspidaria represents the last stage in the development. In the ventral chamber there appears nothing at all corresponding to a branchia; the surface of the partition appears perfectly uniform, but on careful examination three little separate orifices, remains of the three groups of orifices in Silenia, are observed.[276]

Relation between Branchiae and Heart.—The object of the branchiae being, as has been already stated, to aerate the blood on its way to the heart, we find that the heart and the branchiae stand in very important structural relations to one another. When the branchiae are in pairs, we find that the auricles of the heart are also paired, the auricle on the right and left sides being supplied by the right and left branchiae respectively. This is the case with the Dibranchiate Cephalopods (Argonauta, Octopus, Loligo, etc.), the Zygobranchiate Prosobranchs (Fissurella, Haliotis), and all Pelecypoda. In the Amphineura (Chiton, etc.) there are two auricles corresponding to the two sets of multiple branchiae. In the case of the Tetrabranchiate Cephalopods (Nautilus) there are four auricles corresponding to each of the four branchiae. Compare Fig. 79, A, B, C, D, E.

On the other hand, when the branchia is single, or when both branchiae are on the same side, and one is aborted and functionless, the auricle is single too, and on the same side as the branchia. This is the case with the Tectibranchiate Opisthobranchs (Philine, Scaphander, etc.), all the Pectinibranchiate Prosobranchs (Rachiglossa, Taenioglossa, and Ptenoglossa), and the other Azygobranchiate Prosobranchs (Trochidae, Neritidae, etc.). In the last case the right auricle exists, as well as the left, but is simply a closed sac, the coalescing of the two gills on the left side having thrown all the work upon the left auricle. Compare Fig. 79, F, G, H.

Fig. 79.—Diagram illustrating the relations between branchiae, heart, and aorta in the Mollusca: A, In Chiton; B, Pelecypoda; C, Dibranchiate Cephalopoda; D, Tetrabranchiate Cephalopoda; E, Prosobranchiata Zygobranchiata; F, Prosobranchiata Azygobranchiata; G, Prosobranchiata Monotocardia; H, Opisthobranchiata Tectibranchiata: 1, Ventricle; 2, Auricle; 3, Aorta; 3a, Cephalic aorta; 3b, Visceral aorta; 3c, Posterior aorta. (From A. Lang.)

All Mollusca, without exception, possess a circulatory system of more or less complexity. The centre of the system is the heart, which receives the aerated blood from the breathing organs, and propels it to every part of the body. In the Scaphopoda alone there appears to be no distinct heart.

The heart may consist simply of a single auricle and ventricle, and an aorta opening out of the ventricle. From the aorta the blood is conveyed to the various parts of the body by arteries. Veins convey the blood back to the breathing organs, after passing over which it returns by the branchial or pulmonary vein to the heart, thus completing the circuit.

As regards position, the heart is situated within the pericardium, a separate chamber which in the Pelecypoda, Cephalopoda, and the bilaterally symmetrical Gasteropoda lies on the median line, while in the asymmetrical Gasteropoda it is on one or other of the sides of the body, usually the right. The veins connected with the branchiae, and consequently the auricle into which they open, are situated behind the ventricle in the Opisthobranchiata (whence their name), while in the Prosobranchiata they are situated in front of the ventricle.

The number of auricles corresponds to the number of branchiae. Thus there is only one auricle in the great majority of Prosobranchiata (which are accordingly classified as Monotocardia), and also in the Opisthobranchiata, while the Pulmonata have a single auricle corresponding to the pulmonary chamber. There are two auricles in the Amphineura, in a small group of Gasteropoda, hence known as Diotocardia, in all Pelecypoda, and in the Dibranchiate Cephalopoda. In the Tetrabranchiate Cephalopoda alone there are four auricles corresponding to the four branchiae.

A single aorta occurs only in the Amphineura and in the Tetrabranchiate Cephalopoda. In all the other groups there are two aortae, leading out of the anterior and posterior ends of the ventricle in Pelecypoda and Dibranchiate Cephalopoda, while a single aorta leads out of the posterior end alone, and subsequently bifurcates, in most of the Gasteropoda. One aorta, the cephalic, supplies the front part of the body, the oesophagus, stomach, mantle, etc.; the other, the visceral aorta, supplies the posterior part, the liver and sexual organs.

The general circulatory system in the Mollusca has not yet been thoroughly investigated. As a general rule, the blood driven from the ventricle through the aorta into the arteries, passes, on reaching the alimentary canal and other adjacent organs, into a number of irregular spaces called lacunae. These in their turn branch into sinuses, or narrow tubes covered with muscular tissue, which penetrate the body in every direction. In the Dibranchiate Cephalopoda true capillaries are said to occur, which in some cases form a direct communication between the arteries and veins. According to some authorities[277] capillaries and veins exist in certain Pelecypoda in connexion with the intestinal lacunae, but this again is regarded by others as not established. A similar difference of opinion occurs with regard to the precise function of the foot-pore which occurs in many Mollusca, some holding that it serves as a means for the introduction of water into the blood-vascular system, while others regard it as a form of secretion gland, the original purpose of which has perhaps become lost.

Blood.—As a rule, the blood of the Mollusca—i.e. not the corpuscles but the liquor sanguinis—is colourless, or slightly tinged with blue on exposure to the air. This is due to the presence of a pigment termed haemocyanin, in which are found traces of copper and iron, the former predominating. Haemoglobin, the colouring matter of the blood in Vertebrates, is, according to Lankester,[278] of very restricted occurrence. It is found—(1) in special corpuscles in the blood of Solen legumen (and Arca Noae); (2) in the general blood system of Planorbis; (3) in the muscles of the pharynx and jaws of certain Gasteropoda, e.g. Limnaea, Paludina, Littorina, Chiton, Aplysia. This distribution of haemoglobin is explained by Lankester in reference to its chemical activity; whenever increased facilities for oxidisation are required, then it may be present to do the work. The Mollusca, being as a rule otiose, do not possess it generally diffused in the blood, as do the Vertebrata. The actively burrowing Solen possesses it, and perhaps its presence in Planorbis is to be explained from its respiring the air of stagnant marshes. Its occurrence in the pharyngeal muscles and jaws of other genera may be due to the constant state of activity in which these organs are kept.[279]

According to Tenison-Woods[280] a species of Arca (trapezia Desh.) and two species of Solen, all Australian, have red blood. It is suggested that in these cases the habits of the animal (the Solen burrowing deeply in sand, the Arca in mud) require some highly oxidising element, surrounded as the creature is by ooze. In Arca pexata (N. America) the blood is red, the animal being familiarly known as the ‘bloody clam.’ Burrowing species, however, are not all distinguished by this peculiarity. Tenison-Woods finds red fluids in the buccal mass of many Gasteropoda, e.g. in species of Patella, Acmaea, Littorina, Trochus, Turbo, giving the parts the appearance of raw meat.

On the dorsal side of the typical molluscan body, between the visceral sac and the shell, lies a duplicature of the integument, generally known as the mantle. The depending sides of the mantle, which are usually somewhat thickened, enclose between themselves and the body mass a chamber of varying size and shape, called the mantle cavity, which communicates freely with the external air or water, and encloses and furnishes a protection for the organ or organs of respiration. On its upper or dorsal surface the mantle is closely applied to the shell throughout its whole extent, the cells with which it is furnished secreting the materials from which the shell is formed (see p. 255). The whole mantle is capable, to some degree, of secreting shelly matter, but the most active agent in its production is the mantle edge or margin.

In the Prosobranchiata the mantle cavity, for reasons which have already been explained, is found on the left side of the animal, its front portion being in many cases produced into a tubular siphon. Within the mantle cavity are found, besides the branchia, the anus, the apertures of the kidneys, and the osphradium. In the pulmonata the mantle fold encloses a so-called lung-cavity. The front edge of the mantle coalesces with the integument of the neck in such a way as to enclose the cavity very completely, the only communication with the outer air being by means of the contractile breathing or pulmonary aperture on the right side. In the Tectibranchiate Opisthobranchs the mantle fold is inconsiderable, and is usually not of sufficient extent to cover the branchia, while in the Nudibranchs, which have no true branchiae, it disappears altogether.

In the Pelecypoda the mantle cavity is equally developed on each side, enclosing the two sets of branchiae. The mantle may thus be regarded as consisting of two equal portions, which form a sort of lining to the two valves. The lower or ventral portion of the mantle edges may be simple, or provided with ocelli (Pecten, Arca), tentacles, cilia (Lima, Lepton), or doubled folds. The two portions of the mantle touch one another along the whole line of the edge of the two valves, and, although thus in contact, may remain completely separate from one another, or else become permanently united at one or more points. This fusion of the mantle edges corresponds to important changes in the organisation of the animal as a whole. The anal and branchial siphons are no more than prolongations of the mantle edges on the posterior side into a tubular form. These ‘siphons’ exhibit the siphonal form more distinctly according as the adjacent portions of the mantle become more definitely fused together.

Fig. 80.—Diagram illustrating the various stages in the closing of the mantle in Pelecypoda: A, mantle completely open; B, rudiments of siphons, mantle still completely open; C, mantle closed at one point; D, mantle closed at two points, with complete formation of siphonal apertures; E, development of siphons, ventral closure more extended; F, mantle closed at three points, with fourth orifice: f, foot; s.a, s.b, anal and branchial siphons; 1, 2, 3, first, second, and third points of closure of mantle. (After A. Lang.)

This progressive fusion of the mantle edges may be taken as indicating definite stages in the development of the Pelecypoda. A perfectly free mantle edge, joined at no point with the edge of the adjacent mantle, occurs in Nucula, Arca, Anomia, and Trigonia (see Fig. 80, A, B). Here there is nothing in the nature of a siphon, either anal or branchial; in other words, no contrivance exists to prevent the spent water which has passed over the branchiae from becoming mixed with the fresh water which is to reach them. When the mantle edges are fused at one point only, this is invariably on the middle part of the posterior side, thus separating off an anal opening which may become prolonged into a tube-like form. At the same time the adjacent underlying portions of the mantle edges draw together, without actually coalescing, to form an opening for the incurrent stream of water, the rudiments of the ‘branchial siphon’ (Fig. 80, C). This is the case with most Mytilidae (see Fig. 75) with Cardita, Astarte, and Pisidium. In the next stage the branchial opening is separated off by the concrescence of the mantle edges beneath it, and we have the mantle united in two places, thus forming three openings, the ventral of which is the opening for the protrusion of the foot (Fig. 80, D). This is the case in Yoldia, Leda, the majority of the Eulamellibranchiata (e.g. Lucina, Cyrena, Donax, Psammobia, Tellina, Venus, Cardium, Mactra), and all Septibranchiata. In Chama and Tridacna the fused portions of the mantle become more extended, and in Pholas, Xylophaga, Teredo, Pandora, and Lyonsia this concrescence takes place over the greater length of the whole mantle edge, so that the mantle may be regarded as closed, with the exception of the three apertures for the foot and the two siphons (Fig. 80, E).

In certain genera there occurs, besides these three apertures, a fourth, in the line of junction between the pedal and branchial orifices. It appears probable that this fourth orifice (which has been regarded by some as an inlet for water when the siphons are retracted), stands in relation to the byssal apparatus (Fig. 80, F). In Lyonsia, for instance, a thick byssus protrudes through the orifice, which is large and open. In Solen, Lutraria, Glycimeris, Cochlodesma, Thracia, Aspergillum, and a few more genera, which have no byssus, the orifice is very small and narrow. It is possible that in these latter cases, the byssal apparatus having become atrophied, the orifice has been correspondingly reduced in size.[281]

Mantle Reflected over the Shell.—It is sometimes the case that the mantle edges tend to double back over the external surface of the shell, and to enclose it to a greater or less extent. When this process is carried to an extreme, the edges of the reflected mantle unite, and the shell becomes completely internal. We see an incipient stage of this process in Cypraea and Marginella, where the bright polish on the surface of the shell is due to the protection afforded by the lobes of the mantle. A considerable portion of the shell of Scutus is concealed in a similar way, while in Cryptochiton, Lamellaria, and Aplysia the shell is more or less completely enclosed. Among Pulmonata, it is possible that in forms like Vitrina, Parmacella, Limax, and Arion, we have successive stages in a process which starts with a shell completely external, as in Helix, and ends, not merely by enveloping the shell in the mantle, but by effecting its disappearance altogether. In Vitrina and some allied genera we have a type in which the mantle lobes are partly reflected over the shell, which at the same time exhibits rather less of a spiral form than in Helix. In the stage represented by Parmacella, the mantle edges have coalesced over the whole of the shell, except for a small aperture immediately over the spire; the nucleus alone of the shell is spiral, the rest considerably flattened. In Limax the shell has become completely internal, and is simply a flat and very thin plate, the spiral form being entirely lost, and the nucleus represented by a simple thickening at one end of the plate. In Arion, the final stage, we find that the shell, being no longer needed as a protection to the vital organs, has either become resolved into a number of independent granules, or else has entirely disappeared.

Some indications of a similar series of changes occur in the Pelecypoda. The mantle edge of Lepton is prolonged beyond the area of the valves, terminating in some cases in a number of filaments. In Galeomma and Scintilla the valves are partially concealed by the reflected mantle lobes, and in a remarkable form recently discovered by Dall[282] (Chlamydoconcha) the shell is completely imbedded in the mantle, which is perforated at the anterior end by an orifice for the mouth, and at the posterior end by a similar orifice for the anus. In all these cases, except Lepton, it is interesting to notice that the hinge teeth have completely disappeared, the additional closing power gained by the external mantle rendering the work done by a hinge unnecessary. It is quite possible, on the analogy of the Gasteropoda mentioned above, and also, it may be added, of the Cephalopoda and other groups, that we have here indicated the eventual occurrence of a type of Pelecypoda altogether deprived of valves, a greatly thickened mantle performing the part of a shell.[283]

The following works will be found useful for further study of this portion of the subject:—

CHAPTER VII ORGANS OF SENSE: TOUCH, SIGHT, SMELL, HEARING—THE FOOT—THE NERVOUS SYSTEM — Organs of Sense : I. Touch — II. Sight — III. Smell — IV. Hearing — The Foot — Nervous System

Tactile organs, although occurring in some of the Mollusca, do not appear to attain special or marked development, except in a few cases. The whole surface of the skin, and particularly of the foot, is very sensitive to the slightest impression. Nearly all Gasteropoda are furnished with at least two cephalic tentacles, projecting like horns from each side of the fore part of the head. At or near the base of these are generally situated the eyes. In the Helicidae the eyes are situated, not at the base, but at the apex of the tentacles, and in that case—except in Vertigo—a second pair of shorter tentacles appears beneath the longer pair. It frequently happens that several senses are centred in a single organ; thus the upper tentacles of snails not only carry the eyes and serve to a certain extent as tactile organs, but they also carry the organs of smell.

The edges of the mantle, which are sometimes specialised into lobes, appear to be keenly sensitive to touch in all Gasteropoda.

In Cypraea (Fig. 81) these lobes, or tentaculae, are a prominent feature of the animal, and also in certain genera of the Trochidae (Fig. 82). In most of the carnivorous land Pulmonata—e.g. Testacella, Rhytida, Ennea—there are developed, under the lower pair of tentacles, and close to the mouth, large labial palps or feelers. These are connected with the cerebral ganglion by a very large nerve, and may therefore be supposed to be of extreme sensitiveness. In some of the large carnivorous forms (Glandina, Aerope, compare Fig. 21, p. 54) these palpae are of great size, and curl upwards like an enormous pair of moustaches. When a Glandina seizes its prey, the palpae (see Fig. 83) appear to enfold it and draw it in towards the mouth.

Fig. 81.—Cypraea moneta L., showing tentaculae at edge of mantle, which partly envelops the shell: Si, siphon; M, M, mantle; F, foot; T´, tentaculae at edge of mantle. (After Quoy and Gaimard.) × ³/₂.

Fig. 82.—Monodonta canalifera Lam., New Ireland, showing mantle lobes. (After Quoy and Gaimard.)

Fig. 83.—Glandina seizing its prey, with buccal papillae turned back. (Strebel.)

It is in the Opisthobranchiata that the organs of touch attain their maximum development. Many of this group are shell-less or possess a small internal shell, and accordingly, in the absence of this special form of defence, a multiplied sense of touch is probably of great service. Thus we find, besides the ordinary cephalic tentacles, clusters or crowns of the same above the head of many Nudibranchiata, with lobe-like prolongations of the integument, and tentacular processes in the neighbourhood of, or surrounding the branchiae (see Figs. 58 and Fig. 84, or even projecting from the whole upper surface of the body (Fig. 5, C).

In the Pelecypoda, the chief organs of touch are the foot, which is always remarkably sensitive, especially towards its point, the labial palps on each side of the mouth, and the siphons. In certain cases the mantle border is prolonged into a series of threads or filaments. These are particularly noticeable in Pecten, Lepton, and Lima (Fig. 85), the mantle lobes of the common L. hians of our own coasts being very numerous, and of a bright orange colour. In many genera—e.g. Unio, Mactra—this sensibility to touch appears to be shared by the whole mantle border, although it is not furnished with any special fringing. The ‘arms’ of the Cephalopoda appear to be keenly sensitive to touch, and this is particularly the case with the front or tentacular pair of arms, which seem to be employed in an especial degree for exploration and investigation of strange objects.

Fig. 84.—Idalia Leachii A. and H., British seas; br, branchiae. (After Alder and Hancock.)

Fig. 85.—Lima squamosa Lam., Naples, showing tentacular lobes of mantle (t, t); a, anus; ad.m, adductor muscle; br, br, branchiae; f, foot; sh, shell.

Taste.—The sense of taste is no doubt present, to a greater or less extent, in all the head-bearing Mollusca. In many of these a special nerve or nerves has been discovered in the pharynx, connecting with the cerebral ganglion; this no doubt indicates the seat of the faculty of taste. The Mollusca vary greatly in their likings for different kinds of food. Some seem to prefer decaying and highly odoriferous animal matter (Buccinum, Nassa), others apparently confine themselves to fresh meat (Purpura, Natica, Testacella), others again, although naturally vegetarian, will not refuse flesh on occasion (Limax, Helix).

Mr. W. A. Gain[284] has made some interesting experiments on the taste of British land Mollusca, as evidenced by the acceptance or rejection of various kinds of food. He kept twelve species of Arion and Limax, and eight species of Helix in captivity for many months, and tried them with no less than 197 different kinds of food, cannibalism included. Some curious points came out in his table of results. Amalia gagates appears to be surprisingly omnivorous, for out of 197 kinds of food it ate all but 25; Arion ater came next, eating all but 40. Limax arborum, on the other hand, was dainty to a fault, eating only seven kinds of food, and actually refusing Swedes, which every other species took with some avidity. Certain food was rejected by all alike, e.g. London Pride, Dog Rose, Beech and Chestnut leaves, Spruce Fir, Common Rush, Liverwort, and Lichens; while all, or nearly all, ate greedily of Potatoes, Turnips, Swedes, Lettuces, Leeks, Strawberries, Boletus edulis, and common grasses. Few of our common weeds or hedgerow flowers were altogether rejected. Arion and Limax were decidedly less particular in their food than Helix, nearly all of them eating earthworms and puff-balls, which no Helix would touch. Arion ater and Limax maximus ate the slime off one another, and portions of skin. Cyclostoma elegans and Hyalinia nitida preferred moist dead leaves to anything else.

Position of Eyes.—In the majority of the head-bearing Mollusca the eyes are two in number, and are placed on, or in the immediate neighbourhood of the head. Sometimes they are carried on projecting tentacles or ‘ommatophores,’ which are either simple (as in Prosobranchiata) or capable of retraction like the fingers of a glove (Helix, etc.). Sometimes, as in a large number of the marine Gasteropoda, the eyes are at the outer base of the cephalic tentacles, or are mounted on the tentacles themselves, but never at the tip (compare Fig. 60, p. 153 and Fig. 98, p. 199). In other cases they are placed somewhat farther back, at the sides of the neck. The Pulmonata are usually subdivided into two great groups, Stylommatophora and Basommatophora (Fig. 86), according as the eyes are carried on the tip of the large tentacles (Helix, and all non-operculate land shells), or placed at the inner side of their base (Limnaea, Physa, etc.). In land and fresh-water operculates, the eyes are situated at the outer base of the tentacles.

Fig. 86.—A, Limnaea peregra Müll.; e, e, eyes; t, t, tentacles; B, Helix nemoralis Müll.; e, e, eyes; t, t, tentacles; p.o, pulmonary orifice.

In the Helicidae, careful observation will show that the eyes are not placed exactly in the centre of the end of the tentacle, but on its upper side, inclining slightly outwards. The eye is probably pushed on one side, as it were, by the development of the neighbouring olfactory bulb. The sense of smell being far more important to these animals than the sense of sight, the former sense develops at the expense of the latter.

Organisation of the Molluscan Eye.—The eye in Mollusca exhibits almost every imaginable form, from the extremely simple to the elaborately complex. It may be, as in certain bivalves, no more than a pigmented spot on the mantle, or it may consist, as in some of the Cephalopoda, of a cornea, a sclerotic, a choroid, an iris, a lens, an aqueous and vitreous humour, a retina, and an optic nerve, or of some of these parts only.

In most land and fresh-water Mollusca the eye may be regarded, roughly speaking, as a ball connected by an exceedingly fine thread (the optic nerve) with a nerve centre (the cerebral ganglion). In Paludina this ball is elliptic, in Planorbis and Neritina it is drawn out at the back into a conical or pear shape. In Helix (Fig. 87) there is a structureless membrane, surrounding the whole eye, a lens, and a retina, the latter consisting of a nervous layer, a cellular layer, and a layer of rods containing pigment, this innermost layer (that nearest the lens) being of the thickness of half the whole retina.

Fig. 87.—Eye of Helix pomatia L., retracted within the tentacle; c, cornea; ep, epithelial layer; l, lens; op.n, optic nerve; r, retina. (After Simroth.)

Comparing the eyes of different Gasteropoda together, we find that they represent stages in a general course of development. Thus in Patella the eye is scarcely more than an invagination or depression in the integument, which is lined with pigmented and retinal cells. The next upward stage occurs in Trochus, where the depression becomes deeper and bladder-shaped, and is filled with a gelatinous or crystalline mass, but still is open at the top, and therefore permits the eye to be bathed in water. Then, as in Turbo, the bladder becomes closed by a thin epithelial layer, which finally, as in some Murex, becomes much thicker, while the ‘eyeball’ encloses a lens (Fig. 88), which probably corresponds with the ‘vitreous humour’ of other types.

Fig. 88.—Eyes of Gasteropoda, showing arrest of development at successive stages: A, Patella; B, Trochus; C, Turbo; D, Murex; ep, epidermis; l, lens; op.n, optic nerve; r, retina; v.h, vitreous humour. (After Hilger.)

