Opening…

Chapter I

INTRODUCTION--POSITION OF MOLLUSCA IN THE ANIMAL KINGDOM--CLASSIFICATION--ORIGIN OF LAND AND FRESH-WATER MOLLUSCA 1

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Chapter II

LAND AND FRESH-WATER MOLLUSCA, THEIR HABITS AND GENERAL ECONOMY 23

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Chapter III

ENEMIES OF THE MOLLUSCA--MEANS OF DEFENCE--MIMICRY AND PROTECTIVE COLORATION--PARASITIC MOLLUSCA--COMMENSALISM--VARIATION 56

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Chapter IV

USES OF SHELLS FOR MONEY, ORNAMENT, AND FOOD--CULTIVATION OF THE OYSTER, MUSSEL, AND SNAIL--SNAILS AS MEDICINE--PRICES GIVEN FOR SHELLS 96

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Chapter V

REPRODUCTION--DEPOSITION OF EGGS--DEVELOPMENT OF THE FERTILISED OVUM--DIFFERENCES OF SEX--DIOECIOUS AND HERMAPHRODITE MOLLUSCA--DEVELOPMENT OF FRESH-WATER BIVALVES 123

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Chapter VI

RESPIRATION AND CIRCULATION--THE MANTLE 150

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Chapter VII

ORGANS OF SENSE: TOUCH, SIGHT, SMELL, HEARING--THE FOOT--THE NERVOUS SYSTEM 177

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Chapter VIII

THE DIGESTIVE ORGANS, JAW, AND RADULA: EXCRETORY ORGANS 209

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Chapter IX

THE SHELL, ITS FORM, COMPOSITION, AND GROWTH--DESIGNATION OF ITS VARIOUS PARTS 244

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Chapter X

GEOGRAPHICAL DISTRIBUTION OF LAND AND FRESH-WATER MOLLUSCA--THE PALAEARCTIC, ORIENTAL, AND AUSTRALASIAN REGIONS 277

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Chapter XI

GEOGRAPHICAL DISTRIBUTION OF LAND MOLLUSCA (continued)--THE ETHIOPIAN, NEARCTIC, AND NEOTROPICAL REGIONS 328

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Chapter XII

DISTRIBUTION OF MARINE MOLLUSCA--DEEP-SEA MOLLUSCA AND THEIR CHARACTERISTICS 360

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Chapter XIII

CLASS CEPHALOPODA 378

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Chapter XIV

CLASS GASTEROPODA--AMPHINEURA AND PROSOBRANCHIATA 400

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Chapter XV

CLASS GASTEROPODA (continued): OPISTHOBRANCHIATA AND PULMONATA 427

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Chapter XVI

CLASSES SCAPHOPODA AND PELECYPODA 444

BRACHIOPODA (RECENT)

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Chapter XVII

INTRODUCTION--SHELL--BODY--DIGESTIVE SYSTEM--BODY CAVITY--CIRCULATORY SYSTEM--EXCRETORY ORGANS--MUSCLES--NERVOUS SYSTEM--REPRODUCTIVE SYSTEM--EMBRYOLOGY--HABITS--DISTRIBUTION--CLASSIFICATION 463

BRACHIOPODA (FOSSIL)

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Chapter XVIII

INTRODUCTION--DIVISION I. ECARDINES--EXTERNAL CHARACTERS--INTERNAL CHARACTERS--DIVISION II. TESTICARDINES--EXTERNAL CHARACTERS--INTERNAL CHARACTERS--SYNOPSIS OF FAMILIES--STRATIGRAPHICAL DISTRIBUTION--PHYLOGENY AND ONTOGENY 491

SCHEME OF THE CLASSIFICATION ADOPTED IN THIS BOOK

MOLLUSCA

Class Order Sub-order Section { OCTOPODA (p. 382). { { Phragmophora (p. 386). { =Dibranchiata= { { Sepiophora (p. 388). { { { { { DECAPODA { Chondrophora { Myopsidae (p. 389). { { { { Oigopsidae (p. 390). =CEPHALOPODA= { { { NAUTILOIDEA { Retrosiphonata (p. 393). { { { Prosiphonata (p. 395). { =Tetrabranchiata= { { { AMMONOIDEA { Prosiphonata (p. 397). { { { Prosiphonata (p. 397).

{ =Amphineura= { POLYPLACOPHORA (p. 400). { { APLACOPHORA (p. 404). { { { { Docoglossa (p. 405). { { DIOTOCARDIA { { Zygobranchiata { { { Rhipidoglossa { (p. 406). { { { { Azygobranchiata { =Prosobranchiata= { { { (p. 407). { { { { { Ptenoglossa (p. 411). { { { { Platypoda (p. 411). { { { Taenioglossa { Heteropoda (p. 420). { { MONOTOCARDIA { =GASTEROPODA= { { { Gymnoglossa (p. 422). { { { { { { Toxoglossa (p. 426). { { { { Bulloidea (p. 429). { { TECTIBRANCHIATA { Aplysioidea (p. 430). { { { Pleurobranchoidea (p. 431). { { { Siphonarioidea (p. 431). { { { =Opisthobranchiata= { ASCOGLOSSA (p. 431). { { { { NUDIBRANCHIATA { Cladohepatica (p. 432). { { { Holohepatica (p. 433). { { { { PTEROPODA { Thecosomata (p. 435). { { { Gymnosomata (p. 437). { { { { =Pulmonata= { BASOMMATOPHORA (p. 438). { { STYLOMMATOPHORA (p. 439).