In Nautilus the eye is of a very simple type. It consists of a cup-shaped depression, with a small opening which is not quite closed by the integument. The retina consists of cells which line the interior of the depression, and which communicate directly with the branches of the optic nerve, there being no iris or lens. This type of eye, it will be observed, corresponds exactly with that which occurs in Patella. It appears also to correspond to a stage in the development of eyes in the Dibranchiata (e.g. Octopus, Sepia, Loligo). Lankester has shown[285] that in Loligo the eye first appears as a ridge, enclosing an oval area in the integument. By degrees the walls of this area close in, and eventually join, enclosing the retinal cells within the chamber in which the lens is afterwards developed (Fig. 89). It thus appears that in some cases the development of the eye is arrested at a point which in other cases only forms a temporary stage towards a higher type of organisation.

Fig. 89.—Three stages in the development of the eye of Loligo; r, r, ridge, enclosing p.o.c, primitive optic chamber; or, orifice between the closing ridges; s.o.c, secondary optic chamber; ci, ci, ciliary body; l, rudimentary lens; R, retina. (After Lankester.)

Fig. 90.—Eye in A, Loligo; B, Helix or Limax; C, Nautilus: a.o.c, anterior optic chamber; c, cornea; int, integument; ir, iris; l, lens; l´, external portion of lens; op.n, optic nerve; op.g, optic ganglion; p.o.c, posterior optic chamber; r, retina. (After Grenacher.)

The developed eye in the dibranchiate Cephalopods consists of a transparent cornea, which may or may not be closed over the front of the lens. Behind the cornea is a narrow chamber (the anterior optic chamber) which is continued for three parts round the whole circle of the eye, and into which project the front portion of the lens and the folds of the iris. Throughout its whole extent, the anterior optic chamber is lined by the integument, the portion of which on the inner side is the choroid. The lens is divided into an outer and inner segment by a thin membrane, and is supported by the ciliary body which forms a continuation of the retina. The main portion of the lens lies within the posterior optic chamber, at the back and sides of which is found the retina (Grenacher).

There can be no doubt that the Cephalopoda use their eyes to observe, but there is nothing to show that any other Mollusca use their eyes for this purpose, the sense of smell in their case largely taking the place of visual observation. Madame Jeannette Power once saw[286] the Octopus in her aquarium holding a fragment of rock in one of its arms, and watching a Pinna which was opening its valves. As soon as they were perfectly open, the Poulpe, with incredible address and promptitude, placed the stone between the valves, preventing the Pinna from closing again, upon which it set about devouring its victim. The next day the Poulpe was seen, after crushing some Tellina, to stretch himself down close by a Triton nodiferus, and watch it attentively. After four hours the Triton emerged from its shell, when the Octopus sprang upon it, and surrounded it with its arms.

Powers of Vision in Land Mollusca.—The Helicidae are undoubtedly very short-sighted. Seldom emerging from their retreats except in twilight and darkness, they are naturally myopic, and see better in a subdued than in a bright light. Experiment has shown that a Helix can perceive an object better at 6 centimetres distance in a weak light than at 4 or 5 millimetres in a strong one. Cyclostoma elegans and Paludina vivipara are comparatively long-sighted, perceiving objects at a distance of 20 to 30 centimetres.[287] The increased power of vision is due, in these two cases, to increased elaboration in the construction of the eye, Paludina possessing a large and almost spherical lens, to which the vitreous humour closely adheres, while in Cyclostoma the lens is remarkably hard, and the aqueous humour very abundant. According to V. Willem,[288] the Pulmonata are very sensitive to the slightest movement of the air or jarring of the surface on which they crawl, but are so short-sighted as only to perceive a confused image of a large object at about 1 cm., and to distinguish the form of objects at not more than 1 or 2 mm. The senses of touch and smell are far more active than that of sight. A bean-pod enclosed in a narrow glass case and placed before a hungry snail was not noticed, but when taken out of the case and placed 8 cm. behind the snail, the latter at once turned towards it to devour it.

Some interesting experiments were conducted by the same author with the view of ascertaining whether snails avoid or court the light. He placed a number of species in different wooden boxes, which were divided into a light and a dark compartment, having previously well soaked the boxes in water to secure a humid atmosphere and surface, and so induce the snails to move about. The result showed that nearly all species have a marked predilection one way or the other, but not all in the same way. Helix aspersa, Arion empiricorum, six species of Limax, and three of Planorbis, are lovers of darkness, while H. nemoralis, Succinea putris, and two species of Limnaea are lovers of light. Physa fontinalis stands alone in being quite indifferent.

M. Willem endeavoured further to discover whether any of the Mollusca possessed ‘dermatoptic perception,’ or the faculty of perceiving variation of light by means of the skin alone. He accordingly repeated the above-mentioned experiments, having previously extirpated the eyes in all cases. The result was remarkable. In a few instances the experiment was not conclusive, but H. aspersa, A. empiricorum, several species of Limax, and one Limnaea shunned or sought the light just as they had done when their eyes were present. A few marine Mollusca (Littorina littorea, Trochus cinerarius, T. umbilicatus, Patella vulgata) were also shown to be exceedingly sensitive to the impact of a shadow, whether with or without their eyes.

Blind and Eyeless Mollusca.—In a large number of marine Mollusca which habitually creep about half buried in wet sand (Bullia, Sigaretus, Scaphander, Philine), eyes are altogether absent. In some species of Natica and Sigaretus, and in Doris, eyes are developed, but are enclosed in a thick layer of skin, through which they can probably do little more than faintly appreciate different degrees of light and darkness. Chiton has cephalic eyes in the embryo, but loses them in the adult stage. The two great Auricula, A. auris Judae and A. auris Midae, which habitually creep about in the liquid mud of mangrove swamps, have entirely lost their eyes. Certain pelagic Mollusca seem to have a tendency, which is not easily explained, to lose their eyes or the power of seeing with them. Thus Ianthina has no eyes at all. Pteropoda as a rule have no eyes, and the few that have (Creseis, Cavolinia) possess only certain pigmented spots placed near to the nervous centres. In the Heteropoda, however, and the Cephalopoda, many of which are pelagic, the eyes are unusually large.

Fig. 91.—Sigaretus laevigatus Lam., a species frequenting wet sand, and destitute of external eyes: F, anterior portion of foot. (After Souleyet.)

Eyes in Deep-sea and Underground Mollusca.—Deep-sea Mollusca, as a rule, possess no visual organs, or possess them only in a rudimentary state, but this rule has its exceptions. Dr. Pelseneer found[289] no trace of eyes in two species of Pleurotoma from 1850 and 1950 fath., none in a Fossarus from 1400 f., none in a Puncturella from 1340 f. A remarkable form of Voluta (Guivillea) from 1600 f. possessed eyes which could hardly be functional, as they were destitute of pigment, and exhibited other changes of structure. On the other hand, it is remarkable to notice that in three different species of Trochus from 450 f., 565 f., and 1375 f., the eyes were pigmented and well developed.

In land Mollusca which live beneath the surface of the ground or in absolute darkness, the eyes are generally more or less modified. Thus in Testacella, which usually burrows deeply in the soil, but occasionally emerges into the open air, the eyes are very small, but distinct and pigmented. Our little Caecilianella acicula, which is never seen above the surface, is altogether destitute of eyes. A species of Zospeum, a Helix, and a Bithynella from dark caves in Carniola have suffered a similar loss. On the other hand, a small Hyalinia from a dark cave in Utah (probably a recent addition to the cave fauna) has the eyes normally developed.

Eyes of Onchidium.—Many species of Onchidium, a naked land pulmonate which creeps on rocks near high-water mark, are provided with dorsal eyes of various degrees of organisation, and in numbers varying up to nearly one hundred. The tropical Onchidium are the prey of a fish (Periophthalmus) which skips along the beach by the aid of its large ventral fins, and feeds principally on insects and Onchidium. Karl Semper suggests[290] that the eyes are of service to Onchidium as enabling it to apprehend the shadow of the approaching Periophthalmus, and defend itself by suddenly contracting certain glands on the skin and expressing a liquid secretion which flies into the air like shot and frightens the Periophthalmus away. This theory for it is no more than theory—may or may not be true, but it is remarkable that Onchidium with dorsal eyes have precisely the same geographical distribution as Periophthalmus, and that where no Periophthalmus exists, e.g. on our own S.W. coasts, the Onchidium are entirely destitute of dorsal eyes. In those species of Onchidium which have no dorsal eyes, the latter are on the tips of the tentacles, as in Helix. The eyes are developed on the head, and afterwards ascend with the growth of the ommatophores, while in Helix the ommatophores are formed first, and the eyes developed upon them.[291]

Dorsal Eyes in the Chitonidae.—The remarkable discoveries of Moseley with regard to the dorsal eyes of Chiton were first published in 1884.[292] He happened to notice, while examining a specimen of Schizochiton incisus, a number of minute black dots on the outer surface of the shell, which appeared to refract light as if composed of glass or crystal. These ‘eyes,’ in all the species of Chiton yet examined, are restricted to the outer surface of the exposed area of the shell, never being on the laminae of insertion or on the girdle. In certain sub-genera of Chiton the eyes are scattered irregularly over the surface, in others they are arranged symmetrically in rows diverging from the apex of each plate, but in old specimens the eyes towards the apices are generally rubbed off by erosion or abrasion. Moseley regarded the occurrence of scattered eyes as indicating an original stage of development, when the eyes were at first disposed irregularly all over the surface of the shell; the gathering into regular rows showing a later stage.

Fig. 92.—Dorsal eyes of Chitonidae, showing the various forms of arrangement in the first and fourth valves of 1, 1a, Acanthopleura spinigera Sowb., E. Indies, × 2; 2, 2a, Tonicia suezensis Reeve, Suez, × 3; 3, 3a, Acanthopleura granulata Gmel., W. Indies, × 2; 4, 4a, Tonicia lineolata Fremb., Chili, × 2. From specimens in the Museum of Zoology, Cambridge.

The eyes appear to be invariably more numerous on the anterior plate. Thus in Corephium aculeatum there are about 12,000 in all, of which more than 3000 are on the anterior plate. In Schizochiton they are arranged in very symmetrical rows, six of which are situated on the anterior, and only two, sometimes only one, on the central plates. In Tonicia marmorata the eyes are sunk in little cup-shaped depressions of the shell, possibly to escape abrasion. As regards shape and size, in Ch. incisus they are circular, and about ¹/₃₅ inch in diameter, this being the largest size known; in Ch. spiniger and Ch. aculeatus they are oval, measuring about ¹/₁₀₀ x ¹/₆₀₀ inch. There are no eyes in Chiton proper, nor in Mopalia, Maugeria, Lorica, and Ischnochiton.[293] None of our English species appear to possess them.[294]

Eyes in Bivalve Mollusca.—Some, possibly most, of the Pelecypoda possess, in the larval state, true paired eyes at the oral end of the body. These become aborted as the animal develops, since that part of the body becomes entirely screened from the light by the growth of the shell. To compensate for their loss, numerous ocelli, or pigmented spots sensitive to the action of light, are in many cases developed on different parts of the mantle, functionally corresponding to the ‘eyes’ of Chiton described above. As in Chiton, too, we have here an interesting series of instances in which true eyes have suffered total obliteration, through disuse, and, as if to restore to the animal in some measure its lost sense, visual organs of a low power have subsequently been developed and are now observed in various stages of specialisation.

Concentration of Eyes in Special Parts of the Mantle.—Sharp has shown[295] that in several species of Ostrea, Cardium, Anomia, Lima, Avicula, Arca, and Tellina pigmented cells, with a highly refractive cuticle, are scattered over a considerable portion of the mantle. Experiment has proved the powers of ‘vision,’ i.e. of sensitiveness to different degrees of light, possessed by these organs. In Dreissena polymorpha, Tapes decussatus, and two species of Venus these cells are concentrated on that particular part of the mantle which is not always covered by the shell, i.e. the siphon, but since the siphon can be completely retracted within the shell, there is no special provision for their protection. A further step is shown in the case of Mya arenaria, where the siphon is scarcely capable of complete retraction. Here, while some of the pigment cells are scattered about over the surface of the siphon, the majority are placed in grooves at the base of the siphonal tentacles, forming an intensely black band round them. A higher stage still is shown in Solen vagina, S. ensis, and Mactra solidissima, where the cells are situated only in the siphonal grooves, which are more or less specialised in numbers and complexity.

Arca Noae, according to Patten, is very sensitive to any sudden change in the amount of light falling upon its mantle-edge. A faint shadow cast upon it by the hand is sufficient to cause it to close its valves quickly, but always one or two seconds afterwards, the promptitude in all cases depending upon the depth of the shadow. Sensitiveness in this direction was found to depend greatly upon the vitality of the animals themselves, since it always became less in those specimens which had been kept for long in confinement. A shadow was not always necessary to make them close. An ordinary black pencil, if approached within two or three inches with extreme caution, produced the same result, while a glass rod brought within the same distance, and even moved rapidly to and fro, appeared to cause no alarm. Sensitiveness to change in intensity of light was experimentally noticed by the same author in the case of Ostrea, Mactra, Avicula (to a special extent), and Cardium. It is very remarkable to find that increased elaboration in the structure of the eyes does not necessarily carry with it increased sensitiveness, i.e. higher visual powers. Avicula, which is only provided with a few scattered ommatidia, which would entirely escape the notice of any one who had not seen them better developed elsewhere, was considerably more sensitive to light and shade than Arca, with its eyes of conspicuous size and much more perfect organisation, instantly contracting the mantle upon the impact of a shadow so faint as to be invisible to the experimenter.[296]

Visual Faculties of Solen and Ostrea.—The visual power of Solen may be exemplified by any one who is walking along almost any of our sandy bays at extreme low-water mark. If the day be warm and sunny, numbers of Solen will be seen raising themselves an inch or two out of their holes; but if you wish to catch them you must approach very cautiously, and on no account allow your shadow to fall upon them, or they will pop down into their burrows in an instant, and it is vain to attempt to dig them out. ‘How sensitive,’ remarks Mr. W. Anderson Smith, with reference to oysters,[297] ‘the creatures are to the light above them; the shadow [of the boat] as it passes overhead is instantaneously noted, and, snap! the lips are firmly closed.’

Ocelli of Pecten.—In Pecten and Spondylus the ocelli are remarkably large and prominent, shining like precious stones, and are placed along the two edges of the mantle so as to receive the light when the shell gapes (Fig. 93). In Pecten opercularis, jacobaeus, and maximus their number varies from 80 to 120. In Spondylus gaederopus, a very inequivalve shell, 60 have been counted on the right or fixed valve, and 90 on the left or upper valve. Each ocellus is connected, by means of its optic nerve, with the large circumpalleal nerve, and so with the branchial ganglion. They possess a cornea, lens, choroidea, and optic nerve, and, according to Hickson,[298] bear a considerable resemblance to the vertebrate type of eye. In spite of this, the power of vision in these genera does not appear at all superior to that of other Pelecypoda.

Fig. 93.—Pecten opercularis L., showing the ocelli on the two edges of the mantle.

Fig. 94.—Compound eyes (c.e) of Arca barbata L.; m.l, mantle fold; omm, ommatidia. (After Patten.)

According to the elaborate investigations of Patten, the ‘eyes’ in Arca occur upon the middle or ‘ophthalmic’ fold of the mantle-edge, which is thickened at the end to admit of their reception. Along this is ranged a row of dark brown spots of various sizes, which are larger at the anterior and posterior ends of the mantle-edge, but smaller and more numerous towards the middle. These brown spots, or ‘eyes,’ are many of them compound, being made up of the fusion of a number of ommatidia (from 10 to 80) into one large round eye, which is generally elevated above the surface of the surrounding epithelium. Sometimes these eyes themselves tend to fuse together. In one specimen of Arca Noae, 133 of these faceted eyes were counted in one mantle border, and 102 in the other.

There can be little doubt that the development of these functional eyes, or sensitive spots, in bivalve Mollusca, is due to special needs. They appear to be entirely absent in fresh-water bivalves (with the exception of Dreissensia, which is obviously a marine genus recently become fresh-water), while they are most abundant in genera living between tide marks (Solen, Mya, Mactra), and most highly specialised in a genus that is, for a bivalve, of singularly active habits (Pecten). Now genera living in sand between tide marks, as the three above-mentioned genera are in the habit of doing, and also protruding their siphons, and occasionally a considerable portion of their shells, out of their burrow, are manifestly very much at the mercy of their watchful enemies the gulls, and anything which would enable them to apprehend the approach of their enemies would be greatly to their advantage. Here, perhaps, lies the explanation of the greater elaboration of these pigmented spots in littoral genera, as compared with those inhabiting deeper water. Pecten, again, a genus distinguished by great activity, which can ‘fly’ for considerable distances in the water by flapping its valves together and expelling the water from the apertures at either side of the hinge, may be greatly assisted by its ocelli in directing its flight so as to escape its enemies.

The sense of smell—touch at a distance, as Moquin-Tandon has called it—is probably the most important sense which the Mollusca possess, and is unquestionably far more valuable to them than that of sight. Any one who has ever enjoyed the fun of hauling up lobster pots will recollect that part of the contents was generally a plentiful sprinkling of Buccinum, Nassa, and Natica, attracted by the smell of the stinking piece of fish with which the trap was baited. According to Mr. J. S. Gibbons,[299] Bullia rhodostoma congregates in hundreds on gigantic medusae which are stranded on the sandy bays near the Cape of Good Hope. Dr. J. G. Jeffreys says[300] that quantities of the common Neptunea antiqua “are procured on the Cheshire coast by the fishermen placing a dead dog on the sands at low-water mark during spring tides. The bait is then completely covered with stones, which are piled up like a cairn. On the next turn of the tide the heap of stones is visited, and the whelks are found on the surface in great numbers, having been apparently attracted by the smell of the bait, but unable to get at it.” Mr. W. A. Lloyd kept specimens of Nassa reticulata in a tank in the sand, at the bottom of which they usually remained buried. If a piece of meat of any kind were drawn over the sand, the Nassa would appear above the surface in a few minutes. Half-picked beef or mutton bones, if placed in the tank, were covered in a few minutes. In fact, no animal matter, whether living or dead, could be introduced without the Nassa smelling it, and coming up to see what they could get.[301]

Any one can experiment for themselves on the olfactory powers of our common snails or slugs. Moquin-Tandon records[302] two interesting cases, one communicated to him by letter, the other occurring to himself. His correspondent, a M. Parenteau, was one day walking along a dusty high-road, when he noticed, near the middle of the road, an empty bean-pod and two Arions eating it. Attributing the meeting of feeders and food to mere chance, he was walking on, when he noticed a second bean-pod, and, about two yards away from it, a third Arion, hurrying straight towards it. When the Arion had yet more than a yard to traverse, M. Parenteau picked up the bean and put it in his pocket. The Arion stopped, raised its head, and turned in every direction, waving its tentacles, but without advancing. M. Parenteau then carried the bean to the other side of the road, and put it in a small hole behind a piece of stone. The Arion, after a moment’s indecision, started off straight for the bean. Again the position of the precious morsel was changed, and again the Arion made for it, this time without being further tantalised. M. Moquin-Tandon noticed, one rainy day in the botanical gardens at Toulouse, two Limax maximus approaching a rotten apple from different directions. He changed the position of the apple several times, placing it at a sufficient distance, to be sure they could not see it, but they always hit it off correctly, after raising their heads and moving their long tentacles in every direction. It then occurred to him to hold the apple in the air, some centimetres above the head of the Limax. They perceived where it was, raised their heads and lengthened their necks, endeavouring to find some solid body on which to climb to their food.

Several of the land Mollusca have the power of exhaling a disagreeable smell, Hyalinia alliaria smelling strongly of garlic, and Stenogyra decollata of laudanum; but this need not be any argument for the sense of smell in the creatures themselves.

Position of Olfactory Organs in Pulmonata.—Most authorities are of opinion that the olfactory organs are situated in the tentacles. Moquin-Tandon considered that in the Helicidae and Limacidae the sense of smell is confined to the little knob or elevation at the end of the longer tentacles, close to the eye. He found that when he cut off these tentacles both in Limax and Arion, the creatures were quite unable to discover the whereabouts even of strongly-scented food. The same author believed that in the Basommatophora the sense of smell was present in the whole of the tentacle, which is covered with an exceedingly sensitive ciliated epithelium. Lacaze-Duthiers, however, places the olfactory sense in this group at the outer side of the base of the tentacles, near to the eyes. Some authorities[303] deny that the Helicidae have the olfactory organ at the tip of the tentacles, and locate it in a pedal gland near the mouth, which contains conspicuous sensitive cells. A Helix whose tentacles had been removed manifested its repulsion to the smell of spirits of turpentine, while another Helix, which was unmutilated, did not object to the turpentine being held between its tentacles. Altogether, then, the exact position of the smell-organ in the Helicidae must be considered as not yet thoroughly determined. Simroth holds that the sense of smell is distributed over the whole soft integument, and is especially concentrated in the feelers, and in the neighbourhood of the respiratory orifice.[304]

In nearly all marine Mollusca yet examined, the organ of smell or osphradium is in situation intimately connected with the breathing organs, being generally placed near their base, with the object, apparently, of testing the quality of the water before it passes over the branchiae. It consists of a patch of the epithelium, modified in a special manner, and connected by its own nerve with one of the visceral ganglia.

An osphradium does not necessarily occur in all genera; for instance, it has not been detected in Fissurella. It is most highly specialised in the Conidae, and in the carnivorous Gasteropoda generally. In Buccinum undatum, for instance, it is very large indeed, and, from its plumed form, has sometimes been mistaken for an accessory branchia (Fig. 95). In Haliotis it is paired, one lying in close proximity to each of the two branchiae, but in Turbo it is single, corresponding to the single branchia. In Chiton there is an osphradium at the base of each separate gill filament, making a total of twenty or more on each side. Its position in Physa and in Cyclostoma will be seen by reference to Figs. 103 and 104 (p. 205). In the Pelecypoda the osphradia are paired, and lie adjacent to the posterior adductor muscle, close to the hinder end of the axis of the branchiae. In the Tetrabranchiate Cephalopoda there are two osphradia, placed between the bases of the two pairs of gills. In the Dibranchiates on the other hand, a groove above the eyes has been regarded as the seat of the organ of smell. This groove contains sensory and ciliated cells, and appears to be connected with a special nerve centre of its own, which ultimately is derived from the cerebral ganglion.

Fig. 95.—Buccinum undatum L., deprived of its shell, showing the relative position of branchia (br) and osphradium (os); m, mucous glands; s, siphon. The portion of the mantle covering the osphradium has been removed.

Scarcely any instances of the exercise of the sense of smell on the part of bivalve Mollusca have been recorded. Something of the sort, however, seems to have been present in a case related by Mr. R. L. King.[305] A skull of a fox had been placed in a small ditch in order to soak, and after a few days, when taken out, was found to be covered with Pisidium pusillum to the number of at least two hundred, which had been probably attracted from the water in the immediate neighbourhood by the smell of the decaying flesh.

Experiments made with a view to ascertain whether the Mollusca are sensitive to noises have usually led to the conclusion that they are deaf to very loud sounds. This is the more curious, because an undoubted auditory apparatus has been discovered in a large number of genera. In the case of an experiment, it is not easy to be sure that the animal is not affected, at least in part, by the shock or jar, rather than by the actual sound. In some experiments, however, conducted at the Plymouth Marine Biological Laboratory, Mr. Bateson found[306] that Anomia could be made to shut its shell by smearing the glass of the tank with the finger in such a way as to make a creaking sound. It was evident that the cause of alarm was not the jarring of the solid framework of the tank, for the same result occurred when the object on which the Anomia were fixed was suspended in the water by a thread. It was found that the sound had to be of a particular pitch to excite the attention of the mollusc.

As a rule the organ of hearing is nothing more than a small vesicle or sac (the otocyst), filled with a fluid secretion, in which are suspended one or usually more calcareous concretions known as otoliths. By means of cilia, which connect with sense-cells, these otoliths are given a peculiar movement or oscillation in the medium in which they are suspended. The number of the otoliths varies in different genera and species; there are several hundreds in Arion and Limax, about a hundred in Helix pomatia, nemoralis, hispida, arbustorum, rotundata, Succinea putris, and Limnaea stagnalis; about fifty in Planorbis contortus and Physa fontinalis, only one in Cyclostoma elegans. The number increases with age. In young specimens of Limn. stagnalis as few as ten, nine, and seven have been noticed.[307]

The otocysts are always paired, and, in Gasteropoda, are placed close to the pedal ganglia. The acoustic nerve, however, has been shown by Lacaze-Duthiers to connect with the cerebral ganglia in certain cases. The otocysts are never on the surface of the body and are rarely connected with it by any passage or tube; it is probable therefore that sound reaches them simply through the medium of the tissues.

In the Pelecypoda the otocyst is similarly situated near the pedal ganglion, and is probably (though this has not yet been proved) similarly connected with the cerebral. There is only a single otolith. Pelseneer finds[308] in Nuculidae alone a free communication between the otocyst and the exterior. Anodonta has been observed[309] to withdraw its foot into the shell at the noise of an opening door, a loud voice, or a shrill whistle, whether in a basin of water or lying on a study table.

Fig. 96.—Illustrating the otocyst in A, Anodonta, B, Cyclas; ot, otolith; a, b, c, c´, cellular layers surrounding the chamber; ci, cilia on interior walls of chamber: C, an otolith crushed. (After Simroth.)

Delage extirpated the otocysts in certain Octopoda, and obtained some unexpected results. He found that remarkable effects were produced upon the animal’s powers of locomotion, so that it was unable to preserve its proper balance in the water when in rapid motion, but its body was forced to undergo a form of rotation more or less pronounced. He concluded that the otocysts must possess, besides their auditory functions, a power which stands in some relation to the proper orientation of the body in locomotion, a power which is not wholly supplied by sight and touch alone. The otocysts may thus regulate locomotion by stimulating muscular acts which tend to keep the body in the straight line during the process of movement.[310]

One of the most characteristic organs of the Mollusca is the foot, which, under one form or another, occurs throughout the whole phylum. The foot is a thickening, on the ventral side, of a portion of the integument of the animal, modified to serve different forms of motion. It attains its maximum relative area in the Chitonidae, many Nudibranchs, and the slugs generally, in nearly all of which there is no portion of the body which is not subtended by the foot. Here too it presents the form of a regular disc or ellipse, which is more or less produced. In many cases, however, the foot becomes modified in such a way that we are enabled to recognise well-marked anterior and posterior portions, which have received the name of propodium and metapodium respectively, while the intervening central portion is termed the mesopodium.

Fig. 97.—Sigaretus laevigatus Lam., showing excessive development of the propodium (pr) and metapodium (met) in a mollusc living in sand (the shell, which covers only the liver and adjacent parts, has been removed); l, liver; s.ap, aperture of proboscis, here deflected from the median line; t, t, tentacles. (After Quoy and Gaimard.)

The propodium is most strongly developed in genera which crawl about in wet sand, e.g. Natica, Sigaretus, Oliva, Harpa, Scaphander (Figs. 97 and 98, and compare Fig. 91). In such cases it seems to serve as a sort of fender or snow-plough, to push the sand away on both sides of the path the animal is traversing. In some species of Sigaretus the propodium becomes as it were banked up against the head and proboscis, which are thus unnaturally elevated, or tend to disappear altogether. Bullia (Fig. 62), which crawls about rapidly on wet sand, appears to attain its object by a wide extension of the foot on all sides, and so slides over the sand instead of ploughing through it; the little lappets at the end of the ‘tail’ probably serve as rudders.

In Melampus and Pedipes the propodium is marked off by a groove across the ventral surface. When the animal is in motion it first advances the propodium and then pulls the rest of the foot after it with the looping gait of certain caterpillars. In many Cyclostomatidae this groove, instead of being transverse, is longitudinal, and the animal advances first the right and then the left segment of the foot, which gives it a swaying motion from side to side.

Upon the metapodium lies the operculum, when it occurs. As a rule the metapodium is not sharply marked off from the rest of the foot. In Strombus (Fig. 99) it becomes erected into a sort of hump or column, on the top of which the operculum is situated.

Fig. 98.—Oliva textilina Lam., showing how the front part of the foot (f) is developed into a sort of fender, the propodium (pr); e, e, eyes; m.ap, front appendage of mantle; m.ap´, hinder appendage of mantle, folded into the suture when the animal is at rest; si, siphon; t, t, tentacles. (After Quoy and Gaimard.)

The epipodium is a prominent fold or border, which occurs upon the upper edge of the foot in most Diotocardia. In Haliotis it is of considerable breadth, and is covered by a number of lobes which spring from a moss-like prolongation of the skin. From the epipodium are developed the lateral tentaculae of Monodonta (Fig. 82, p. 178), and of other sub-genera of the Trochidae.[311]

In the Opisthobranchiata the lateral edges of the foot (the parapodia) are frequently produced into broad folds or wing-like extensions, which in many cases tend to fold over the shell, and, in conjunction with the mantle, eventually imbed it altogether. By the wavy motion of the parapodia the animal is enabled to progress through the water. The paired natatory lobes of the Pteropoda are simply the parapodia of the Tectibranchs modified for swimming purposes.

Fig. 99.—Strombus lentiginosus Lam., showing the modified form of the foot (f): e, e, eyes on their pedicels; mp, metapodium; op, operculum; p, penis; pr, proboscis; t, t, tentacles. (After Quoy and Gaimard.)

It is in the Heteropoda, Pteropoda, and most of all, the Cephalopoda, groups which have, for the most part, exchanged a crawling for a swimming life, that the modifications of the foot are most considerable. In Oxygyrus and Atlanta, for instance, the propodium and metapodium are sharply distinguished from the mesopodium, and no doubt have acquired, as a means of propulsion, the power of separate movement, the animal swimming with these portions of the foot uppermost. In Carinaria and Pterotrachea the metapodium has probably become continuous with the long axis of the body, while the so-called ‘foot’ with its sucker represents only the original propodium. In the Cephalopoda the arms and funnel represent the modified foot, the sides of which are prolonged into a number of very long specialised tentaculae. In the adult Cephalopod some of the arms have assumed a position in advance of the mouth, the latter being in fact surrounded by a circle of arms. But in the Cephalopod embryo the mouth opens as in the Gasteropoda, i.e. in advance of the arms, and it is only gradually that it becomes encircled by them. Arms and funnel alike are found to be innerved from the pedal ganglion.[312]

The pointed axe-shaped foot, which is characteristic of the majority of Pelecypoda, is doubtless derived from a form more akin to the flattened ‘sole’ of the Gasteropoda. A foot with something of this disc-shaped base actually occurs in some of the Nuculidae, the parapodia being furnished with pleats which recall similar formations in other Orders (Fig. 100). The principal modifications of the foot are due to its employment as a burrowing organ. In genera which burrow but slightly it is small and feebly developed, while in genera which habitually excavate, it becomes the largest and strongest organ of the body. At the same time it has a tendency to shift its position from the ventral to the anterior margin, accompanied by a corresponding narrowing of the shell, until it arrives at the position seen in Mollusca of the shape of Mya, Pholas, and Solen. In sedentary or attached genera, e.g. Pecten, Chama, Ostrea, the foot tends to become aborted.

Fig. 100.—Yoldia limatula Say, Greenland, showing the short plumed branchiae (br, br), the gasteropodous foot (f), and the large labial palps (l.p, l.p): A, as seen from the ventral margin; B, from the left side, with the mantle turned back; a.m, position of anterior adductor muscle; i, intestine; l, liver; m, m, mantle.

The byssus gland, secreting a byssus of horny threads, is characteristic of many Pelecypoda, and may be observed by any one in the common mussel. It occurs in the larvae of many species which do not possess a byssus in the adult stage. The pedal gland of many Prosobranchiates, which secretes a tough and almost thread-like slime, is possibly homologous with the byssus gland of bivalves.

The Mollusca possess a nervous system, which usually consists of a number of nerve centres or ganglia, linked together by bands (the commissures) and sending out thread-like nerves which ramify into the various organs. The character of the nervous system varies greatly in different groups, ranging as it does from a condition of extreme complexity, in which the ganglia are numerous and the commissures equally so, to that of considerable simplicity, in which ganglia are almost entirely absent.

The most important ganglia are (1) the cerebral,[313] which are always placed above or on either side of the mouth, and from which proceed the nerves of the eyes and tentacles; (2) the pedal, which in Gasteropoda are situated below the oesophagus, in Pelecypoda at the base of the foot, and from which the nerves of the foot and sometimes the acoustic nerve arise; (3) the pleural,[314] whose position varies considerably, but is always below the oesophagus and slightly above the pedal ganglia; these innervate the mantle, branchiae, heart, and viscera generally.

Gasteropoda.—The simplest form of nerve system as thus understood occurs in the Amphineura, and more particularly in the Chitons. Here we find four longitudinal nerve-cords, parallel to one another for nearly the whole length of the mollusc. The two exterior cords probably represent the pleural, the two interior the pedal nervous system. There being no head or tentacles, but simply a mouth at the anterior end, the cerebral ganglia do not exist, but they are represented by the curved ring formed by the massing together of the two nerve-cords on each side. The only distinct ganglia are a pair of buccal ganglia (which are developed on a pair of commissures which pass forward from the cerebral mass and innervate the lips and buccal region), and a much smaller group, the sublingual. The two pedal cords are united by a number of transverse parallel connectives, which recall similar modes of connexion in the Chaetopod worms and in Arthropoda.

This quadruple set of nerve-cords is characteristic of all the Amphineura, but the absence of ganglia is most marked in the Chitons. In Proneomenia and Neomenia there is a distinct cerebral ganglion, formed by the massing of the two ganglia into one, while in Proneomenia the lateral cords are joined to the pedal, as well as the pedal to one another, by connectives. In Chaetoderma the cerebral ganglia, though adjacent, are distinct, and both the pedal and lateral cords connect directly with them, while there are no transverse connectives.

The remaining three great divisions of Gasteropoda, namely, the Prosobranchiata, Opisthobranchiata, and Pulmonata, may be regarded as comprising two distinct types of nervous condition, according as the loop formed by the two visceral nerve-cords is twisted over itself, forming a figure of 8, or continues straight and uncrossed. In the former case, we get the condition known as streptoneurous, in the latter that as euthyneurous.[315] The Euthyneura include the whole of the Opisthobranchiata[316] and Pulmonata, the Streptoneura all the Prosobranchiata.

Fig. 101.—Nervous system of the Amphineura: A, Proneomenia; B, Neomenia; C, Chaetoderma; D, Chiton; c, cerebral ganglia; l, l, lateral cords; pc, posterior commissure; s, sublingual commissure or ring, with ganglia; v, v, pedal cords. (Alter Hubrecht.)

The simplest form of nervous system in the euthyneurous Gasteropoda occurs in the Opisthobranchiata. The cerebral, pleural, and pedal ganglia tend to become concentrated in a ring-like form, united by short commissures at the posterior end of the pharynx. The visceral loop is in some cases long, and the two or three visceral ganglia are then situated at its posterior extremity. The nervous system of the Pulmonata is of a similar type, the visceral loop being often much shorter, and tending to draw in towards the central group of ganglia. The tentacular and optic nerves are, as usual, derived from the cerebral ganglion, with which also the octocysts are probably connected by rather long nerves. A pair of buccal ganglia innervate the buccal mass, and are united by commissures with the right and left cerebral ganglia. The osphradial nerve springs from one of the ganglia on the visceral loop, the osphradium itself being situated (in Limnaea) immediately above the pulmonary orifice and adjacent to the anus (Fig. 102). This massing of the ganglia is still better illustrated by the accompanying figure of Physa (Fig. 103), in which the animal is represented as if transparent, so that the ganglia and nerves are seen through the tissues.

Fig. 102.—I. Nervous system of Limnaea stagnalis L. The oesophagus has been cut and pulled forwards through the nerve-collar, so as to expose the lower surface of the buccal mass(dissected by F. B. Stead)

• B.M, buccal mass.
• B.G, buccal, C.G, cerebral, Os.G, osphradial, Pe.G, pedal ganglia.
• Pl.G, pleural ganglia.
• Op.N, optic, Os.N, osphradial, Te.N, tentacular nerve.
• Ot, otocyst; V.L, visceral loop.
• R, rectum, dotted in to show its position relative to the osphiadium.

II. Right side of the head of Limnaea stagnalis. The overhanging flap of the mantle has been cut in the middle line, and the right half twisted back, so as to expose the pulmonary orifice, etc. The points A A on the mantle edge were continuous before the mantle was cut; the line BA is part of the free edge of the mantle.

An, anus; F, female generative orifice; J, portion of jaw; M, male generative orifice under right tentacle; Os, osphradium; P.O, pulmonary orifice.

Of the streptoneurous Gasteropoda, the nervous system of Fissurella and Haliotis shows distinct points of similarity to that of the Amphineura. The pedal nerves are united by transverse commissures throughout their entire length, while a double commissure unites the cerebral ganglia to the mass from which the pedal nerves proceed. In the great majority of the Streptoneura the ganglia (except the visceral) are more concentrated and the commissures are consequently much shorter. The accompanying figure of Cyclostoma, in which the animal is represented as in that of Physa just described, illustrates this grouping of the ganglia, the twist of the visceral loop, and the position of the visceral ganglia at its posterior end.

Fig. 103.—Nervous system of Physa acuta Drap., showing the massing of the ganglia at the hinder end of the pharynx: e, e, eyes; m, mouth; m.l, m.l, mantle lappets; o.f, female generative orifice; o.m, male generative orifice; os, osphradium. (After Lacaze-Duthiers.)

Fig. 104.—Example of a streptoneurous Gasteropod (Cyclostoma elegans Drap.): c.g, c.g, cerebral ganglia; e, e, eyes; os, osphradium; ot, ot, otocysts; p.g, p.g, pedal ganglia; pl.g, pl.g, pleural ganglia; sp.g, supra-intestinal ganglion; sb.g, sub-intestinal ganglion; t.n, tentacle nerve; v.g, visceral ganglion. (After Lacaze-Duthiers.)

Scaphopoda.—In the Scaphopoda the nervous system resembles that of the Pelecypoda. The cerebral and pleural ganglia lie close together, while the pedal ganglia are placed in the anterior part of the foot, connected with the cerebral ganglia by long commissures; the visceral loop is rather long, and the two visceral ganglia are adjacent to the anus.

Pelecypoda.—The nervous system in the Pelecypoda is the simplest type in which well-marked ganglionic centres occur. The ganglia are few, symmetrically placed, and are usually at a considerable distance apart. There are, as a rule, three distinct pairs of ganglia, the cerebral (cerebro-pleural), pedal, and visceral. The cerebral are formed by the fusion of the cerebral and pleural ganglia, which, however, in some cases (Protobranchiata) continue distinct.[317] They lie above or on each side of the mouth, united by a commissure of varying length. Another pair of commissures unites them with the pedal ganglia, which are placed at the base of the foot, and are usually very close together, sometimes (as in Anodonta) becoming partially fused. The length of these commissures depends upon the distance between mouth and foot; thus they are very long in Mya and Modiola, and very short in Pecten. In cases where the foot is rudimentary or becomes aborted through disuse (e.g. Ostrea), the pedal ganglia may dwindle or disappear altogether. The visceral ganglia are on the ventral side of the posterior adductor muscle, beneath the rectum, and innervate the branchiae, osphradia, and the whole of the visceral sac. A pair of cerebro-visceral commissures traverses the base of the foot, surrounding it with a comparatively short loop (compare Fig. 106, c.v.c´), while a long commissure, which runs round the entire edge of the mantle, and supplies branching nerves to the mantle border and siphons (Fig. 106, c.v.c), may also connect the visceral and cerebral ganglia.

Fig. 105.—Nervous system of Pelecypoda: A, Teredo; B, Anodonta; C, Pecten; a, a, cerebral ganglia; b, pedal ganglia; c, visceral ganglia. (After Gegenbaur.)

Cephalopoda.—In the Cephalopoda the concentration of ganglia attains its maximum, and may perhaps be regarded as approaching the point at which a definite brain may be said to exist. Another point of distinction is the formation of special small ganglia upon the nerve-cords in different parts of the body. In the Tetrabranchiata (Nautilus) the cerebral and pedal ganglia form a broad ring which surrounds the oesophagus, the former giving out the optic nerves, with their special optic ganglion, and a pair each of buccal and pharyngeal ganglia, the latter the nerves of the arms and funnel. The visceral loop is still present in the form of a separate band, which innervates the branchiae, osphradia, and viscera generally, forming a special genital ganglion in connexion with the reproductive organs. The principal ganglia of the Dibranchiata are still more concentrated, even the visceral loop being possibly united with the rest in forming an unbroken mass in which scarcely any trace of commissures can be detected. The pedal ganglion becomes separated into two portions, one of which innervates the arms, the other the funnel. Two peculiar ganglia (the stellate ganglia) supply a number of branching nerves to the mantle.

Fig. 106.—Nervous system of Cardium edule L.: a.m, anterior adductor muscle; br, branchiae; br.n, branchial nerve; c.g, c.g, cerebral ganglia; c.p.c, cerebro-pedal commissure; c.v.c’, cerebro-visceral commissure; c.v.c, cerebro-visceral commissure of mantle; l.p, labial palps: m, mouth; p.g, pedal ganglion; p.m, posterior adductor muscle; v.g, visceral ganglion. (After Drost, × 3.)

CHAPTER VIII THE DIGESTIVE ORGANS, JAW, AND RADULA: EXCRETORY ORGANS — The Kidneys

The digestive tract, or, as it is often termed, the alimentary canal or gut, is a very important feature of the Mollusca. It may be regarded as consisting of the following parts: (1) a mouth or oral aperture: (2) a throat or pharynx; (3) an oesophagus, leading into (4) a stomach, (5) an intestine and rectum, ending in (6) an anus.

The primitive positions of mouth and anus were presumably at the anterior and posterior ends of the animal, as in the Amphineura and symmetrical Mollusca generally. But the modifications of original molluscan symmetry, which have already been referred to (p. 154, compare pp. 245, 246), have resulted in the anus becoming, in the great majority of Gasteropoda, twisted forward, and occupying a position on some point in the right side in dextral, and in the left in sinistral species.

The process of digestion, as the food passes from one end of the tract to the other, is performed by the aid of the secretions of various glands, which open into the alimentary canal at different points in its course. The principal of these are the salivary glands, situated on the pharynx and oesophagus, and the liver, biliary or hepatic gland, connecting with the stomach. With these may be considered the anal and ink-glands, which, in certain genera, connect with the terminal portion of the rectum.

1. The mouth is generally, as in the common snail and periwinkle, placed on the lower part of the head, and may be either a mere aperture, circular or semicircular, in the head-mass, or, as is more usual, may be carried on a blunt snout (compare Fig. 6, p. 10, and Fig. 68, p. 159), which is capable of varying degrees of protrusion. From the retractile snout has doubtless been derived the long proboscis which is so prominent a feature of many genera (compare Figs. 1, B, and 99), and in some (e.g. Mitra, Dolium) attains a length exceeding that of the whole body. As a rule, Mollusca provided with a proboscis are carnivorous, while those whose mouth is on the surface of the head are Vegetable feeders, but this rule is by no means invariable. The mouth is thickened round the aperture into ‘lips,’ which are often extensile, and appear capable of closing upon and grasping the food. In the Pelecypoda the mouth is furnished, on each side, with a pair of special external lobes, the ‘labial palps,’ which appear to be of a highly sensitive nature, and whose object it is to collect, and possibly to taste, the food before it passes into the mouth.

2. The Pharynx, Jaws, and Radula.—Immediately behind the lips the mouth opens into the muscular throat, pharynx, or buccal mass. The pharynx of the Glossophora, i.e. of the Gasteropoda, Scaphopoda, and Cephalopoda, is distinguished from that of the Pelecypoda,[318] by the possession of two very characteristic organs for the rasping or trituration of food before it reaches the oesophagus and stomach. These are (a) the jaw or jaws, and (b) the radula,[319] odontophore, or lingual ribbon. The jaws bite the food, the radula tears it up small before it passes into the stomach to undergo digestion. The jaws are not set with teeth like our own; roughly speaking, the best idea of the relations of the molluscan jaw and radula may be obtained by imagining our own teeth removed from our jaws and set in parallel rows along a greatly prolonged tongue.[320]

In nearly all land Pulmonata the jaw is single, and is placed behind the upper lip. If a common Helix aspersa be observed crawling up the inside of a glass jar, or feeding on some succulent leaf, the position and action of the jaw can be readily discerned. It shows very black when the creature opens its mouth, and under its operation the edge of a lettuce leaf shows a regular series of little curved indentations, in shape not unlike the semicircular bites inflicted by a schoolboy upon his bread and butter. The jaw of Helix (Fig. 107, B) is arched in shape, and is strengthened by a number of projecting vertical ribs. That of Limax (A) is straighter, and is slightly striated, without vertical ribs. In Bulimulus (C) the arch of the jaw is very conspicuous, and the upper edges are always denticulated; in Orthalicus there is a central triangular plate with a number of overlapping plates on either side; in Succinea (E) there is a large square accessory plate above the jaw proper. The form of the jaw is peculiar not only to the genus but to the species as well. Thus the jaw of H. aspersa is specifically distinct from that of H. pomatia, and that of H. nemoralis is distinct from both. Wiegmann has observed[321] that in young Arion, Limax, and Helix, the jaw consists of two pieces, which coalesce by fusion in the adult, thus indicating a stage of development in advance of the double jaw which is found in most of the non-pulmonate Mollusca. In all fresh-water Pulmonata there are two small accessory side plates besides the jaw proper (Fig. 107, F).