=SCAPHOPODA= (p. 444).

Class Order Suborder { =Protobranchiata= (p. 447). { { ANOMIACEA (p. 448). { =Filibranchiata= { ARCACEA (p. 448). { { MYTILACEA (p. 448). { { =Pseudolamellibranchiata= (p. 449). { =PELECYPODA= { { SUBMYTILACEA (p. 451). { { TELLINACEA (p. 453). { { VENERACEA (p. 454). { =Eulamellibranchiata= { CARDIACEA (p. 454). { { MYACEA (p. 456). { { PHOLADACEA (p. 457). { { ANATINACEA (p. 458). { =Septibranchiata= (p. 459).

BRACHIOPODA

Order Family { { Lingulidae (pp. 487 and 503). { { Obolidae (p. 504). { ECARDINES { Discinidae (pp. 487 and 504). =Brachiopoda= { { Craniidae (pp. 487 and 504). { { Trimerellidae (p. 504). { { { Productidae (p. 504). { { Strophomenidae (p. 505). { { Koninckinidae (p. 505). { { Spiriferidae (p. 505). { { Atrypidae (p. 505). { TESTICARDINES { Rhynchonellidae (pp. 487 and 505). { { Terebratulidae (pp. 487 and 506). { { Argiopidae (p. 506). { { Stringocephalidae (p. 506). { { Thecidiidae (pp. 487 and 506).

LIST OF MAPS

THE GEOGRAPHICAL DISTRIBUTION OF THE LAND OPERCULATE MOLLUSCA Frontispiece

THE GEOGRAPHICAL DISTRIBUTION OF THE LAND MOLLUSCA OF THE EAST INDIAN ARCHIPELAGO Between pp. 308 and 309

THE RELATIONS OF THE LAND MOLLUSCA OF NEW GUINEA WITH THOSE OF NORTH AUSTRALIA To face p. 322

THE GEOGRAPHICAL DISTRIBUTION OF THE LAND MOLLUSCA OF THE WEST INDIES Between pp. 344 and 345

MOLLUSCS

BY

REV. A. H. COOKE, M.A. Fellow and Tutor of King’s College, Cambridge

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Chapter I

INTRODUCTION--POSITION OF MOLLUSCA IN THE ANIMAL KINGDOM--CLASSIFICATION--ORIGIN OF LAND AND FRESH-WATER MOLLUSCA

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.

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

MOLLUSCA | +----------------+----------------------------------+ | | =Glossophora= =Aglossa= | | +--------+----------+-------------------+ | | | | | Cephalopoda Gasteropoda Scaphopoda Pelecypoda

=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.

(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.

(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.

(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

{ Amphineura { Polyplacophora { { Aplacophora { { Prosobranchiata { Diotocardia =Gasteropoda= { { Monotocardia (incl. Heteropoda) { { Opisthobranchiata { Tectibranchiata (incl. Pteropoda) { { Nudibranchiata[17] { { Pulmonata { Stylommatophora { { Basommatophora

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.

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.

=Origin of the Fresh-water Fauna=

(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.

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.

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.

(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).

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.

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.

=Origin of the Land Fauna=

=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.

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.

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]

=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.

Spotted a typo or a problem in this chapter? Tell us

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:--

Number of pulsations per minute Helix hortensis Hyal. cellaria At degrees Fahr. 22 21 52° 14 12 44° 10 11 38° 4 9 30°

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.

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.

Aug. 1882. Eggs hatched; one attained diameter of ⅜ in. before winter; fed on coltsfoot and cabbage.

5th Oct. 1883. Shell 1 in. in diameter, no lip formed.

July 1884. Shell finished; diameter 1⅛ in., including perfect lip.

3rd May 1885. Left winter quarters; companion introduced, with which it was seen in company on 5th August.

9th Aug. „ Laid eggs in soil, which were hatched on 10th September, and feeding on 17th September; in May 1886 the largest of these was 11/16 in. diameter.

13th Oct. 1887. Old snail died, aged 5 years 2 months.

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.

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.

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]

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Chapter III

ENEMIES OF THE MOLLUSCA--MEANS OF DEFENCE--MIMICRY AND PROTECTIVE COLORATION--PARASITIC MOLLUSCA--COMMENSALISM--VARIATION

=Enemies of the Mollusca=

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).

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]

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.

=Means of Defence=

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.

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).

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.

=Mimicry and Protective Coloration.=

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.

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.”

=Parasitic Mollusca=

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.

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.

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.

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.

=Commensalism=

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]

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.

=Variation=

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.

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.

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.

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.

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