Fig. 107.—Jaws of various Pulmonata: A, Limax (gagates Drap., Lancashire, × 15); B, Helix (acutissima Lam., Jamaica, × 15); C, Bulimulus (depictus Reeve, Venezuela, × 20); D, Achatina (fulica Fér., Mauritius, × 7); E, Succinea (elegans Riss., Aral District, × 30); F, Limnaea (stagnalis L., Cambridge, × 30).

Nearly all the non-carnivorous Prosobranchiata, land, fresh-water, and marine alike, are provided with two large lateral jaws. Many of these are sculptured with the most elaborate patterns, and appear to be furnished with raised teeth, like a file. In the Nudibranchiata the jaws are of great size and beauty of ornamentation (Fig. 109).

Fig. 108.—Jaws of A, Triton australis Lam., Sydney; B, Ampullaria fasciata Reeve, Demerara; C, Calliostoma punctulatum Mart., New Zealand; D, Cyclophorus atramentarius Sowb., Sanghir; all × 15.

Fig. 109.—Jaws of A, Chromodoris gracilis Iher., × 15; B, Scyllaea pelagica L., × 7; C, Pleurobranchus plumula Mont., × 10; D, Pleurobranchaea Meckelii Lam., × ⁵/₂.

The carnivorous genera, whether marine (e.g. Conus, Murex, Buccinum, Nassa) or land (e.g. Testacella, Glandina, Streptaxis, Ennea), are entirely destitute of jaws, the reason probably being that in all these cases the teeth of the radula are sufficiently powerful to do the work of tearing up the food without the aid of a masticatory organ as well. Jaws are also wanting in the Heteropoda, and in many of the Nudibranchiata and Tectibranchiata.

In the Cephalopoda the jaws, or ‘beaks,’ as they are called, are most formidable weapons of attack. In shape they closely resemble the beaks of a parrot, but the hook on the dorsal side of the mouth does not, as in birds, close over the lower hook, but fits under it. Powerful muscles govern these mandibles, which must operate with immense effect upon their prey (Fig. 110).

Fig. 110.—Jaws of Sepia: A, in situ within the buccal mass, several of the arms having been cut away; B, removed from the mouth and slightly enlarged.

Fig. 111.—Patella vulgata L., showing the normal position of the radula, which is doubled back in a bow; the shell has been removed, and the whole visceral mass is turned forward, exposing the dorsal surface of the muscular foot: gr, longitudinal groove on this surface; i, i, intestine; l, liver; m, m, mantle edge; mu, muscles (cut through) fastening the visceral mass to the upper sides of the foot; ov, ovary; r, radula; u.f, upper or dorsal surface of the foot.

The Radula.[322]—When the food has passed beyond the operation of the jaw, it comes within the province of the radula, the front part of which perhaps co-operates to a certain extent with the jaw in performing the biting process. The function of the radula as a whole is to tear or scratch, not to bite; the food passes over it and is carded small, the effect being very much the same as if, instead of dragging a harrow over the surface of a field, we were to turn the harrow points upwards, and then drag the field over the harrow.

The radula itself is a band or ribbon of varying length and breadth, formed of chitin, generally almost transparent, sometimes beautifully coloured, especially at the front end, with red or yellow.[323] It lies enveloped in a kind of membrane, in the floor of the mouth and throat, being quite flat in the forward part, but usually curving up so as to line the sides of the throat farther back, and in some cases eventually forming almost a tube. The upper surface, i.e. the surface over which the food passes, is covered with teeth of the most varied shape, size, number, and disposition, which are almost invariably arranged in symmetrical rows. These teeth are attached to the cartilage on which they work by muscles which serve to erect or depress them; probably also the radula as a whole can be given a forward or backward motion, so as to rasp or card the substances which pass over it.

The teeth on the front part of the radula are often much worn (Fig. 112), and probably fall away by degrees, their place being taken by others successively pushed up from behind. At the extreme hinder end of the radula the teeth are in a nascent condition, and there are often as many as a dozen or more scarcely developed rows. Here, too, lie the cells from which the teeth are originally formed.

The length and breadth of the radula vary greatly in different genera. In Littorina it is very narrow, and several times the length of the whole animal. It is kept coiled away like a watch-spring at the back of the throat, only a small proportion of the whole being in use. I have counted as many as 480 rows in the common Littorina littorea. In Patella it is often longer than the shell itself, and if the radula of a large specimen be freshly extracted and drawn across the hand, the action of the hooks can be plainly felt. In Aerope, the Turbinidae generally, and Haliotis it is very large. In Turritella, Aporrhais, Cylichna, Struthiolaria, and the Cephalopoda it is small in proportion to the size of the animal. In the Pulmonata generally it is very broad, the length not exceeding, as a rule, thrice the breadth; in most other groups the breadth is inconsiderable, as compared to the length.

The Radula is wanting in two families of Prosobranchiata, the Eulimidae and Pyramidellidae, which are consequently grouped together as the section Gymnoglossa. It is probable that in these cases the radula has aborted through disuse, the animals having taken to a food which does not require trituration. Thus several genera contained in both these families are known to live parasitically upon various animals—Holothurians, Echinoderms, etc.—nourishing themselves on the juices of their host. In some cases, the development of a special suctorial proboscis compensates for the loss of radula (see pp. 76–77). In Harpa there is no radula in the adult, though it is present in the young form. No explanation of this fact has yet been given. It is also absent in the Coralliophilidae, a family closely akin to Purpura, but invariably parasitic on corals, and probably nourished by their exudations. There is no radula in Entoconcha, an obscure form parasitic on the blood-vessels of Synapta, or in Neomenia, a genus of very low organisation, or in the Tethyidae, or sea-hares, or in one or two other genera of Nudibranchiata.

Fig. 112.—Example of a front portion of a radula (Cantharus ringens Reeve, Panama), much worn by use. × 70.

The number of teeth in the radula varies greatly. When the teeth are very large, they are usually few in number, when small, they are very numerous. In the carnivorous forms, as a rule, the teeth are comparatively few and powerful, while in the phytophagous genera they are many and small. Large hooked and sickle-shaped teeth, sometimes furnished with barbs like an arrow-head, and poison-glands, are characteristic of genera which feed on flesh; vegetable feeders, on the contrary, have the teeth rounded, and blunter at the apex, or, if long and narrow, so slender as to be of comparatively little effect. Genera which are normally vegetarian, but which will, upon occasion, eat flesh, e.g. Limax and Hyalinia, exhibit a form of teeth intermediate between these two extremes (see Fig. 140, A).

In Chaetoderma there is but one tooth. In Aeolis coronata there are about 17, in A. papillosa and Elysia viridis about 19, in Glaucus atlanticus about 21, in Fiona nobilis about 28. In the common whelk (Buccinum undatum) there are from 220 to 250, in the common periwinkle about 3500. As many as 8343 have been counted in Limnaea stagnalis, about 15,000 in Helix aspersa (that is, about 400,000 to the square inch), about 30,000 in Limax maximus, and as many as 40,000 in Helix Ghiesbreghti, a large species from Mexico; they are very numerous also in Nanina, Vitrina, Gadinia, and Actaeon. But Umbrella stands far and away the first, as far as number of teeth is concerned. In both U. mediterranea and U. indica they entirely baffle calculation, possibly 750,000 may be somewhere near the truth.

The teeth on the radula are almost invariably disposed in a kind of pattern, exactly like the longitudinal rows of colour in a piece of ribbon, down the centre of which runs a narrow stripe, and every band of colour on one side is repeated in the same relative position on the other side. The middle tooth of each row—the rows being counted across the radula, not longitudinally—is called the central or rachidian tooth; the teeth next adjacent on each side are known as the laterals, while the outermost are styled uncini or marginals. As a rule, the distinction between the laterals and marginals is fairly well indicated, but in the Helicidae and some of the Nudibranchiata it is not easy to perceive, and in these cases there is a very gradual passage from one set to the other.

The central tooth is nearly always present. It is wanting in certain groups of Opisthobranchiata, some of the carnivorous Pulmonata, and in the Conidae and Terebridae, which have lost the laterals as well. Voluta has lost both laterals and marginals in most of the species, and the same is the case with Harpa. In Aeolis, Elysia, and some other Nudibranchiata the radula consists of a single central row. Other peculiarities will be described below in their proper order.

The extreme importance of a study of the radula depends upon the fact, that in each species, and a fortiori in each genus and family, the radula is characteristic. In closely allied species the differences exhibited are naturally but slight, but in well-marked species the differences are considerable. The radula, therefore, serves as a test for the distinction of genera and species. For instance, in the four known recent genera of the family Strombidae, viz. Strombus, Pteroceras, Rostellaria, and Terebellum, the radula is of the same general type throughout, but with distinct modifications for each genus; and the same is true, though to a lesser extent, for all the species hitherto examined in each of the genera. These facts are true for all known genera, differences of the radula corresponding to and emphasising those other differences which have caused genera to be constituted. The radula therefore forms a basis of classification, and it is found especially useful in this respect in dealing with the largest class of all, the Gasteropoda, and particularly with the chief section of this order, the Prosobranchiata. Thus we have—

(a) Toxoglossa.—Only three families, Terebridae, Conidae, and Cancellariidae, belong to this section. There is no central tooth, and no laterals, the radula consisting simply of large marginals on each side. In Conus these are of great size, with a blunt base which contains a poison-gland (see p. 66), the contents of which are carried to the point by a duct. The point is always singly and sometimes doubly barbed (Fig. 116). When extracted, the teeth resemble a small sheaf of arrows (Figs. 113, 115). A remarkable form of radula, belonging to Spirotropis (a sub-genus of Drillia, one of the Conidae), enables us to explain the true history of the radula in the Toxoglossa. Here there are five teeth in a row, a central tooth, and one lateral and one marginal on each side, the marginals being very similar in shape to the characteristic shafts of the Conidae (Fig. 114). It is evident, then, that the great mass of the Toxoglossa have lost both their central and lateral teeth, and that those which remain are true uncini or marginals. Spirotropis appears to be the solitary survival of a group retaining the primitive form of radula.

Fig. 113.—Radula of Bela turricula Mont. × 70.

Fig. 114.—Portion of radula of Spirotropis carinata Phil., Norway. × 70.

Fig. 115.—Eight teeth from the radula of Terebra caerulescens Lam. × 60.

The arrangement of teeth in all these sections is expressed by a formula applicable to each transverse row of the series. The central tooth, if present, is represented by 1, and the laterals and marginals, according to their number, on each side of the central figure. Thus the typical formula of the Toxoglossa is 1.0.0.0.1, the middle 0 standing for the central tooth which is absent, and the 0 on each side of it for the absent laterals; the 1 on each extreme represents the one uncinus in each row. Thus the formula for Spirotropis, which has also one lateral on each side and a rachidian or central tooth, is 1.1.1.1.1. Often the formula is given thus: where 30 and 42 stand for the average number of rows of teeth in Conus and Spirotropis respectively; the same is sometimes expressed thus: 1.0.0.0.1 × 30; 1.1.1.1.1 × 42.

Fig. 116.—A tooth from the radula of Conus imperialis L., S. Pacific, × 50, showing barb and poison duct.

Fig. 117.—Portion of the radula of Melongena vespertilio Lam., Ceylon. × 30.

Fig. 118.—Portion of the radula of Eburna japonica Sowb., China. × 30.

Fig. 119.—Portion of the radula of Murex regius Lam., Panama. × 60.

(b) The Rachiglossa comprise the 12 families Olividae, Harpidae, Marginellidae, Volutidae, Mitridae, Fasciolariidae, Turbinellidae, Buccinidae, Nassidae, Columbellidae, Muricidae, and Coralliophilidae. Certainly most and probably all of these families are or have been carnivorous, the Coralliophilidae being a degraded group which have become parasitic on corals, and have lost their teeth in consequence. The characteristics of the group are the possession of a central tooth with from one cusp (Boreofusus) to about fourteen (Bullia), and a single lateral more or less cuspidate, the outer cusp of all being generally much the largest. Thus in Melongena respertilio (Fig. 117) the central tooth is tricuspid, the central cusp being the smallest, while the laterals are bicuspid; in Eburna japonica (Fig. 118) the central tooth is 5-cusped, the two outer cusps being much the smallest. The teeth, on the whole, are sharp and hooked, with a broad base and formidable cutting edge. In the Olividae, Turricula, Buccinopsis, and the Muricidae the laterals are unicuspid and somewhat degraded (Fig. 119). In Mitra and the Fasciolariidae they are very broad and finely equally toothed like a comb (Figs. 120, 121). The whole group is destitute of marginals.

Fig. 120.—Portion of the radula of Imbricaria marmorata Swains. × 80.

Fig. 121.—Three rows of teeth from the radula of Fasciolaria trapezium Lam. × 40.

Fig. 122.—Six teeth from the radula of Cymbium diadema Lam., Torres Strait. × 25.

Fig. 123.—Examples of degraded forms of radula: A, Cantharus pagodus Reeve, Panama (nascent end), × 40; A´, same radula, central and front portion; B, Columbella varia Sowb., Panama, × 50.

Fig. 124.—Three rows of the radula of Sistrum spectrum Reeve, Tonga, × 80. The laterals to the right are not drawn in.

Several remarkable peculiarities occur. Harpa loses the radula altogether in the adult. In the young it has lost only the laterals, and consists of nothing but the central tooth. Marginella has no laterals; the central tooth is small and comb-shaped, with blunt cusps. In Voluta the laterals are generally lost, but in Volutomitra and one species of Voluta[325] they are retained. The central tooth usually has three strong cusps, and is very thick and coloured a deep red or orange (Fig. 122); in the sub-genus Amoria it is unicuspid, in shape rather like a spear-head with broadened wings; in Volutolyria it is of a different type, with numerous unequal denticulations, something like the laterals of Mitra or Fasciolaria. Of the Mitridae, Cylindromitra has lost the laterals. Among the Buccinidae, Buccinopsis possesses a curiously degraded radula, the central tooth having no cusps, but being reduced to a thin basal plate, while the laterals are also weakened. This degradation from the type is a remarkable feature among radulae, and appears to be characteristic, sometimes of a whole family, e.g. the Columbellidae (Fig. 123, B), sometimes of a genus, sometimes again of a single species. Thus in Cantharus (a sub-genus of Buccinum) the radula is typical in the great majority of species, but in C. pagodus Reeve, a large and well-grown species, it is most remarkably degraded, both in the central and lateral teeth (Fig. 123, A). This circumstance is the more singular since C. pagodus lives at Panama side by side with C. ringeus and C. insignis, both of which have perfectly typical radulae. It is probable that the nature of the food has something to do with the phenomenon. Thus Sistrum spectrum Reeve was found to possess a very aberrant radula, not of the common muricoid type, but with very long reed-like laterals. This singularity was a standing puzzle to the present writer, until he was fortunate enough to discover that S. spectrum, unlike all other species of Sistrum, lives exclusively on a branching coral.

The dental formula for the Rachiglossa is thus 1.1.1, except in those cases where the laterals are absent, when it is 0.1.0.

Fig. 125.—Portion of the radula of Cassis sulcosa Born., × 40. The marginals to the right are not fully drawn.

(c) The Taenioglossa comprise 46 families in all, of which the most important are Tritonidae, Cassididae, Cypraeidae, Strombidae, Cerithiidae, Turritellidae, Melaniidae, Littorinidae, Rissoidae, Paludinidae, Ampullariidae, Cyclophoridae, Cyclostomatidae, and Naticidae. The radula is characterised by a central tooth of very variable form, the prevailing type being multicuspid, the central cusp the largest, on a rather broad base; a single lateral, which is often a broad plate, more or less cusped, and two uncini, rather narrow, with single hooks, or slightly cusped. The accompanying figures of Cassis, Vermetus, and Cypraea, and those of Littorina and Cyclophorus given on pp. 20, 21, are good examples of typical taenioglossate radulae.

Fig. 126.—Four rows of teeth from the radula of Vermetus grandis Gray, Andamans. × 40.

In Homalogyra the radula is much degraded, the central tooth is large and triangular on a broad base, the lateral is represented only by a thin oblong plate, and the uncini are absent. In some species of Jeffreysia the uncini are said to be absent, while present in others. Lamellaria has lost both its uncini, but the radula of the allied Velutina is quite typical. A peculiar feature in this group is the tendency of the marginals to increase in number. A stage in this direction is perhaps seen in Ovula, Pedicularia, and the Cyclostomatidae. Here the outermost of the two marginals is by far the larger and broader, and is strongly pectinated on its upper edge; in the Cyclostomatidae the pectinations are rather superficial; in Ovula (where both marginals are pectinated) they are decidedly deeper; in Pedicularia they are deeper still, and make long slits in the tooth, tending to subdivide it altogether. In Turritella the number of marginals is said to vary from none (in T. acicula) to three (T. triplicata), but the fact wants confirmation. Solarium is an aberrant form, possessing simply a number of long uncini, which recall those of Conus or Pleurotoma, and is therefore hard to classify; the allied Torinia has a radula which appears allied to Ovula or Pedicularia. In Triforis the teeth are identical throughout, very small, about 27 in a row, tricuspid on a square base, cusps short.

The normal formula of the Taenioglossa is 2.1.1.1.2; in Lamellaria, 1.1.1; in Triforis, 13.1.13, or thereabouts.

Fig. 127.—Two rows of the radula of Cypraea tigris L. × 30.

Fig. 128.—Portion of the radula of Ianthina communis Lam. × 40.

(d) Ptenoglossa.—This section consists of two families only, which certainly appear remarkably dissimilar in general habits and appearance, viz., the Ianthinidae and Scalariidae. In all probability their approximation is only provisional. The radula, which in Ianthina is very large, and in Scalaria very small, possesses an indefinite number of long hooked teeth, of which the outermost are the largest. The central tooth, if present (it does not occur in Ianthina), is the smallest in the series, and thus recalls the arrangement in some of the carnivorous Pulmonata (p. 232). In Ianthina the radula is formed of two large divisions, with a gap between them down the middle.

The formula is ∞.1.∞ or ∞.O.∞ according as the central tooth in Scalaria is or is not reckoned to exist.

(e) Gymnoglossa.—In the absence of both jaw and radula it is not easy to classify the two families (Eulimidae and Pyramidellidae) which are grouped under this section. Fischer regards them as modified Ptenoglossa; one would think it more natural to approximate them to the Taenioglossa.

Fig. 129.—Portion of the radula of Margarita umbilicalis Brod., Labrador. × 75 and 300.

(f) Rhipidoglossa.—This section consists of seventeen families, the most important being the Helicinidae, Neritidae, Turbinidae, Trochidae, Haliotidae, Pleurotomariidae, and Fissurellidae. The radula is characterised by—

(1) The extraordinary development of the uncini, of which there are so many that they are always reckoned as indefinitely numerous. They are long, narrow, hooked, and often cusped at the top, and crowded together like the ribs of a fan, those at the extreme edge not being set straight in the row, but curving away backwards as they become smaller; in Solariella alone, where there are from five to ten, can they be counted.

Fig. 130.—Portion of the radula of Nerita albicilla L., Andaman Is., with central tooth highly magnified: c, c, the capituliform tooth. × 40.

(2) The varying number of the laterals. The average number of these is five on each side; in some cases (Livona) there are as many as nine, in some (Neritopsis) only three. The lateral next to the uncini (which is specially large in the Neritidae, and is then known as the capituliform tooth) is regarded by some authorities as the first uncinus, by others as the sole representative of the laterals, the teeth on the inner side of it being reckoned as multiplied central teeth. According to this latter view, Livona will have as many as seventeen central teeth. Taking five as the average number of ‘laterals,’ we shall have the following different ways of constituting the rhipidoglossate formula, the first being that to which preference is given, viz.:—

In the Neritidae and the derived fresh-water genera (Neritina, Navicella) the first lateral, as well as the capituliform tooth, is very large, and in shape rather like the blade bone of a shoulder of mutton; the intervening laterals are very small. In Neritopsis (a degraded form) the central tooth and first lateral are entirely wanting. In the neritiform land-shells (Helicina, Proserpina) the first lateral is no larger than the others, while the capituliform tooth is enormous. Hydrocena is a very aberrant and apparently degraded form; the laterals between the first and the capituliform tooth are all wanting. In Haliotis, Scissurella, and Pleurotomaria the five laterals are of fairly equal size; in Fissurella we again meet with a large capituliform tooth, with very small laterals.

(g) The Docoglossa are in direct contrast with the Rhipidoglossa in possessing few and strong teeth, instead of many and weak. There are only three families, Acmaeidae, Patellidae, and Lepetidae. In some of the Acmaeidae there are not more than two teeth in a row, while in no species are there more than twelve. The radula is, however, very long; there are as many as 180 rows in Patella vulgata. The teeth are thick, generally of a very deep red horn colour, rather opaque. The cartilage in which they are set is remarkably thick, and in some species whose teeth are very few a considerable portion of this cartilage is left quite bare.

Fig. 131.—Portion of the radula of Patella cretacea Reeve, seen in half profile. × 40.

Although the teeth are so few, the arrangement is by no means simple. The special feature of the group is the multiplication of identical centrals. Of these there are two in Acmaea, and four, as a rule, in Patella. Thus in these two genera there is seldom an absolutely central tooth. Either laterals or marginals are liable to be lost, but there are never more than two of either in Acmaea, and never more than two laterals and three marginals in Patella. Thus the formula varies from 0.0.(1 + 0 + 1).0.0 in Pectinodonta, 2.2.(1 + 0 + 1).2.2 in Collisellina (both Acmaeidae), to 3.2.(1 + 0 + 1).2.3 in Patinella, and 3.1.(2 + 0 + 2).1.3 in Patella proper. In the Lepetidae there is an absolutely central tooth, which appears to be made up of the coalescence of several teeth, no laterals, and about two marginals; formula, 2.0.1.0.2.

Fig. 132.—Two rows of the radula of Pterotrachea mutica Les., Naples. × 60.

The radula of the Heteropoda is quite characteristic, and shows no sign of affinity with any other Prosobranchiate. The central tooth is large, broad, tricuspid, and denticulated on a broad base; the single lateral is strong, often bicuspid; the two marginals simple, long, falciform; formula, 2.1.1.1.2 (Fig. 132).

Fig. 133.—A, Portion of the radula of Chiton (Acanthopleura) spiniger] Sowb., Andamans, × 30; B, portion of the radula of Dentalium entalis L., Clyde, × 50.

Amphineura.—(a) Polyplacophora.—The radula of the Chitonidae is quite unique. It resembles that of the Docoglossa in being very long, and composed of thick and dark horn-coloured teeth. The number of teeth, however, is considerably greater, amounting almost invariably to seventeen in each row. There are three rather small central teeth, the two outer of these being similar; next comes a very large lateral (the major lateral), usually tricuspid, which is followed by two much smaller laterals, which are scarcely more than accessory plates; then a very large and arched marginal (the major uncinus), at the outer side of which are three accessory plates. Some consider there is only one central tooth, and count the two small teeth on each side of it as laterals.

Thus the formula is either (3 + 1).(2 + 1).3.(1 + 2).(1 + 3) or (3 + 1).(2 + 1 + 1).1.(1 + 1 + 2).(1 + 3).

(b) Aplacophora.—Of this rather obscure order, Chaetoderma has a single strong central tooth, Neomenia has no radula, Proneomenia and Lepidomenia have about twenty falciform teeth, much larger at one end of the radula than the other; formula, 0.1.0.

Opisthobranchiata.—The radula of the Opisthobranchiata is exceedingly variable in shape, size, and number and character of teeth. Not only do allied families differ greatly from one another, but allied genera often possess radulae widely distinct in plan. Thus, among the Polyceridae, Goniodoris has no central tooth, one large lateral and one marginal (form. 1.1.0.1.1); Doridunculus the same, with five marginals (form. 5.1.0.1.5); Lamellidoris one each of median, laterals, and marginals (1.1.1.1.1); Idalia, Ancula, and Thecacera the same as Goniodoris; Crimora several each of laterals and marginals. Even species of the same genus may differ; thus the formula for Aeolis papillosa is 0.1.0, but for Ae. Landsbergi 1.1.1; for Philine aperta 1.0.1, but for Philine pruinosa 6.0.6.

Fig. 134.—Two teeth from the radula of Aeolis papillosa L. × 55.

It must not be forgotten, however, that a simple repetition of the same tooth, whether lateral or marginal, does not necessarily constitute an important characteristic, nor does the presence or absence of a central tooth. In most of the cases mentioned above, the difference in the number of laterals and marginals is due to the multiplication of identical forms, while the central tooth, when present, is often a mere plate or narrow block without cusps, whose presence or absence makes little difference to the character of the radula as a whole.

There appear to be three well-marked types of radula among the Opisthobranchiata.

(a) Radula with a single strong central tooth, rows few. This form is characteristic of the Aeolididae, Fionidae, Glaucidae, Dotoidae, Hermaeidae, Elysiidae (Fig. 135), and Limapontiidae. In the Aeolididae it is sometimes accompanied by a single lateral. The same type occurs in Oxynoe, and in Lobiger (= Lophocercus).

(b) Radula with the first lateral very strongly developed. This type may take the form of (1) a single lateral, no central or marginals, e.g. Onchidoris, Scaphander (Fig. 137, A), Philine (certain species), Ringicula, or (2) first lateral strongly developed, and repeated in succeeding laterals (2–6) on a smaller scale, e.g. Philine (certain species). A few marginals are sometimes added, e.g. in Polycera, Lamellidoris (where there is a degraded central tooth, Fig. 137, B), Idalia, and Ancula.

Fig. 135.—Radula of Elysia viridis Mont. × 40. Type (a).

Fig. 136.—Portion of the radula of Gadinia peruviana Sowb., Chili. × 250. Type (c).

(c) Radula with an indefinite number of marginals, laterals (if present) merging into marginals, central tooth present or absent, inconspicuous, teeth all very small. This type of radula, among the Nudibranchiata, is characteristic of certain sub-genera of Doris (e.g. Chromodoris, Aphelodoris, Casella, Centrodoris), of Hypobranchiaea and Pleurophyllidia; among the Tectibranchiata, of Actaeon, many of the Bullidae, Aplustrum, the Aplysiidae, Pleurobranchus, Umbrella and Gadinia (Figs. 136 and 137, C).

In the Pteropoda there are two types of radula. The Gymnosomata, which are in the main carnivorous, possess a radula with a varying number (4–12) of sickle-shaped marginals, central tooth present or absent. In the Thecosomata, which feed on a vegetable diet, there are never more than three teeth, a central and a marginal on each side; teeth more or less cusped on a square base.

Pulmonata.—The radula of the Testacellidae, or carnivorous land Mollusca, is large, and consists of strong sickle-shaped teeth with very sharp points, arranged in rows with or without a central tooth, in such a way that the largest teeth are often on the outside, and the smallest on the inside of the row (as in Rhytida, Fig. 139). The number and size of the teeth vary. In Testacella and Glandina, they are numerous, consisting of from 30 to 70 in a row, with about 50 rows, the size throughout being fairly uniform. In Aerope they are exceedingly large, and only eight in a row, the outermost marginal being probably the largest single tooth in the whole of the Mollusca. The central tooth is always obscure, being, when present, simply a weaker form of the weakest lateral; in genera with only a few teeth in a row it is generally absent altogether.

Fig. 137.—Portions of the radula of Opisthobranchiata, illustrating types (b) and (c); A, Scaphander lignarius L.; A´, one of the teeth seen from the other side, × 40; B, Lamellidoris bilamellata L., Torbay, × 60; C, Hydatina physis L., E. Indies, × 75.

The first family of jaw-bearing snails, the Selenitidae, is distinctly intermediate. The possession of a jaw relates it to the main body of Helicidae, but the jaw is not strong, while the teeth are still, with the exception of the central, thoroughly Testacellidan. The central tooth is quite rudimentary, but it is something more than a mere weak reproduction of the marginals. There are no true laterals. The Limacidae show a further stage in the transition. Here the central tooth has a definite shape of its own, tricuspid on a broad base, which is more or less repeated in the first laterals; these, as they approach the marginals, gradually change in form, until the outer marginals are again thoroughly Testacellidan.[326] This is the general form of radula, varied more or less in different genera, which occurs in Nanina, Helicarion, Limax, Parmacella, and all the sub-genera of Zonites. It is certain that some, and probable that all of these genera will, on occasion, eat flesh, although their usual food appears to be vegetable. The jaw is more powerful than in the Selenitidae, but never so large or so strongly ribbed as in Helix proper.

Fig. 138.—Portion of the radula of Glandina truncata Gmel. × 40.

Fig. 139.—Portion of the radula of Rhytida Kraussii Pfr., S. Africa. × 25.

When we reach the Helicidae, we arrive at a type of radula in which the aculeate form of tooth—so characteristic of the Agnatha—disappears even in the marginals, and is replaced by teeth with a more or less quadrate base; the laterals, which are always present, are intermediate in form between the central and the marginals, and insensibly pass into the latter. In size and number of cusps the first few laterals resemble the central tooth; in the extreme marginals the cusps often become irregular or evanescent. As a rule, the teeth are set squarely in the rows, with the exception of the extreme marginals, which tend to slope away on either side. In some Helicidae there is a slight approximation to the Zonitidae in the elongation of the first marginals.

The above is the type of radula occurring in the great family Helicidae, which includes not only Helix proper, with several thousand species, but also Arion, Bulimus, Ariolimax, and other genera. The jaw is almost always strongly transversely ribbed.

In the Orthalicidae (Fig. 140, C) the teeth of the radula, instead of being in straight rows, slope back at an angle of about 45 degrees from the central tooth. The central and laterals are very similar, with an obtuse cusp on rather a long stem; the marginals become bicuspid.

In the Bulimulidae, which include the important genera Placostylus, Amphidromus, Partula, Amphibulimus, and all the groups of South American Bulimulus, the jaw is very characteristic, being thin, arched, and denticulated at the edges, as if formed of numerous narrow folds overlapping one another. The radula is like that of the Helicidae, but the inner cusp of the laterals is usually lengthened and incurved. In Partula the separation between laterals and marginals is very strongly marked.

The remaining families of Pulmonata must be more briefly described. In the Cylindrellidae there are three distinct types of radula: (a) Central tooth a narrow plate, laterals all very curiously incurved with a blunt cusp, no marginals (Fig. 140, D); (b) radula long and narrow, central tooth as in (a), two laterals, and about eight small marginals; (c) much more helicidan in type, central and laterals obtusely unicuspid, marginals quite helicidan. Type (c) is restricted to Central America, types (a) and (b) are West Indian.

Pupidae: Radula long and narrow; teeth of the helicidan type, centrals and laterals tricuspid on a quadrate base, marginals very small, cusps irregular and evanescent. This type includes Anostoma, Odontostomus, Buliminus, Vertigo, Strophia, Holospira, Clausilia, and Balea.

Stenogyridae, including Achatina, Stenogyra, and all its sub-genera: Central tooth small and narrow, laterals much larger, tricuspid, central cusp long, marginals similar, but smaller.

Achatinellidae: Two types occur; (a) teeth in very oblique rows, central, laterals, and marginals all of the same type, base narrow, head rather broad, with numerous small denticles (Achatinella proper, with Auriculella and Tornatellina, Fig. 140, E); (b) central tooth small and narrow, laterals bicuspid, marginals as in Helix (Amastra and Carelia).

Fig. 140.—Portions of the radula of A, Hyalinia nitidula Drap., Yorkshire, with central tooth, first lateral, and a marginal very highly magnified; B, Helix pomatia L., Kent, showing central tooth, laterals, and one extreme marginal, the two former also highly magnified; C, Orthalicus undatus Brug., Trinidad, with three laterals highly magnified; D, Cylindrella rosea Pfr., Jamaica, central tooth and laterals, the same very highly magnified; E, Achatinella vulpina Fér., Oahu, central tooth (c) and laterals, the same highly magnified.

Succineidae: Central and laterals helicidan, bi- or tricuspid on a quadrate plate, marginals denticulate on a narrow base; jaw with an accessory oblong plate.

Janellidae: Central tooth very small, laterals and marginals like Achatinellidae (a).

Vaginulidae: Central, laterals, and marginals unicuspid throughout, on same plan.

Onchidiidae: Rows oblique at the centre, straight near the edges; central strong, tricuspid; laterals and marginals very long, falciform, arched, unicuspid.

Auriculidae: Teeth very small; central narrow, tricuspid on rather a broad base; laterals and marginals obscurely tricuspid on a base like Succinea.

Limnaeidae: Jaw composed of one upper and two lateral pieces; central and lateral teeth resembling those of Helicidae; marginals much pectinated and serriform (Fig. 141, A). In Ancylus proper the teeth are of a very different type, base narrow, head rather blunt, with no sharp cusps, teeth similar throughout, except that the marginals become somewhat pectinated (Fig. 141, B); another type more resembles Limnaea.

Fig. 141.—Portions of the radula of A, Limnaea stagnalis L., with the central tooth and two first laterals, and two of the marginals, very highly, magnified; B, Ancylus fluviatilis Müll., with two of the marginals very highly magnified; C, Physa fontinalis L., with central tooth and two of the marginals very highly magnified.

Physidae: Jaw simple, but with a fibrous growth at its upper edge, which may represent an accessory plate; radula with very oblique rows, central tooth denticulate, laterals and marginals serriform, comb-like, with a wing-like appendage at the superior outer edge (Fig. 141, C).

Chilinidae: Central tooth small, cusped on an excavated triangular base, marginals five-cusped, with a projection as in Physa, laterals comb-like, serrations not deep.

Amphibolidae: Central tooth five-cusped on a broad base, central cusp very large; two laterals only, the first very small, thorn-like, the second like the central tooth, but three-cusped; laterals simple, sabre-shaped.

Scaphopoda.—In the single family (Dentaliidae) the radula is large, and quite unlike that of any other group. The central tooth is a simple broad plate; the single lateral is strong, arched, and slightly cusped; the marginal a very large quadrangular plate, quite simple; formula, 1.1.1.1.1 (Fig. 133, B).

Cephalopoda.—The radula of the Cephalopoda presents no special feature of interest. Perhaps the most remarkable fact about it is its singular uniformity of structure throughout a large number of genera. It is always very small, as compared with the size of the animal, most of the work being done by the powerful jaws, while the digestive powers of the stomach are very considerable.

The general type of structure is a central tooth, a very few laterals, and an occasional marginal or two; teeth of very uniform size and shape throughout. In the Dibranchiata, marginals are entirely absent, their place being always taken, in the Octopoda, by an accessory plate of varying shape and size. This plate is generally absent in the Decapoda. The central tooth is, in the Octopoda, very strong and characteristic; in Eledone and Octopus it is five-cusped, central cusp strong; in Argonauta unicuspid, in Tremoctopus tricuspid. The laterals are always three in number, the innermost lateral having a tendency to assume the form of the central. In Sepia the two inner laterals are exact reproductions of the central tooth; in Eledone, Sepiola, Loligo, and Sepia, the third lateral is falciform and much the largest.

Fig. 142.—Portion of the radula of Octopus tetracirrhus D. Ch., Naples, × 20.

In Nautilus, the only living representative of the Tetrabranchiata, there are two sickle-shaped marginals on each side, each of which has a small accessory plate at the base. The two laterals and the central tooth are small, very similar to one another, unicuspid on a square base.

Fig. 143.—Alimentary canal of Helix aspersa L.: a, anus; b.d, b.d´, right and left biliary ducts; b.m, buccal mass; c, crop; h.g, hermaphrodite gland; i, intestine; i.o, opening of same from stomach (pyloric orifice); l, l´, right and left lobes of liver; m, mouth; oe, oesophagus; r, rectum; s.d, salivary duct; s.g, salivary gland; st, stomach; t, left tentacle. (After Howes and Marshall, slightly modified.)

Salivary glands are found in most Glossophora. They occur in one or two pairs on each side of the pharynx and oesophagus, the duct usually leading forwards and opening into the anterior part of the pharynx (see Figs. 143, 144). They are exceptionally large in the carnivorous Gasteropoda. In certain genera, e.g. Murex, Dolium, Cassis, Pleurobranchus, the secretions of these glands are found to contain a considerable proportion (sometimes as much as 4·25 per cent) of free sulphuric acid. This fact was first noticed by Troschel, who, while handling a Dolium galea at Messina, saw the creature spit a jet of saliva upon a marble slab, which immediately produced a brisk effervescence. A number of the genera thus provided bore through the shells of other Mollusca and of Echinoderms, to prey upon their soft tissues, and it is possible that the acid assists in the piercing of the shell by converting the hard carbonate of lime into sulphate of lime, which can easily be removed by the action of the radula.[327] In the majority of the Cephalopoda there are two pairs of salivary glands, one lying on each side of the mouth, the other on the middle of the oesophagus.

Fig. 144.—Alimentary canal, etc., of Sepia officinalis L.: a, anus; b.d, one of the biliary ducts; b.m, buccal mass; c, coecum; i, ink-sac; i.d, duct of same; j, jaws; l.l, lobes of the liver; oe, oesophagus; p, pancreatic coeca; r, rectum; s.g, salivary glands; st, stomach. (From a specimen in the British Museum.)

Fig. 145.—Gizzard of Scaphander lignarius L.: A, showing position with regard to oesophagus (oe) and intestine (i), the latter being full of comminuted fragments of food; p, left plate; p´, right plate; p.ac, accessory plate; B, the plates as seen from the front, with the enveloping membranes removed, lettering as in A. Natural size.

Fig. 146.—Section of the stomach of Melongena, showing the gastric plates (g.p, g.p,) for the trituration of food; b.d, biliary duct; g.g, genital gland; i, intestine; l, liver; oe, oesophagus; st, stomach. (After Vanstone.)

3. The Oesophagus.—That part of the alimentary canal which lies between the pharynx and the stomach (in Pelecypoda between the mouth and stomach) is known as the oesophagus. Its exact limits are not easy to define, since in many cases the tube widens so gradually, while the muscular structure of its walls changes so slowly that it is difficult to say where oesophagus ends and stomach begins. As a rule, the oesophagus is fairly simple in structure, and consists of a straight and narrow tube. In the Pulmonata and Opisthobranchiata it often widens out into a ‘crop,’ which appears to serve the purpose of retaining a quantity of masticated food before it passes on to the stomach. In Octopus and Patella the crop takes the form of a lobular coecum. In the carnivorous Mollusca the oesophagus becomes complicated by the existence of a varying number of glands, by the action of which digestion appears to begin in some cases before the food reaches the stomach proper.

4. The Stomach.—At the posterior end of the oesophagus lies the muscular pouch known as the stomach, in which the digestion of the food is principally performed. This organ may be, as in Limax, no more than a dilatation of the alimentary canal, or it may, as is usually the case, take the form of a well-marked bag or pocket. The two orifices of the stomach are not always situated at opposite ends; when the stomach itself is a simple enlargement of the wall of one side of the alimentary canal, the cardiac or entering orifice often becomes approximated to the orifice of exit (pyloric orifice).

The walls of the stomach itself are usually thickened and strengthened by constrictor muscles. In some Nudibranchs (Scyllaea, Bornella) they are lined on the inside with chitinous teeth. In Cyclostoma, and some Bithynia, Strombus, and Trochus there is a free chitinous stylet within the stomach.[328] In Melongena (Fig. 146) the posterior end of the oesophagus is provided with a number of hard plate-like ridges, while the stomach is lined with a double row of cuticular knobs, which are movable on their bases of attachment, and serve the purpose of triturating food.[329] Aplysia has several hard plates, set with knobs and spines, and similar organs occur in the Pteropoda. But the most formidable organ for the crushing of food is possessed by the Bullidae, and particularly by Scaphander (Fig. 145). Here there is a strong gizzard, consisting of several plates connected by powerful cartilages, which crush the shells, which are swallowed whole.

Into the stomach, or into the adjacent portions of the digestive tract, open the ducts which connect with the so-called liver. The functions of this important organ have not yet been thoroughly worked out. The liver is a lobe-shaped gland of a brown-gray or light red colour, which in the spirally-shelled families usually occupies the greater part of the spire. In the Cephalopoda, the two ducts of the liver are covered by appendages which are usually known as the pancreatic coeca; the biliary duct, instead of leading directly into the stomach, passes into a very large coecum (see Fig. 144) or expansion of the same, which serves as a reservoir for the biliary secretions. At the point of connexion between the coecum and stomach is found a valve, which opens for the issue of the biliary products into the stomach, but closes against the entry of food into the coecum. In most Gasteropoda the liver consists of two distinct lobes, between which are embedded the stomach and part of the intestine. In many Nudibranchiata the liver becomes ‘diffused’ or broken up into a number of small diverticula or glands connecting with the stomach and intestine. The so-called cerata or dorsal lobes in the Aeolididae are in effect an external liver, the removal of which to the outside of the body gives the creature additional stomach-room.

The Hyaline Stylet.—In the great majority of bivalves the intestine is provided with a blind sac, or coecum of varying length. Within this is usually lodged a long cylindrical body known as the hyaline or crystalline stylet. In a well-developed Mytilus edulis it is over an inch in length, and in Mya arenaria between two and three inches. The bladder-like skin of the stylet, as well as its gelatinoid substance, are perfectly transparent. In the Unionidae there is no blind sac, and the stylet, when present, is in the intestine itself. It is said to be present or absent indifferently in certain species.

The actual function performed by the hyaline stylet is at present a matter of conjecture. Haseloff’s experiments on Mytilus edulis tend to confirm the suggestion of Möbius, that the structure represents a reserve of food material, not specially secreted, but a chemical modification of surplus food. He found that under natural conditions it was constantly present, but that specimens which were starved lost it in a few days, the more complete the starvation the more thorough being the loss; it reappeared when they were fed again. Schulze, on the other hand, believes that it serves, in combination with mucus secreted by the stomach, to protect the intestine against laceration by sharp particles introduced with the food. W. Clark found that in Pholas the stylet is connected with a light yellow corneous plate, and imagined therefore that it acts as a sort of spring to work the plate in order to comminute the food, the two together performing somewhat the function of a gizzard.[330]

5. and 6. The Intestine, Rectum, and Anus.—The intestine, the wider anal end of which is called the rectum, almost invariably makes a bend forward on leaving the stomach. This is the case in the Cephalopoda, Scaphopoda, and the great majority of Gasteropoda. The exceptions are the bilaterally symmetrical Amphineura, in which the anus is terminal, and many Opisthobranchiata, in which it is sometimes lateral (Fig. 68, p. 159), sometimes dorsal (Fig. 67). The intestine is usually short in carnivorous genera, but long and more or less convoluted in those which are phytophagous. In all cases where a branchial or pulmonary cavity exists, the anus is situated within it, and thus varies its position according to the position of the breathing organ. Thus in Helix it is far forward on the right side, in Testacella, Vaginula, and Onchidium almost terminal, in Patella at the back of the neck, slightly to the right side (Fig. 64, p. 157).

In the rhipidoglossate section of the Diotocardia (Trochus, Haliotis, etc.) the rectum passes through the ventricle of the heart, a fact which, taken in conjunction with others, is evidence of their relationship to the Pelecypoda.

Fig. 147.—Ink-sac of Sepia, showing its relation to the rectum: a, anus; d, duct of sac; i.g, ink-gland; i.r, portion of the sac which serves as a reservoir for the ink; o, orifice of ink-gland; r, rectum; sp, double set of sphincter muscles controlling upper end of duct. (Modified from Girod.)

In nearly all Pelecypoda the intestine is very long and convoluted, being sometimes doubled forward over the mouth. Towards its terminal part it traverses the ventricle of the heart, except in Ostrea, Anomia, Teredo, and a few more. The anus is always at the posterior end of the animal, adjacent to and slightly above the adductor muscle.

Anal glands, which open into the rectum close to the anus, are present in some Prosobranchiata, e.g. Murex, Purpura. In the Cephalopoda the anal gland becomes of considerable size and importance, and is generally known as the ink-sac (Fig. 147); it occurs in all known living genera, except Nautilus. The ink-sac consists of a large bag generally divided into two portions, in one of which the colouring matter is secreted, while the other acts as a reservoir for its storage. A long tube connects the bag with the end of the rectum, the mouth of the tube being controlled, in Sepia, by a double set of sphincter muscles.

The kidneys, nephridia,[331] renal or excretory organs, consist typically of two symmetrical glands, placed on the dorsal side of the body in close connexion with the pericardium. Each kidney opens on the one hand into the mantle cavity, close to the anus (see Fig. 64, p. 157), and on the other, into the pericardium. The venous blood returning from the body passes through the vascular walls of the kidneys, which are largely formed of cells containing uric acid. The blood thus parts with its impurities before it reaches the breathing organs.

The kidneys are paired in all cases where the branchiae are paired, and where the heart has two auricles, i.e. in the Amphineura, the Diotocardia (with the exception of the Neritidae), the Pelecypoda, and all Cephalopoda except Nautilus, which has four branchiae, four auricles, and four kidneys. In other Gasteropoda only one kidney survives, corresponding to the left kidney of Zygobranchiate Gasteropods.

Besides their use as excretory organs the kidneys, in certain groups of the Mollusca, stand in very close relation to the genital glands. In some of the Amphineura the generative products, instead of possessing a separate external orifice of their own, pass from the genital gland into the pericardium and so out through the kidneys (see Fig. 61 C, D, p. 154). In the Diotocardia it is the right kidney alone which serves, besides its excretory functions, as a duct for the emission of the generative products, the left kidney being at the same time greatly reduced in size. Thus in Patella the left nephridium is small, the right being much larger; both function as excretory organs, but the right serves as a mode of conveyance for the seminal products as well. In certain Pelecypoda (e.g. Yoldia, Avicula, Modiola, Pecten, Spondylus) the genital glands communicate directly, and with a similar object, with the renal pouch on the same side of the body, but in the majority of cases the orifices are distinct.

The following memoirs will be found useful for further study of this portion of the subject:—

CHAPTER IX THE SHELL, ITS FORM, COMPOSITION AND GROWTH—DESIGNATION OF ITS VARIOUS PARTS

The popular names of ‘shells,’ ‘shell-fish,’ and the like, as commonly applied to the Mollusca, the intrinsic beauty and grace of the shells themselves, resulting in the passion for their collection, their durability and ease of preservation, as compared with the non-testaceous portion,—all these considerations tend to unduly exalt the value of the shell as part of the organism as a whole, and to obscure the truth that the shell is by no means the most important of the organs.

At the same time it must not be forgotten that the old systems of classification, which were based almost entirely on indications drawn from the shell alone, have been strangely little disturbed by the new principles of arrangement, which depend mainly on structural points in the animal. This fact only tends to emphasise the truth that the shell and animal are in the closest possible connexion, and that the shell is a living part of the organism, and is equally sensitive to external influences.

A striking instance of the comparative valuelessness of the shell alone as a primary basis of classification is furnished by the large number of cases in which a limpet-shaped shell is assumed by genera widely removed from one another in cardinal points of organisation. This form of shell occurs in the common limpet (Patellidae), in Ancylus (Limnaeidae), Hemitoma (Fissurellidae), Cocculina (close to Trochidae), Umbrella and Siphonaria (Opisthobranchiata), while in many other cases the limpet form is nearly approached.

Roughly speaking, about three-quarters of the known Mollusca, recent and fossil, possess a univalve, and about one-fifth a bivalve shell. In Pholas, and in some species of Thracia, there is a small accessory hinge plate; in the Polyplacophora, or Chitons, the shell consists of eight plates (see Fig. 2, p. 8), usually overlapping. A certain proportion of the Mollusca have no shell at all. In many of these cases the shell has been present in the larva, but is lost in the adult.

The shell may be

(1) External, as in the great majority of both univalves and bivalves.

(2) Partly external, partly internal; e.g. Homalonyx, Hemphillia, some of the Naticidae, Scutum, Acera, Aplustrum (Figs. 148 and 149).

Fig. 148.—Aplustrum aplustre L. Mauritius, showing the partly internal shell (S); F, foot; LL, cephalic lappets; TT, double set of tentacles. (After Quoy and Gaimard.)

Fig. 149.—Sigaretus laevigatus Lam., showing shell partially immersed in the foot; F, anterior prolongation of the foot. (After Souleyet.)

(3) Internal; e.g. Philine, Gastropteron, Pleurobranchus, Aplysia, Limax, Arion, Hyalimax, Parmacella, Lamellaria, Cryptochiton, and, among bivalves, Chlamydoconcha.

(4) Absent; e.g. all Nudibranchiata and Aplacophora, many Cephalopoda, a few land Mollusca, e.g. all Onchidiidae, Philomycus, and Vaginula.

The Univalve Shell.—In univalve Mollusca the normal form of the shell is an elongated cone twisted into a spiral form round an axis, the spiral ascending to the left. Probably the original form of the shell was a simple cone, which covered the vital parts like a tent. As these parts tended to increase in size, their position on the dorsal side of the animal caused them gradually to fall over, drawing the shell with them. The result of these two forces combined, the increasing size of the visceral hump, and its tendency to pull the shell over with it, probably resulted in the conversion of the conical into the spiral shell, which gradually came to envelop the whole animal. Where the visceral hump, instead of increasing in size, became flattened, the conical shape of the shell may have been modified into a simple elliptical plate (e.g. Limax), the nucleus representing the apex of the cone. In extreme cases even this plate dwindles to a few calcareous granules, or disappears altogether (Arion, Vaginula).

Varieties of the Spiral.—Almost every conceivable modification of the spiral occurs, from the type represented by Gena, Haliotis, Sigaretus, and Lamellaria, in which the spire is practically confined to the few apical whorls, with the body-whorl inordinately large in proportion, to a multispiral form like Terebra, with about twenty whorls, very gradually increasing in size.

Fig. 150.—Examples of shells with A, a flattened spire (Polygyratia); B, a globose spire (Natica); C, a greatly produced spire (Terebra).

As a rule, the spire is more or less obliquely coiled round the axis, each whorl being partially covered, and therefore hidden by, its immediate successor, while the size of the whorls, and therefore the diameter of the spire as a whole, increases somewhat rapidly. The effect of this is to produce the elevated spire, the shell of six to ten whorls, and the wide aperture, of the normal type of mollusc, the whelk, snail, periwinkle, etc.

Sometimes, however, the coil of the whorls, instead of being oblique, tends to become horizontal to the axis, and thus we have another series of gradations of form, from the excessively produced spire of Terebra to the flattened disc of Planorbis, Polygyratia, Euomphalus, and Ammonites. The shell of many species of Conus practically belongs to the latter type, each whorl folding so closely over its predecessor that the spiral nature of the shell is not perceived until it is looked at at right angles to the spire.

Fig. 151.—Examples of shells with disconnected whorls; A, Cyathopoma cornu Mf., Philippines; B, Cylindrella hystrix Wright, Cuba. (Both × 4.)

Fig. 152.—Example of a shell whose apical whorls alone are coiled, and the remainder produced in a regular curve. (Cyclosurus Mariei Morel., Mayotte.)

In some cases the regularly spiral form is kept, but the whorls are completely disconnected; e.g. some Scalaria, Spirula; among fossil Cephalopoda, Gyroceras, Crioceras, and Ancyloceras; and, among recent land Mollusca, Cylindrella hystrix and Cyathopoma cornu (Fig. 151). Sometimes only the last whorl becomes disconnected from the others, as in Rhiostoma (see Fig. 180, p. 266), Teinostoma, and in the fossil Ophidioceras and Macroscaphites. Sometimes, again, not more than one or two whorls at the apex are spirally coiled, and the rest of the shell is simply produced or coiled in an exceedingly irregular manner, e.g. Cyclosurus, Lituites, Orygoceras, Siliquaria (Fig. 153), Vermetus. In Coecum (Fig. 170, p. 260) the spiral part is entirely lost, and the shell becomes simply a cylinder. In a few cases the last whorl is coiled irregularly backwards, and is brought up to the apex, so that the animal in crawling must carry the shell with the spire downwards, as in Anostoma (Fig. 154), Opisthostoma (Fig. 208, p. 309), Strophostoma, and Hypselostoma (Fig. 202 A, p. 302).

Fig. 153.—Siliquaria anguina Lam., showing scalariform coil of upper whorls and irregular extension of the lower.

Fig. 154.—Anostoma globulosum Lam., Brazil. (After P. Fischer.)

Fig. 155.—Various forms of the internal plate in Capulidae: A, Calyptraea (Mitrularia) equestris Lam., E. Indies; B, Crucibulum scutellatum Gray, Panama; C, Ergaea plana Ad., and Reeve, Japan; D, Galerus chinensis L., Britain; E, Crepipatella dilatata Lam., Callao; F, Trochita maculata Quoy, N. Zealand; G, Crepidula fornicata Lam., N. America.

Some genera of the Capulidae, in which the shell is of a broadly conical form or with scarcely any spire, develop an internal plate or process which serves the purpose of keeping the animal within the shell, and does the work of a strong attachment muscle. In Mitrularia this process takes the form of a raised horse-shoe; in Crucibulum it is cup-shaped, with the edge free all round; in Galerus, Ergaea, Crepipatella, and Trochita we get a series of changes, in which the edge of the cup adheres to the interior of the shell, and then gradually flattens into a plate. In Crepidula proper this plate becomes a regular partition, covering a considerable portion of the interior (Fig. 155 G). Hipponyx secretes a thin calcareous plate on the ventral surface of the foot, which intervenes like an operculum between the animal and the substance to which it adheres.

Sinistral, or Left-handed Shells.—The vast majority of univalve spiral shells are normally dextral, i.e. when held spire uppermost, with the aperture towards the observer, the aperture is to the right of the axis of the spire. If we imagine such a shell to be a spiral staircase, as we ascended it we should always have the axis of the spire to our left.

Sinistral or ‘reversed’ forms are not altogether uncommon, and may be grouped under four classes:—

(1) Cases in which the genus is normally sinistral; (2) cases in which the genus is normally dextral, but certain species are normally sinistral; (3) cases in which the shell is indifferently dextral or sinistral; (4) cases in which both genus and species are normally dextral, and a sinistral form is an abnormal monstrosity.

Fig. 156.—Fulgur perversum L., Florida. ½.

Fig. 157.—Illustration of the gradation of forms in Ampullaria between a dextral (A) and an ultra-dextral species (F).

In all cases of sinistral monstrosity, and all in which a sinistral and dextral form are interchangeable (sections 3 and 4 above), the position of the apertures of the internal organs appears to be relatively affected, i.e. the body is sinistral, as well as the shell. This has been proved to be the case in all specimens hitherto examined, and may therefore be assumed for the rest. The same uniformity, however, does not hold good in all cases for genera and species normally sinistral (sections 1 and 2). As a rule, the anal and genital apertures are, in these instances also, to the left, but not always. In Spirialis, Limacina, Meladomus, and Lanistes the shell is sinistral, but the animal is dextral. This apparent anomaly has been most ingeniously explained by Simroth, Von Ihering, and Pelseneer. The shell, in all these cases, is not really sinistral, but ultra-dextral. Imagine the whorls of a dextral species capable of being flattened, as in a Planorbis, and continue the process, still pushing, as it were, the spire downwards until it occupies the place of the original umbilicus, becoming turned completely ‘inside out,’ and we have the whole explanation of these puzzling forms. The animal remains dextral, the shell has become sinistral. A convincing proof of the truth of this is furnished by the operculum. It is well known that the twist of the operculum varies with that of the shell; when the shell is dextral, the operculum is sinistral, with its nucleus near the columella, and vice versâ. In these ultra-dextral shells, however, where it is simply the method of the enrolment of the spire that comes in question, and not the formation of the whorls themselves, the operculum remains sinistral on the apparently sinistral shell.

The reverse case to this, when the shell is dextral but the orifices sinistral, is instanced by the two fresh-water genera Pompholyx (from N. America), and Choanomphalus (L. Baikal). A similar transition in the enrolment of the whorls may be confidently assumed to have taken place, and the shells are styled ultra-sinistral.

Yet another variation remains, in which the embryonic form is sinistral, but the adult shell dextral, the former remaining across the nucleus of the spire. This is the case with Odostomia, Eulimella, Turbonilla, and Mathilda, all belonging to the Prosobranchiata, with Actaeon, Tornatina, and Actaeonina among the Opisthobranchs, and Melampus alone among Pulmonates.

Monstrosities of the Shell.—Abnormal growths of the shell constantly occur, some of them being scarcely noticeable, except by a practised eye, others of a more serious nature, involving an entire change in the normal aspect of the creature. Scalariform monstrosities are occasionally met with, especially in Helix and Planorbis, when the whorls become unnaturally elevated, and sometimes quite disjoined from one another; carinated monstrosities develop a keel on a whorl usually smooth; acuminated monstrosities have the spire produced to an extreme length (Fig. 158); sinistral monstrosities (see above) have the spire reversed: dwarfs and giants, as in our own race, are occasionally noticed among a crowd of individuals.

More serious forms of monstrosity are those which occur in individual cases. Mr. S. P. Woodward once observed[332] a specimen of an adult Helix aspersa with a second, half-grown individual fixed to its spire, and partly embedded in the suture of the body whorl. The younger snail had died during its first hibernation, as was shown by the epiphragm remaining in the aperture, and its neighbour, not being able to get free of the incubus, partially enveloped it in the course of its growth. In the British Museum two Littorina littorea have become entangled in a somewhat similar way (Fig. 160 B), possibly as a result of embryonic fusion. Double apertures are not uncommon[333] in the more produced land-shells, such as Cylindrella and Clausilia (Fig. 160 A). In the Pickering collection was a Helix hortensis which had crawled into a nutshell when young, and, growing too large to escape, had to carry about this decidedly extra shell to the end of its days. A monstrosity of the cornucopia form, in which the whorls are uncoiled almost throughout, is of exceedingly rare occurrence (Fig. 161).

Fig. 158.—Monstrosities of Neptunea antiqua L., and Buccinum undatum L., with a greatly produced spire (from specimens in the Brit. Mus.).

Fig. 159.—Monstrosities of Littorina rudis Mat, The Fleet, Weymouth. (After Sykes.)

Some decades ago ingenious Frenchmen amused themselves by creating artificial monstrosities. H. aspersa was taken from its shell, by carefully breaking it away, and then introduced into another shell of similar size (H. nemoralis, vermiculata, or pisana). At the end of several days attachment to the columella took place, and then growth began, the new shell becoming soldered to the old, and the spiral part of the animal being protected by a thin calcareous envelope. A growth of from one to two whorls took place under these conditions. The individuals so treated were always sordid and lethargic, but they bred, and naturally produced a normal aspersa offspring.[334] In the British Museum there is a specimen of one of these artificial unions of a Helix with the shell of a Limnaea stagnalis.

Fig. 160.—Monstrosities with two apertures: A, Cylindrella agnesiana C. B. Ad., Jamaica; B, Littorina littorea (from specimens in the British Museum).

Fig. 161.—Cornucopia-shaped monstrosity of Helix aspersa, from Ilfracombe. (British Museum.)

Composition of the Shell.—The shell is mainly composed of pure carbonate of lime, with a very slight proportion of phosphate of lime, and an organic base allied to chitin, known as conchiolin. The proportion of carbonate of lime is known to vary from about 99 p.c. in Strombus to about 89 p.c. in Turritella. Nearly 1 p.c. of phosphate of lime has been obtained from the shell of Helix nemoralis, and nearly 2 p.c. from that of Ostrea virginica. The conchiolin forms a sort of membranous framework for the shell; it soon disappears in dead specimens, leaving the shell much more brittle than it was when alive. Carbonate of magnesia has also been detected, to the extent of ·12 p.c. in Telescopium and ·48 p.c. in Neptunea antiqua. A trace of silica has also occasionally been found.

When the shell exhibits a crystalline formation, the carbonate of lime may take the form either of calcite or aragonite. The calcite crystals are rhombohedral, optically uniaxal, and cleave easily, while the aragonite cleave badly, belong to the rhombic system, and are harder and denser, and optically biaxal. Both classes of crystal may occur in the same shell.

Two main views have been held with regard to the formation and structure of the shell—(1) that of Bowerbank and Carpenter, that the shell is an organic formation, growing by interstitial deposit, in the same manner as the teeth and bones of the higher animals; (2) that of Réaumur, Eisig, and most modern writers, that the shell is of the nature of an excretion, deposited like a cuticle on the outside of the skin, being formed simply of a number of calcareous particles held together by a kind of ‘animal glue.’ Leydig’s view is that the shell of the Monotocardia is a secretion of the epithelium, but that in the Pulmonata it originates within the skin itself, and afterwards becomes free.[335]

According to Carpenter, when a fragment of any recent shell is decalcified by being placed in dilute acid, a definite animal basis remains, often so fine as to be no more than a membranous film, but sometimes consisting of an aggregation of ‘cells’ with perfectly definite forms. He accordingly divides all shell structure into cellular and membranous, according to the characteristics of the animal basis. Cellular structure is comparatively rare; it occurs most notably in Pinna, where the shell is composed of a vast multitude of tolerably regular hexagonal prisms (Fig. 162 B). Membranous structure comprises all forms of shell which do not present a cellular tissue. Carpenter held that the membrane itself was at one time a constituent part of the mantle of the mollusc, the carbonate of lime being secreted in minute ‘cells’ on its surface, and afterwards spreading over the subjacent membrane through the bursting of the cells.

The iridescence of nacreous shells is due to a peculiar lineation of their surface, which can be readily detected by a lens. According to Brewster, the iridescence is due to the alternation of layers of granular carbonate of lime and of a very thin organic membrane, the layers very slightly undulating. Carpenter, on the other hand, holds that it depends upon the disposition of a single membranous layer in folds or plaits, which lie more or less obliquely to the general surface, so that their edges show as lines. The nacreous type of shell occurs largely among those Mollusca which, from other details in their organisation, are known to represent very ancient forms (e.g. Nucula, Avicula, Trigonia, Nautilus). It is also the least permanent, and thus in some strata we find that only casts of the nacreous shells remain, while those of different constitution are preserved entire.

Porcellanous shells (of which the great majority of Gasteropoda are instances) usually consist of three layers, each of which is composed of a number of adjacent plates, like cards on edge. The inclination of the plates in the different layers varies, but that of the plates in the inner and outer layer is frequently the same, thus if the plates are transverse in the middle stratum, they are longitudinal in the inner and outer strata, and, if longitudinal in the middle, they are transverse in the other two. Not uncommonly (Fig. 163 B) other layers occur. In bivalves the disposition and nature of the layers is much more varied.

Fig. 162.—A, Section of shell of Unio: a, periostracal layer; b, prismatic layer; c, nacreous layer. B, Horizontal section of shell of Pinna, showing the hexagonal prisms.

In Unio the periostracal or uppermost layer is very thin; beneath this is a prismatic layer of no great depth, while the whole remainder of the shell is nacreous (Fig. 162 A). Many bivalves show traces of tubular structure, while in the Veneridae the formation and character of the layers approaches closely to that of the Gasteropoda. Further details may be gathered from Carpenter’s researches.[336]

Formation of Shell.[337]—The mantle margin is the principal agent in the deposition of shell. It is true that if the shell be fractured at any point, the hole will be repaired, thus showing that every part of the mantle is furnished with shell-depositing cells, but such new deposits are devoid of colour and of periostracum, and no observation seems to have been made with regard to the layers of which they are composed. As a rule the mantle, except at its margin, only serves to thicken the innermost layer of shell.

It is probable that the carbonate of lime, of which the shell is mainly composed, is separated from the blood by the epithelial cells of the mantle margin, and takes the crystalline or granular form as it hardens on exposure after deposition. The three layers of a porcellanous shell are deposited successively, and the extreme edge of the mouth, when shell is forming, will contain only one layer, the outermost; a little further in, two layers appear, and further still, three. The pigment cells which colour the surface are situated at the front edge of the mantle margin.

Fig. 163.—Sections of shells. A, Conus: a, outer layer; b, middle prismatic layer, with obliquely intersecting laminae above and below; c, inner layer. B, Oliva: a, outer layer; b, layer of crossed and curved laminae; c, prismatic layer, succeeded by layer of laminae at right angles to one another; d, inner layer. C, Cypraea: a, outer layer; b, middle layer; c, inner layer.

Shelly matter is deposited, and probably secreted, not only by the mantle, but also in some genera by the foot. This is certainly the case in Cymbium, Oliva, Ancillaria, Cassis, Distortio, and others, in several of which the foot is so large that the shell appears to be quite immersed in it.[338]

The deposition of shell is not continuous. Rest periods occur, during which the function is dormant; these periods are marked off on the edge of the shell, and are known as lines of growth. In some cases (Murex, Triton, Ranella), the rest period is marked by a decisive thickening of the lip, which persists on the surface of the shell as what is called a varix (see p. 263).

Fig. 164.—Murex tenuispina L., Ceylon. × ⅔.

Fig. 165.—Neritina longispina Récl., Mauritius. (Operculum removed.)

The various details of sculpture on the exterior surface of the shell, the striae, ribs, nodules, imbrications, spines, and other forms of ornamentation are all the product of similar and corresponding irregularities in the mantle margin, and have all been originally situated at the edge of the lip. Spines, e.g. those of Murex and Pteroceras, are first formed as a hollow thorn, cleft down its lower side, and are afterwards filled in with solid matter as the mantle edge withdraws. What purpose is served by the extreme elaboration of these spiny processes in some cases, can hardly be considered as satisfactorily ascertained. Possibly they are a form of sculptural development which is, in the main, protective, and secures to its owners immunity from the attacks of predatory fishes.

‘Attached’ genera (e.g. Chama, Spondylus) when living on smooth surfaces have a flat shell, but when affixed to coral and other uneven surfaces they become very irregular in shape. The sculpture of the base on which they rest is often reproduced in these ‘attached’ shells, not only on the lower, but also on the upper valve, the growing edge of which rests on the uneven surface of the base. Oysters attached to the branches of the mangrove frequently display a central convex rib, modelled on the shape of the branch, from which the plaits of sculpture radiate, while specimens fixed to the smooth trunk have no such rib. Crepidula, a genus which is in the habit of attaching itself to other shells, varies in sculpture according to that of its host. Sometimes the fact may be detected that a specimen has lived on a ribbed shell when young, and on a smooth one when old, or vice versâ. A new genus was actually founded by Brown for a Capulus which had acquired ribs through adhesion to a Pecten. A specimen of Hinnites giganteus in the British Museum must at one period of its growth have adhered to a surface on which was a Serpula, the impression of which is plainly reproduced on the upper valve of the Hinnites.[339]

Fig. 166.—A specimen of Anomia ephippium L., Weymouth, taken upon Pecten maximus, the sculpture of which is reproduced on the upper valve of the Anomia, and even on a young Anomia attached to the larger specimen.

Growth of the Shell.—Nothing very definite is known with regard to the rate of growth of the shell in marine Mollusca. Under favourable conditions, however, certain species are known to increase very rapidly, especially if the food supply be abundant, and if there is no inconvenient crowding of individuals. Petit de la Saussaye mentions[340] the case of a ship which sailed from Marseilles for the west coast of Africa, after being fitted with an entirely new bottom. On arriving at its destination, the vessel spent 68 days in the Gambia River, and took 86 days on its homeward voyage. On being cleaned immediately on its return to Marseilles, an Avicula 78 mm. and an Ostrea 95 mm. long (both being species peculiar to W. Africa) were taken from its keel. These specimens had therefore attained this growth in at most 154 days, for at the period of their first attachment they are known to be exceedingly minute. P. Fischer relates[341] that in 1862 a buoy, newly cleaned and painted, was placed in the basin at Arcachon. In less than a year after, it was found to be covered with thousands of very large Mytilus edulis, 100 mm. × 48 mm., the ordinary size on the adjoining banks being only about 50 to 60 × 30 mm.

Some observations have already been recorded (p. 40) on the growth of Helix aspersa. In the summer of 1858, which was very dry, especially in the south of France, the young Helices born that year were still very small in August. About the end of that month abundant rain came on, and in four or five days young H. variabilis, H. pisana, and H. aspersa, eating without cessation, as if to make up for lost time, grew more than a centimetre of shell. The lip of a young H. arbustorum has been observed to have grown, at the end of the first week in the season’s growth, 3 mm., at the end of the second week, 6·25 mm., the third, 11·5 mm., and the fourth 12·5 mm., with a finished lip.[342]

Careful observation has shown that in the growth of the shell of Helix aspersa the periostracum is first produced; it is covered with hyaline globules, 10–12 mm. in diameter, which persist even in the oldest shells. Calcareous matter is deposited on the internal face of the new periostracum, at some distance from the margin. It is secreted by a white zone or band of cells bounding the entire breadth of the mantle as applied to the peristome. Immediately behind the white zone are a series of pigment cells which not only give the shell its colour but complete the calcification of the shelly matter laid down by the white zone. When the animal has attained its full growth and the lip is finished off, the white band and the periostracum cells completely disappear, and only such cells persist as contribute to the internal thickening of the shell. Shell growth, in this species, is very rapid. If a portion of the pulmonary sac is laid bare, by removing a fragment of shell, at the end of 1½ or 2 hours there may be detected a delicate organic membrane covering the hole, and strewn with crystals of carbonate of lime. This thickens with great rapidity, and soon fills up the hole with solid matter. For two consecutive months an animal, deprived of food, has been known to reproduce this membrane daily after its removal every morning.[343] Prof. Schiedt has found that oysters, if deprived of the right valve and exposed to the light, not only develop brown pigment over the whole exposed surface of mantle and branchiae, but actually succeed in part in reproducing the valve and hinge.[344]

Deposit of Additional Layers of Shell.—Mollusca possess the power of thickening the interior of the shell, by the deposit of successive layers. This is frequently done in self-defence against the attacks of boring Mollusca, sponges, and worms. Cases may often be noticed of Ostrea, Spondylus, and other sedentary molluscs, which, unable to escape the gradual assaults of their foes, have provided against them by the deposit of fresh shelly matter. A somewhat similar plan is adopted to provide against intrusion by way of the aperture. Pearls are, in many cases, the result of shell deposition upon the eggs or even the body of some intrusive parasite (Distoma, Filaria, etc.), and are, in some countries, artificially produced by the introduction of fragments of sand, metal, etc., into living Unio and Anodonta. Little joss images are made in India and China, the nacre on which is produced by thrusting them inside living Unionidae.

A specimen of Helix rosacea, in the British Museum, into whose shell a piece of grass somehow became introduced, has partitioned it off by the formation of a sort of shelly tunnel extending throughout its entire length (Fig. 167).

Fig. 167.—A specimen of Helix rosacea Müll., Cape of Good Hope, into which a piece of grass has by some means become introduced. The animal has protected itself by covering the grass with a shelly layer. (From a specimen in the British Museum.)

Absorption of Internal Portions.—Certain genera have the remarkable property of absorbing, when they become adult, the internal portions of the whorls and the greater part of the columellar axis. The effect of this is to make the shell, when the process is complete, no longer a spiral but a more or less produced cone, and it is found that in such cases the viscera of the spire lose their spiral form, and take the shape of the cavity in which they lie. Amongst the genera in which this singular process takes place are Nerita,[345] Olivella, and Cypraea amongst marine forms, and nearly the whole of the Auriculidae[346] (Fig. 168). Conus reduces the internal subdivisions of the spire to extreme thinness. It is noticeable that these genera are all of considerable thickness of shell, and it is perhaps the result of the whole energy of the animal being directed to the formation of its external protection that the internal walls of the spire become atrophied and eventually disappear.

Fig. 168.—Auricula Judae Lam., showing the disappearance of the partitions of the whorls, which are represented by dotted lines. (After Fischer.)

Fig. 169.—A, Decollated (adult) form, and B, perfect (young) form of Cylindrella nobilior Ad., Jamaica; the dotted line shows where decollation takes place.

Fig. 170.—Development of Coecum: A, showing the gradual formation of septa; a, apex; ap, aperture; ss, first septum; s´s´, second septum. (After de Folin.) B, Adult form of C. eburneum Ad., Panama. × 8.

Decollation.—In certain genera, when the shell becomes adult, the animal ceases to occupy the upper whorls, which accordingly die and drop off, the orifice at the top having meanwhile been closed by a shelly deposit. Such shells are termed decollated. In some land genera decollation is the rule, e.g. in Cylindrella (Fig. 169), Eucalodium, and Rumina, as well as in many species of the brackish water genera, Truncatella, Cerithidea, and Quoyia. Stenogyra (Rumina) decollata, a common shell in the south of Europe, has been noticed to bang its upper whorls violently against some hard substance, as if to get rid of them.

Fig. 171.—Four stages in the growth of Fissurella, showing how the spire gradually disappears and the marginal slit becomes an apical hole, A, B, C, highly magnified, D, natural size. (After Boutan.)

Fig. 172.—Three stages in the growth of Cypraea exanthema L. (From specimens taken at Panama.)

Special Points in the Growth of Certain Genera.—In the young of Coecum the apex is at first spiral, but as growth proceeds and the long tube begins to form, a septum is produced at the base of the apex, which soon drops off. Soon afterwards, a second septum forms a little farther down, and a second piece drops off, leaving the shell in the normal cylindrical form of the adult (Fig. 170). The development of Fissurella is of extreme interest. In an early stage it possesses a spiral shell, with a slit on the margin of the outer lip of the last whorl. As growth advances, shelly matter is deposited on both margins, which results in the slit becoming a hole and the spire a mere callosity, until at last they appear to coalesce in the apex of the adult shell (Fig. 171). The singular formations of Magilus and Rhizochilus have already been described (pp. 75, 76). Cypraea, in the young stage, is a thin spiral shell with a conspicuous apex. As growth proceeds, the surface of the whorls, which are nearly enveloped by two large lobes of the mantle, becomes overlaid with new layers of shelly matter, until eventually the spire becomes embedded, and ultimately disappears from view (Fig. 172).

Patella, when young, has a nautiloid shell (see Fig. 45, p. 134), but it is a remarkable fact that we are entirely ignorant, in this commonest of molluscs, of the transition stages which convert the nautiloid into the familiar conical shell. The young shell of Pteroceras is deceptively unlike the adult, and is entirely devoid of the finger-like processes which are so characteristic of the genus (chap. xiv.).

Among the bivalve Mollusca, Anomia in a young stage is not to be distinguished from Ostrea. Soon a small sinus appears on the ventral margin, which gradually deepens and, as the shell grows round it, forms a hole for the byssus, eventually becoming fixed beneath the umbones (see Fig. 173). In Teredo the two valves of the shell proper, which is very small, become lodged in a long calcareous tube or cylinder, which is generally open at both ends (see chap. xvi.). In Aspergillum a somewhat similar cylinder is developed, but the valves are soldered to the tube, and form a part of it, the tube itself being furnished, at the anterior end, with a disc, which is perforated with holes like the rose of a watering-pot. In Clavagella the left valve alone becomes soldered to the tube, while the right valve is free within it (see chap. xvi.). Fistulana encloses the whole of its shell in a long tapering tube, which is not at any point adherent to the shell.

Fig. 173.—Development of the byssus or the plug-hole in Anomia. (After Morse.)

Terms employed to denote various Parts of the Univalve Shell.—The apex is the extreme top of the spire, and generally consists of the embryonic shell, which may often be recognised by its entire want of sculpture. When the embryonic shell happens to be large, the apex is often mammillated, e.g. in Fusus, Neptunea, and some Turbinella; in the Pyramidellidae it is sinistral.

The suture is the line of junction between any two successive whorls. It may be deep, and even channelled, or very shallow, as in Fig. 150 B (p. 246).

The spire is the whole series of whorls except the last or body whorl. A whorl is a single revolution of the spiral cone round the axis. The spire may be subulate (as in Terebra, Fig. 150 C), turreted (Scalaria), depressed (Polygyratia, Fig. 150 A), conical (Trochus), globose (Ampullaria, Natica, Fig. 150 B), with almost all conceivable gradations between these types. The number of whorls is best counted by placing the shell mouth downwards, and reckoning one for every suture that occurs between the extreme anterior point of the shell and the apex.

Fig. 174.—Illustrating the technical terms applied to the various parts of a univalve shell.

The mouth or aperture may be (a) entire, as in Helix, Natica, Ampullaria, when its peristome or margin is not interrupted by any notch or canal, or (b) prolonged at its anterior and sometimes also at its posterior end into a canal. The anterior canal serves as a protection to the siphon,[347] the posterior canal is mainly anal in function, and corresponds, in part, to the hole of Fissurella, the slit in Pleurotoma and Emarginula, and the row of holes in Haliotis. The mouth presents every variety of shape, from the perfect circle in Cyclostoma and Trochus, to the narrow and prolonged slit in Conus and Oliva.

Fig. 175.—Anal slit in Pleurotoma.

The right margin of the mouth (the left, in sinistral shells) is termed the outer lip or labrum, the left margin the inner lip, labium, or columella lip.[348] In young shells the outer lip is usually thin and unfinished, while in the adult it is generally thickened into a rib, or furnished with more or less prominent teeth, or given an inward or outward curve. In some genera, especially the Strombidae, the outer lip of the adult develops long finger-like processes, which sometimes attain an extraordinary size (chap. xiv.). As growth proceeds, these marginal teeth and ribs are either dissolved and disappear, or are permanently incorporated, in the shape of varices, with the framework of the shell. Some shells, e.g. Natica, Turritella, Actaeon, have a permanently unfinished outer lip, even in the adult stage. The columella lip varies in shape with the mouth as a whole; thus it may be straight, as in Conus, or excavated, as in Sigaretus, Struthiolaria, and Bulla. Frequently it is continued by part of the body whorl, as in Ficula, Dolium, and Fasciolaria.

Fig. 176.—Solarium perspectivum Lam., from the under side.

Fig. 177.—Section of Turbinella pyrum L., showing the plicae on the columella and the growth of successive whorls.

The folds or plaits on the columella, which are often characteristic of the genus or even family (e.g. Fasciolariidae, Mitridae, Turbinellidae) are not merely external, but continue down the whole spire (see Fig. 177, which also shows how successive fresh growths have thickened the columella).

The whorls may be wound in a spiral, which is either hollow, as in Solarium, or quite compact, as in Oliva, Terebra, Cypraea, with every possible intermediate grade. This concavity, which varies in depth and width, is known as the umbilicus, and shells are accordingly spoken of as deeply (e.g. most Trochidae and Naticidae), narrowly (e.g. Lacuna, Littorina), or widely (e.g. Solarium) umbilicated. When the spiral is quite flat, as in Planorbis and some Helix, the umbilicus vanishes entirely. Shells in which the whorls are so compactly coiled on an ascending spiral that there is no umbilicus, are termed imperforate.

Fig. 178.—The slit in A, Hemitoma, B, Emarginula, C, Macroschisma, D, Craniopsis, E, Puncturella, F, Fissurella.

The Slit.—Many shells are furnished with a slit in the last whorl, which opens, in most cases, on the outer lip, and is sometimes of considerable depth, at others a mere notch. In the patelliform shells it is always in front of the apex. The function of the slit appears to be mainly anal, the excretory products being thus allowed to escape by a passage of their own, without soiling the clean water taken in by the branchiae. The posterior canal of some Gasteropoda probably performs a similar function. In the adult Fissurella the slit becomes an apical hole (see Fig. 178 F), in the allied genera it is either immediately in front of the spire (Puncturella), or half-way between the spire and the anterior margin (Rimula), or on the margin and well marked (Emarginula), or a mere indentation of the margin (Hemitoma). In Pleurotomaria it is exceptionally long, and is well marked in Bellerophon, Schismope, Scissurella, Murchisonia, and Pleurotoma (where it is sutural). In Haliotis and Polytremaria it is replaced by a series of holes, which are closed up as the animal grows past them. Some of these holes (at least in Haliotis) certainly serve the purpose of admitting water to the branchiae, while others are anal. In Trochotoma there are only two holes, united by a narrow fissure.

The Tubed Land Operculates.—A group of the Cyclophoridae, which is restricted to Further India and the great Malay Islands, has developed a remarkable sutural tube on the exterior of the last whorl, near the aperture, A similar tube, but more obscure, exists in Alycaeus. Several stages in the development of this tube may be noticed, beginning with the elevation of part of the peristome into a simple irregular shelly plate, which is continued, first into a short, and then into a long tube, which becomes soldered to the shell; finally, the tube becomes free, and the anterior part of the last whorl is disconnected from the spire (Fig. 180 A-D).

Fig. 179.—The slit in A, Bellerophon, B, Pleurotomaria, C, Schismope, D, Polytremaria, E, Haliotis (not drawn to scale).

Fig. 180.—Development of the tube in the tube operculates: A, Pterocycus rupestris Bens.; B, Opisthoporus birostris Pfr.; C, Spiraculum travancoricum Bedd.; D, Rhiostoma Housei Pfr.

It is singular that the tube does not appear to be of any use to the animal, since its internal extremity, in the complete form, is closed, and does not communicate with the interior of the whorl. It may be presumed, however, that in origin the tube served as a means of conveying air to the animal when the operculum was closed. The holes in the peristome of Pupina, Cataulus, and Anostoma (Fig. 154) may be compared.

Fig. 181.—Eburna spirata Lam., E. Indies. F, foot; OP, operculum; P, penis; S, siphon; T, tentacles, with eyes at their base. (After Souleyet.)

The Operculum.—The operculum is a cuticular development of a group of cells situated on the dorsal side of the foot, exactly over the terminal point of the fibres of the columellar muscle. It is so situated that in crawling it is generally carried free of the shell, sometimes at the extreme upper end of the foot, more usually somewhat nearer to the shell (Fig. 181). In Pterocyclus it is pushed back into the umbilicus when the animal is in motion.

The operculum is present in nearly all land, fresh-water, and marine Prosobranchiata, absent in all Opisthobranchiata in the adult state, except Actaeon, and in all Pulmonata, except Amphibola. It has been lost in the following marine Prosobranchiata: many Cancellariidae and Conidae, Oliva (though present in Olivella and Ancilla), Harpidae, Marginellidae, Voluta proper (though present in V. musica), nearly all Mitridae, Cypraeidae, Doliidae, Ianthinidae; and, of land genera, in Proserpinidae. It is evident, therefore, that its presence or absence is of limited value in classification. In some species of Ampullaria and Natica it is horny, in others shelly. Dall found that in a number of specimens of Volutharpa ampullacea, 15 p.c. had opercula, 10 p.c. traces of the operculigenous area, but no operculum, the rest no trace of either. Monstrosities of Buccinum undatum sometimes occur, which have two, or in rare case three opercula.

As a rule, the operculum exactly fits the mouth of the shell. But in cases where the mouth is very large (e.g. Conus, Strombus, Concholepas, some Bullia), it only covers a very small portion and is quite inadequate as a protection (Fig. 62, p. 155). Again, when the shell has assumed a more or less limpet-shaped form, and habitually adheres to flat surfaces without much occasion for locomotion, the operculum becomes degraded and is probably on the way to being lost altogether. This is the case with Navicella (a modified Nerita, see Fig. 13, p. 17), Concholepas (a modified Purpura), Sigaretus (a modified Natica). Probably the more completely patelliform shells of Crepidula, Haliotis, Fissurella, and Patella have reached the stage at which the operculum has been lost entirely. In Navicella, besides becoming degraded, the operculum has actually become partly internal, and apparently serves the purpose of separating the viscera from the upper part of the foot, something like the shelly plate in Crepidula. This explains why the operculum in this genus is polished on both sides.[349]

Some authors have imagined that the operculum is homologous (a) to the second valve in Pelecypoda, (b) to the byssus. It differs, however, morphologically from the former in the essential point of not being produced by the mantle, and from the latter in not being produced by a special gland.

Turbo (Sarmaticus) Turbo (Callopoma) Livona Ampullaria Natica

Fig. 182.—Various forms of opercula.

As regards shape and formation, the operculum has usually a more or less well-marked nucleus which may be central (e.g. Livona), sub-central (Ampullaria), lateral (Purpura), or terminal (Pyrula). As a rule, both the inner and outer surfaces are fairly flat, but in Torinia, Cyathopoma, and Pterocyclus the outer surface is elevated and conically spiral, in some Turbo (e.g. Sarmaticus) it is covered with raised tubercles resembling coral, while in others (e.g. Callopoma) it is scored with a deep trench. Aulopoma, a land genus peculiar to Ceylon, has a paucispiral operculum with hollow whorls, deceptively like a Planorbis; it fits over the aperture instead of into it. In Livona and most Trochidae the operculum is cartilaginous and multispiral. In Strombus it is narrow, curved, and often serrated like a leaf on one of the edges; in Conus it is narrowly oblong and rather featureless; in Littorina, paucispiral and always cartilaginous. In many cases (e.g. Paludina) there is no true spiral form, but the striae are concentric to a nearly central nucleus, and thus give the appearance of a spiral. The evolution of the operculum in Navicella from Nerita has already been illustrated (p. 10). Neritopsis has a very remarkable operculum, the striated appendage of which locks behind the columella of the shell, like the tooth in the opercula of the Neritidae.

Pyrula Purpura Littorina Aulopoma × 3 Torinia × 2 Neritopsis Strombus Conus × ³/₂

Fig. 183.—Various forms of opercula.

Terms employed to denote various parts of the Bivalve Shell.—The umbo, or beak, is the apex of the hollow cone, of which each valve may be regarded as consisting. This apex is usually more or less twisted: it is markedly spiral in Isocardia, Diceras, some Chama, and especially Requienia, while in Pecten, Lepton, and others the spiral is altogether absent. As a rule the umbones point forward, i.e. towards the anterior end of the shell. In Donax, Nucula, and Trigonia, however, they point backward. The umbones are generally more or less approximated, but in Arca they are widely separated.

An equilateral shell is one in which the umbones are more or less central with regard to its anterior and posterior portion, while in an inequilateral shell the umbones are much nearer one end than the other. On the other hand, equivalve and inequivalve are terms used to express the relation of the two valves to one another as a whole. Thus nearly all bivalve shells are more or less inequilateral, but a comparatively small proportion are inequivalve.

The dorsal margin is adjacent to, the ventral margin opposite to, the umbones. The anterior and posterior margins are respectively the front and hinder edges of the shell.

The muscles which serve to close the valves leave impressions on the inner surface of each valve. These, when both muscles are present, are known as the anterior and posterior adductor impressions. The impression produced by the muscular edge of the mantle, which curves downwards and backwards from the anterior adductor impression, is known as the pallial line. In shells with only one muscle it is represented by an irregular row of small marks, or disappears altogether (Ostrea). The pallial sinus is produced by the muscles which retract the siphons, and is most marked in those genera in which the muscles are powerful and the siphons large (e.g. Tellina, Mya). It is entirely absent in genera possessing no retractile siphons.

Fig. 184.—Left valve of Venus gnidia L.: A, anterior, B, posterior, C, dorsal, D, ventral margin, AB, length, CD, breadth of shell.

a.m, anterior; p.m, posterior adductor muscle; p, pallial line; p.s, pallial sinus; l, ligament; lu, lunule; u, umbo; c, cardinal teeth; a.l, anterior lateral tooth; p.l, posterior lateral tooth.

Fig. 185.—Right valve of Lucina tigerina L.: A, anterior, B, posterior, C, dorsal, D, ventral margin; AB, length, CD, breadth of shell.

a.m, anterior; p.m, posterior adductor muscles; p, pallial line; l, ligament; u, umbo; c, cardinal teeth; a.l, p.l, anterior and posterior lateral tooth.

Right and Left Valve.—The simplest way of distinguishing the valves as right and left is to hold the shell in such a way that the siphons point towards the observer, and the mouth away from him; in this position the valve to the right is called the right valve, and the valve to the left the left valve. If, however, the animal is not present, it may be remembered that the ligament is nearly always behind the beaks, and that the beaks, as a rule, point forward, thus the right and left valves can generally be named by observation of the beaks and ligament. When the ligament is median to the valves (e.g. Ostrea, Pecten), and the beaks are not curved, the valves may be recognised by noting the fact that the impression of the adductor muscle (in these cases always single) is nearer to the posterior than to the anterior side. In a similar way the pallial impression, which only forms a sinus on the posterior side, furnishes a guide to the valves of Donax, in which the beaks point backward, and of Tellina, in which the beaks are frequently central.

In the fixed inequivalves (e.g. Chama) it is sometimes the right, sometimes the left valve which is undermost, but the fixed valve, whether right or left, is always deep, and the free valve flat. Ostrea and Anomia are always fixed by the left valve.

The lunule is a well-marked area in front of and close to the umbones, usually more or less heart-shaped, and limited by a ridge. Generally it is shallow, but sometimes, as in Dosinia, Opis, and some Cardium, deeply impressed. A corresponding area behind the umbones, enclosing the ligament, is called the escutcheon (Fig. 186), but it seldom occurs.

The ligament is a more or less elastic band, which unites the two valves along a line adjacent to the umbones. As a rule, the greater part of the ligament is external to the shell, but it may be entirely internal. It is placed, normally, behind the umbones, but in a few cases, when the hinge line is very long (Arca, Pectunculus), it extends in front of them as well. The edges of the valves, when the ligament is mainly external, are more or less excavated for its reception. When internal it is generally contained in a groove or spoon-shaped pit, known as the fossette (compare Fig. 187).

Fig. 186.—Venus subrostrata Lam.: es, escutcheon; li, ligament; lu, lunule; u, u, umbones.

The ligament consists of two distinct parts, which may occur together or separately, the external, or ligament proper, and the internal, or cartilage. Only the external portion can be seen when the valves are closed. As a rule, the two portions are intimately connected with one another, the ligament folding over the cartilage, but in some cases, e.g. Mya, Mactra, where the cartilage is lodged within the hinge, they are completely disconnected (Fig. 187).

In Pecten the external ligament is very thin, and runs along the dorsal margin, while the internal ligament is large, solid, and situated in a shallow pit. In Perna, where the hinge is toothless, the ligament is folded into a number of transverse ridges, which fit into corresponding grooves in the shell.

The ligament proper is inelastic and insoluble in caustic potash. The cartilage is very elastic, composed of parallel fibres, slightly iridescent, and soluble in caustic potash.

The operation of the ligament—using the word as including the whole ligamental process—is in opposition to that of the adductor muscles. When the latter close the valves, they compress the ligament, an action which its elasticity resists: thus its operation tends in part towards keeping the valves open. But when ligament and cartilage are both fully developed, they work in opposition to one another, the ligament, by its resistance to compression, preventing any straining of the adductor muscles when the valves are open, and the cartilage, for the same reason, preventing the ventral margins of the shell from closing too rapidly upon one another when the valves are being shut.

Fig. 187.—Hinge of A, right valve, and B, left valve of Mulinia edulis King; ca, cardinals; l.a, anterior laterals; l.p, posterior laterals; f, fossette; c, cartilage; l, ligament.

The Hinge.—The valves of Pelecypoda are generally articulated, below the umbones, by a hinge which is furnished, in the majority of cases, with interlocking teeth, small pits or depressions in each valve corresponding to the teeth in the other. The teeth are distinguished as cardinal, or those immediately below the umbo, and lateral, or those to either side of the cardinals, the latter being also distinguished as anterior and posterior laterals, according as they are before or behind the umbo (Fig. 184). In shells which are tolerably equilateral there is no difficulty in distinguishing between cardinal and lateral teeth. But when they are very inequilateral, the whole hinge may share in the inequality of growth, and an anterior lateral may be thrown backward and simulate a cardinal, or a cardinal may be thrown backward and simulate a posterior lateral (e.g. Cardita, Unio, Fig. 188). In many Chama the cardinals are pushed up into the umbo and become a mere ridge, while the strong anterior lateral becomes nearly central and simulates a cardinal.

Fig. 188.—Hinges of A, Cardita semiorbiculata Brug., and B, Unio rectus Lam., showing how, in inequilateral shells, the lateral teeth tend to shift their position. a.m, anterior adductor, p.m, posterior adductor muscle; c, c, cardinal teeth; p.l, posterior lateral teeth; l, ligament.

Some bivalves, e.g. Anodonta, Ostrea, Pedum, many Mytilus, have no hinge teeth at all, in others the laterals are wanting (Psammobia, Diplodonta). In the Arcadae the hinge consists of a number of very similar denticles, which are often serrated like the teeth of a comb (Fig. 189).

Fig. 189.—The hinge in Arcadae: A, Nacula Loringi Ang. × ⁴/₃; B, Arca granosa L.; u.a, umbonal area.

Fig. 190.—A, Tridacna scapha Lam.; B, Cardium enode Sowb., showing the interlocking of the ventral margins.

Hinge-teeth are probably, in origin, derived from the crenulations or ribbings of the surface of the shell, the upper ends of which impinge upon the dorsal margin and mark it in a way which is quite recognisable when the shell is thin. Similar crenulations, resulting in interlocking of the valves, are not uncommon upon the ventral margin in certain genera (Fig. 190). The mechanical effect of these continued riblets, when fitted together on the opposing valves, would be to prevent the valves sliding upon one another while closing, or after being closed. Thus there would be a probability of their surviving, even after the ribbing had disappeared from the surface of the shell, the increased strength given by the hinge compensating for, and making it possible to do without, the extra strength supplied by the ribs. It is therefore possible that the teeth of the Nuculidae and Arcadae, which have no distinction between cardinals and laterals, represent a very ancient type, from which have been evolved the various forms of hinge in which cardinals and laterals are distinguished. Even in some forms of Arcadae (comp. Pectunculus) we get a hint how the transverse teeth of the typical Arca may have become transformed into the longitudinal tooth of the normal lateral.[350]

The developed hinge-teeth, then, ensure the opening of the valves in one direction; they also secure their accurate closure upon one another in exactly the same plane. Exposed shells and gaping siphons matter little to animals which are protected by their burrowing propensities, but to those which live in material which can be easily penetrated by their foes, it must be of advantage to be able to buckle their armour absolutely tight. The edentulous hinge of Anodonta is a degeneration from a dentate type, which retains its teeth (in Unio, etc.) when subject to the jar of rapid streams, but tends to lose them in the stiller waters of canals, lakes, and ponds.

Other processes in the bivalve shell.—In Anatina each umbo is fissured and strengthened on the inside by a kind of umbonal plate which carries the ligament. Some forms of Liligna develop a strong internal umbonal rib, which serves as a buttress to strengthen the shell. In Pholas, the so-called falciform process serves as a point of attachment for the muscles of the foot and viscera. There is no ligament or hinge-teeth, the place of the latter being taken by the anterior adductor muscle, which is attached to the hinge-plate, the latter being reflected back into the shell.

In Septifer the anterior adductor muscle is carried on a sort of shelf or myophore, and in Cucullaea the posterior adductor is partly raised on a similar and very prominent formation.

Length and breadth of bivalve shells is variously measured. Most authorities measure length, or ‘antero-posterior diameter,’ by a straight line drawn from the extreme anterior to the extreme posterior margin, and breadth by a similar line, drawn from the umbones to a point, not very clearly marked, on the opposite ventral margin (see Figs. 184 and 185). Others, less correctly, reverse these terms. Thickness is measured by the extreme distance of the opposite faces of the closed valves. As a rule, the length exceeds, and often greatly exceeds, the thickness, but in a few cases—e.g. the Cardissa section of Cardium—this is reversed.

The periostracum.—Nearly all shells are covered, at some period of their growth, by a periostracum,[351] or surface skin, which serves the purpose of protecting the shell against the destructive effects of the chemical action set up by water or air. It also, in some cases (see p. 258), acts as a kind of base upon which the shell is deposited. In old shells it is commonly worn away, especially at those parts which are likely to become abraded.

The form and composition of the periostracum varies greatly. Sometimes (e.g. Oliva) it is a mere transparent film, at others (Zonites) it is transparent, but stout and solid. It is corneous in Solenomya, covered with fine hairs in many Helicidae, in Conus, Velutina, and Cantharus it is thick, fibrous, and persistent; in Trichotropis and some Triton it is furnished with long bristles on a thick ground (Fig. 191). In fresh-water shells it is usually rather thick, in order to protect the shell from the erosive powers of certain kinds of water. In some cases (Mya, Anatina) the periostracum is continued over the siphons, so as to form a protection throughout their whole length.

Fig. 191.—Triton olearium L., Mediterranean, an example of a shell with a stout and hairy periostracum. × ½.

Erosion.—The fresh-water Mollusca generally, and marine mollusca in a few rare cases (Purpura, Littorina) are subject to erosion, or decay in the shell substance. In univalves erosion usually sets in near the apex (Fig. 192), where the life of the shell may be regarded as weakest, and in bivalves near the umbones. It is commonest in old shells, and rarely occurs in the very young. So long as the periostracum is present to protect the shell, erosion cannot set in, but when once it has been removed the shell is liable to the chemical changes set up in its substance by water. There is abundant evidence to show that erosion is caused by pollution of water. Out of many instances one must suffice. In a certain stream near Boston, U.S., numbers of Mollusca occurred, the shells of which were very perfect and free from disease. Some little way down stream an alkaline manufactory drained its refuse into the water. At and below this point for some distance every shell was more or less eroded, most of them seriously. Farther down, when the alkali refuse became diluted away, the shells retained their normal condition.[352]

Fig. 192.—Example of an eroded fresh-water shell (Melania confusa Dohrn, Ceylon).

A small percentage of lime in the water appears to produce erosion. The result of some experiments by G. W. Shrubsole, in the investigation of this point, may be tabulated as follows:[353]—

CHAPTER X GEOGRAPHICAL DISTRIBUTION OF LAND AND FRESH-WATER MOLLUSCA—THE PALAEARCTIC, ORIENTAL, AND AUSTRALASIAN REGIONS — A. The Palaearctic Region — B. The Oriental or Palaeotropical Region — C. The Australasian Region

The Mollusca afford specially valuable evidence on problems of geographical distribution. This fact is largely due to their extreme susceptibility to any change in the conditions of life. Genera which are accustomed to live in a certain temperature and on certain food, cannot sustain life if the temperature falls or rises beyond certain limits, or if the required food be not forthcoming. There is therefore a marked contrast between the Mollusca of the tropics and of the temperate zones, while different regions in the same latitude, whether within or without the tropics, often show great diversity in their fauna. Every region is thus characterised by its Mollusca. The Mollusca, for instance, of Australia or of South Africa characterise those countries quite as much as do the kangaroo and the emu, the hartebeest and the ostrich; there is nothing like them anywhere else in the world. In the Greater Antilles the Mollusca stand out beyond all other forms of life as characteristic of the islands as a whole, and of each separate island in particular.

The geographical distribution of the land and fresh-water Mollusca must be considered quite apart from that of the marine Mollusca. The sea offers no such serious barriers to the spread of the latter as the land does to the spread of the former. If we were to journey to the Azores, and turn our attention to the land-snails, we should find them almost wholly peculiar, while amongst the sea-shells we should recognise many as occurring also on our southern coasts, and few that were different from those of the Mediterranean. The marine Mollusca of the Sandwich Islands, in spite of the enormous intervening distance, are not very different from those of Natal, but the land Mollusca of the two countries are as widely different as is possible to imagine.

Land Mollusca are, as has been remarked, fettered to the soil. Quadrupeds, birds, fishes, and reptiles are provided with organs of motion which enable them to overpass barriers of various kinds. Even plants, although themselves incapable of motion, may be conveyed in every direction by means of seeds, which are either wafted by the wind or adhere to the skin of animals. But the Mollusca have no such regular means of transport, and are, in a large number of instances, limited to districts of a certain character of soil, or producing certain kinds of vegetation.

The localisation, both of genera and species, occurs all over the world. The genus Achatinella, which is peculiar to the Sandwich Islands, is found there in a profusion of species. It lives in the mountain valleys which radiate from the central ridge of each island, and each valley is characterised by its own peculiar set of species. The great carnivorous Glandina is restricted to Central America and the adjacent parts of the two continents, with one or two species in Southern Europe. Bulimus proper is restricted to South America; Achatina to Africa south of the Sahara; Tornatellina to the Pacific Islands; Cochlostyla to the Philippines; Cylindrella and Bulimulus are peculiar to the New World; Buliminus, Nanina, Scarabus, and Cassidula to the Old.

Extreme cases of this restriction of habitat sometimes occur. Thus Limnaea involuta is found only in a single small mountain tarn in Ireland; Clausilia scalaris along a narrow strip of limestone in Malta; Strophia nana is confined to a few square rods on an island that is itself a mere dot in the Caribbean Sea; the genus Camptonyx occurs only in the neighbourhood of Mt. Girnar, in Gujerat; and Lantzia in moss on the top of a mountain in Bourbon.

Attempts to colonise snails in strange localities have usually resulted in failure, especially when the attempt has involved serious changes of environment. The common Cochlicella acuta of our coasts resists all endeavours to establish it beyond a certain distance from the sea. Snails brought from the Riviera and placed under almost similar conditions of climate on our own southern coasts have lived for a while, but have very rarely taken permanent root. Mr. H. W. Kew[354] has collected a good many of these attempts to acclimatise species, the general success of which seems to depend almost entirely on a restoration of the old conditions of life.

At the same time there are certain species which exhibit a curiously opposite tendency, and which seem capable of flourishing in almost any part of the world, and under the most varied surroundings. Our own common garden snail (Helix aspersa) is a striking instance of this adaptability to new conditions. It has been established, by art or by accident, in Nova Scotia, Maine, South Carolina, New Orleans, California, Mexico city, Cuba, Hayti, Cayenne, Brazil, Valparaiso, Cape Town, the Azores, St. Helena, Mauritius, Loyalty Islands, and Australia. The great Achatina fulica of East Africa has been established first in Mauritius, and from thence has been carried to the Seychelles and Calcutta. Helix lactea, a common Mediterranean species, has been carried to Teneriffe and Montevideo; Helix similaris, whose fatherland is Eastern Asia, has been transported to Mauritius, Bourbon, West Africa, West Indies, Brazil, and Australia; Ennea bicolor (Eastern Asia) to India, Bourbon, Mauritius, West Indies; Stenogyra decollata (Mediterranean basin) to South Carolina; S. Goodallii (West Indies) to British pineries; Helix Hortensis to New Jersey. Seven common English species (Limax gagates, Hyalinia cellaria, H. alliaria, Helix aspersa, H. pulchella, Pupa umbilicata) have become naturalised in St. Helena,[355] and as many as nineteen in Australia.[356]

Cases of artificial transport of this kind are readily detected; they follow the lines of trade. The snails themselves or their ova have been accidentally enclosed with plants or mould, or have adhered to packing-cases, or to hay and grass used in packing. Thus they constitute no disturbance to the general rule of the persistent localisation of species and genera, and there is little fear that the evidence which the geographical distribution of the Mollusca brings to bear upon the general problems of distribution will be confused by any intermixture of fauna naturally distinct.

Land Mollusca: Barriers to Dispersal.—The chief natural barriers to dispersal are extremes of temperature, the sea, mountain ranges, and deserts. Rivers, however large, seem of little effect in checking dispersal. There is no appreciable difference between the land Mollusca north and south of the Ganges, or north and south of the Amazon. Living snails, or their ova, are no doubt transported from one bank to another on floating débris of various kinds. The barrier offered by the sea is obvious, and at first sight appears insurmountable; but the facts with regard to oceanic groups of islands like the Azores and Canaries (see p. 297) show that even a stretch of salt water many hundred miles in breadth may be ineffectual in preventing the dispersal of Mollusca.

Mountain ranges, provided they are too high to be scaled, and too long to be turned in flank, offer a far more effective barrier than the sea. Every thousand feet upward means a fall of so many degrees in the mean temperature, and a change, more or less marked, in the character of the vegetation. There is generally, too, a considerable difference in the nature of the climate on the two sides of a great mountain range, one side being often arid and cold, the other rainy and warm. The combined effect of these influences is, as a rule, decisive against the dispersal of Mollusca. Thus the Helices of California are almost entirely peculiar; one or two intruders from states farther east have succeeded in threading their way through the deep valleys into the Pacific provinces, but not a single genuine Californian species has been able to scale the heights of the Cascade Mountains. The land Mollusca of India are numbered by hundreds; not one penetrates north of the Himalayas. According to Mr. Nevill,[357] the change from the Indo-Malayan to the so-called European molluscan fauna at the northern watershed of the Kashmir valley is most abrupt and distinct; in two days’ march northward, every species is different. Ranges of inferior altitude, such as the Pyrenees, the Carpathians, or the Alleghanies, may be turned in flank as well as scaled, and we find no such marked contrast between the Mollusca on their opposite sides.

The most effective barrier of all, however, is a desert. Its scorching heat, combined with the absence of water and of vegetable life, check dispersal as nothing else can. The distribution of the Mollusca of the Palaearctic Region is an excellent instance of this. Their southern limit is the great desert which stretches, with scarcely a break, from the west coast of Africa to the extreme east coast of Asia. The Mediterranean offers no effectual barrier; shells of southern Europe are found in profusion in Morocco, Tunis, and Egypt, while all through Siberia to the extreme of Kamschatka the same types, and even the same species, of Mollusca occur.

A detailed examination of the means, other than voluntary, by which Mollusca are transported from one place to another hardly comes within the scope of this work. Ocean currents, rivers, floods, cyclonic storms of wind, birds, and even beetles and frogs, play a part, more or less considerable, in carrying living Mollusca or their ova, either separately or in connexion with floating débris of every kind, to a distance from their native home. Accidental locomotion, of one or other of these kinds, combined with the well-known tenacity of life in many species (p. 37), may have contributed to enlarge the area of distribution in many cases, especially in the tropics, where the forces of nature are more vigorous than in our latitudes. The ease with which species are accidentally spread by man increases the probability of such cases occurring without the intervention of human agency, and numbers of instances may be collected of their actual occurrence.[358]

A point, however, which more concerns us here is to remark on the exceedingly wide distribution of the prevailing forms of fresh-water Mollusca. It might have been expected that the area of distribution in the fresh-water forms would be greatly restricted, since they cannot migrate across the land from one piece of water to another, and since the barriers between pond and pond, lake and lake, and one river system and another are, as far as they are concerned, all but insuperable. We might have expected, therefore, as Darwin and Wallace have remarked, to find a great multiplicity of species confined to very restricted areas, since the possibility of communication with the parent stock appears, in any given case, to be so exceedingly remote.

As is well known, the exact reverse occurs. The range, not merely of genera, but even of individual species, is astonishingly wide. This is especially the case with regard to the Pulmonata and Pelecypoda. The genera Limnaea, Planorbis, Physa, Ancylus, Unio, and Cyclas are world-wide. Out of about ten genera of fresh-water Mollusca in New Zealand, one of the most isolated districts known, only one is peculiar. In South Africa and the Antilles no genus is peculiar. In the latter case, this fact is remarkable, when we consider that the same sub-region has at least ten peculiar genera of operculate land Mollusca alone.

To give a few instances of the distribution of particular species:—

Limnaea stagnalis L. occurs in the whole of Europe, and northern Asia to Amoorland, Turkestan, Afghanistan, North Persia, and Kashmir; Greenland, North America from the Atlantic to the Pacific, and from North Canada and British Columbia as far south as Texas. The distribution of L. peregra Müll., L. truncatula Müll., and L. palustris Müll, is almost equally wide.

Planorbis albus occurs in the whole of Europe, and northern Asia to Amoorland, Kamschatka, and Japan; Turkestan, the Altai-Baikal district, Alaska and Greenland, North Canada, and the whole of eastern North America.

The distribution of Anodonta anatina L., Cyclas cornea L., and Pisidium pusillum Gmel. is almost equally wide.

It is evident that the accidental means of transport mentioned above are insufficient to account for the facts as we find them; we are therefore compelled to seek for further explanation. Anything in the nature of a current furnishes a ready means of transport for Mollusca which have obtained a footing in the upper waters of a river, and there is no difficulty in imagining the gradual spread of species, through the agency of floods or otherwise, over a whole river system, when once established at any point upon it. The feeble clinging power of newly-hatched Limnaea has often been noticed as contributing to the chances of their range of distribution becoming extended. Fresh-water Mollusca, too, or their ova, are exceedingly likely, from their extreme abundance, to be transported by water-birds, which fly without alighting from one piece of water to another. Again, the isolation of one river system from another is, in many instances, by no means well marked or permanent, and a very slight alteration of level will frequently have the effect of diverting the supplies of one watershed into another. When we know what enormous oscillations in level have taken place over practically the whole surface of the globe, we can recognise the probability that the whole river system of the earth has been mixed up and reconstructed again and again, with a very thorough blending of adjacent fauna.

It is possible that the very uniform conditions under which fresh-water Mollusca live may have something to do with the uniformity of their distribution and the comparative sameness in their development. There can scarcely be any question that the environments of fresh-water species are in themselves less varied and less liable to fluctuation than those of species whose home is the land. Water is very like water, all the world over; it may be running or motionless, warm or cold, clear or muddy, but the general tendency is for it to be free from extremes of any kind. Even if the surface water of a lake or river freezes, or becomes unusually hot, there is generally plenty of water at a lower stratum which maintains a less extreme temperature, and to which creatures can retire on the first symptoms of a change. From this two results will follow. Not only will the inhabitants of a piece of water not be inclined to vary much from the type, since their whole surroundings, food, etc., continue very much the same, but, if transported by any accident or cataclysm elsewhere, they will be exceedingly likely to arrive at a place which closely resembles their former home in all essentials. Thus the tendency for new types to be formed would be constantly checked, or rather would very seldom arise.

Mr. Belt, while recognising the importance of changes of level as affecting the distribution of fresh-water species, appears to regard the operations of such changes from a rather different point of view to that described above. “I think it probable,” he writes,[359] “that the variation of fresh-water species of animals and plants has been constantly checked by the want of continuity of lakes and rivers in time and space. In the great oscillation of the surface of the earth, of which geologists find so many proofs, every fresh-water area has again and again been destroyed.... Thus species of restricted range were always exposed to destruction, because their habitat was temporary and their retreat impossible, and only families of wide distribution could be preserved.”

The terrestrial surface of the globe has been divided, as indicating the facts of geographical distribution, into six regions—the Palaearctic, Oriental, Australasian, Ethiopian, Nearctic, and Neotropical. To these is sometimes added a seventh, the Neantarctic, consisting of Chili and Patagonia (and certain islands of the south Atlantic); but since the Mollusca of Chili unmistakably form a part of the Neotropical fauna, it seems hardly worth while to recognise a separate region for those of the extreme south of South America, which have no peculiar characteristics.

In certain points the exact limits of these regions, as indicated by the Mollusca, will probably not correspond to those which are marked out by other zoological classes. Wallace’s line, for instance, does not exist, as far as the Mollusca are concerned.

These regions may be further subdivided into sub-regions, thus:—

Regions		Sub-regions
Palaearctic		Septentrional
		Mediterranean
		Central Asiatic
Oriental		Indo-Malay
		Chinese
Australasian		Papuan
		Australian
		Polynesian
Ethiopian		Central African
		South African
		Malagasy
Nearctic		American
		Californian
Neotropical		Antillean
		Central American
		Colombian
		Brazilian
		Chilian

The southern boundary of this region is the northern limit of the African Sahara, the Mediterranean forming no break whatever in its continuity. In Asia this boundary is less well marked, but roughly corresponds to the southernmost of the vast ranges of mountains which border the great tablelands of central Asia. Across Africa the line of desert is well defined; but in the north-east, as the desert approaches more nearly to the sea, the African extent of the region is correspondingly narrowed until it becomes little more than a strip of coast land, scarcely widening even in Lower Egypt. On the Morocco coast, Palaearctic land forms penetrate as far south as Cape Nun.[360] At its eastern extremity the line becomes less well defined, but probably proceeds along the snowy mountains west of Setchouan, the Pe-ling and Tan-sia-shan ranges, so as to include all the high ground of Thibet and of the upper waters of the Hoang-ho, and ultimately reaches its eastern limit at some point on the shores of the Sea of Japan.

The region thus includes all Europe, Africa north of the Sahara, with the Atlantic islands (the Azores, Canaries, etc.), North Arabia, Asiatic Turkey, the greater part of Persia, Afghanistan, Thibet, all Asiatic Russia, and a very large portion of the Chinese empire.

The principal characteristics of the region as a whole are:—

(1) The rich development of Helix, Arion, Limax, Buliminus, and Clausilia.

(2) The comparative absence of land operculates (see map, frontispiece).

(3) The uniform character of the fresh-water fauna.

It is in the southern portion of the region that Helix (in the sub-genera Macularia, Iberus, Pomatia, and Xerophila) and Buliminus (Zebrina, Chondrula, Ena) attain their maximum. In the north, Fruticicola is the characteristic group; in the mountainous districts of the south-east, Campylaea, with Clausilia. The Arionidae have their headquarters in the damp and warm regions of western Europe, but are rare in the south. They only approach the Mediterranean coast in Algeria, near Gibraltar, and in the region between the base of the Pyrenees and the Maritime Alps, and are very poor in species throughout Italy and Sardinia. They are absent from almost the whole of northern Africa, the Mediterranean islands (except Sardinia), the whole Balkan district, the Crimea, Caucasus, and western Asia.[361]