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MANN LIBRARY AT CORNELL UNIVERSITY Cornell University Library QL 403.ws9 - | itil | THE LIFE OF THE MOLLUSCA UNIFORM WITH THIS VOLUME THE LIFE OF CRUSTACEA BRITISH FRESHWATER FISHES THE OX AND ITS KINDRED THE SNAKES OF EUROPE PLATE I PLATE I The Pearly Nautilus (Nautilus pompilius, Linn., after Owen). The chambered shell has been laid open down to the central plane. uw. The mantle. n. Hood b. The dorsal fold of the 0,0,0. Pedal lobes. mantle. p. Tentacles. e. Nidamental gland. s. Eye. ~ -g. Shell muscle. x, x. Septa. Bett Shell siphuncle. z. Body chamber. ~ &. Funnel: Frontispiece THE LIFE OF THE MOLLUSCA BY iB. B. WOODWARD, F.L.S., ETC. OF THE BRITISH MUSEUM (NATURAL HISTORY) PAST-PRESIDENT OF THE MALACOLOGICAL SOCIETY OF LONDON WITH THIRTY-TWO PLATES AND A MAP METHUEN & CO. LTD. 36 ESSEX STREET W.C. LONDON fs First Published in ror3 PREFACE Vee object of the present work is to give a succinct account of what is known concerning the life of that branch of the animal kingdom to which the Snail, the Oyster, and the Cuttlefish belong—the MoLtusca. Especial attention has, therefore, been given to their history, relationships, and everyday life (ecology), with only general notes on their anatomy, classifica- tion and distribution, etc. The numerous illustrations it is hoped will prove helpful to students. Many figures have been specially drawn by Miss G. M. Woodward, but most of them first appeared in the late Dr. S. P. Woodward’s “ Manual of the Mollusca,” the original drawings and engravings for which are in the possession of the writer. Cordial thanks are due to the Trustees of the British Museum for their courteous permission Vv vi THE LIFE OF THE MOLLUSCA to reproduce illustrations from their publications or taken from specimens on exhibition in the Natural History Museum. Other illustrations have been kindly lent by Mr. W. M. Webb, F.L.S. To his colleagues in the Natural History Museum, Mr. E. A. Smith, 1.S.0., and Mr. G. C. Crick, the author is indebted for valuable assistance in regard to points of nomenclature. CONTENTS CHAPTER I. II TIL. IV. VI Vil. VII. GENERAL INTRODUCTORY CLASSIFICATION GEOLOGICAL HISTORY - PRESENT HISTORY AND DISTRIBUTION + FOOD, HABITS, ETC. REPRODUCTION - EVOLUTION INSTINCT, INTELLIGENCE, AND USES INDEX - vii PAGE 14 44 51 67 87 100 137 145 LIST OF ILLUSTRATIONS PLATE I, THE SHELL (IN SECTION) AND ANIMAL OF THE PEARLY Navtitus (Nautilus pompilius). II. Various GaAsTROPODS, SHOWING THE ANIMALS CRAWLING, TIT. DiaGRAMMaTIC SKETCHES OF THE STRUCTURE OF THE GILLs AND Nervous SysTEM OF MOLLUSscA. DRAWINGS OF THE DIFFERENT FORMS OF THE MANDIBLES AND RADULA OF MOLLUSGCA, ETC, IV. Diacrammatic SKETCHES TO ILLUSTRATE (a) THE ToRSION OF THE GastROPOD Bopy, (b) THE TRAN- SITION FROM A FLAT-COILED GASTROPOD SHELL TO A RIGHT- OR a LEFT-HANDED SPIRAL ONE, (c) THE ManTLE OPENINGS IN BIVALVES, AND (d) THE DIFFERENT ForMs OF GILLS IN BIVALVES. V. DIFFERENT Forms oF COILING, OF APERTURE, AND ORNAMENTATION IN GASTROPOD SHELLS, VI, TorpoGRAPHyY oF GASTROPOD SHELLS, AND FORMATION OF CALLUS IN THE UmBILIcUS oF NAT7IcaA, VII. NoTcHED AND PERFORATED GASTROPOD SHELLS. SHELL STRUCTURE AND REMOVAL OF PoRTIONS OF THE SHELL, VIII, Various Devices For ExcLuDING ENEMIES FROM GastRoPpop SHELLS, IX. Various Forms oF SHELL AND ANIMAL IN THE (4) PotypLacorHora, (B) APLACOPHORA, AND (C) PROSOBRANCHIA (DOCOGLOSSA AND RHIPIDOGLOsSA), ix x THE LIFE OF THE MOLLUSCA PLATE X. Various Forms oF SHELLS IN THE GASTROPODA PROSOBRANCHIA (T#NIOGLOSSA). XI. Various Forms oF SHELLS IN THE GASTROPODA PROSOBRANCHIA (RHACHIGLOSSA). XII. Various Forms or SHELLS IN THE GASTROPODA PRoSOBRANCHIA (TOXOGLOSSA), THE TECTI- BRANCHIA, AND OF THE ANIMALS OF NUDIBRANCHIA. XIII. Various Forms oF SHELLS IN THE PULMONATA. XIV. A Worm-EatineG Stue (Testacella scutulim). XV. StructURE AND SHELL OF THE SCAPHOPODA (ELEPHANTS’-TusK SHELLS). Various BIVALVES, SHOWING THE ANIMALS EXTENDED, XVI. TopocraPpHy oF BivaLvE SHELLS, XVII. Various Forms oF BivaLtvE SHELLS: PRoro- BRANCHIA AND FILIBRANCHIA, XVIII. Various Forms oF BIVALVE SHELLS: EULAMELLI- BRANCHIA (A. OstRACEA, B, SUBMYTILACEA, AND C, TELLINACEA). XIX. Various Forms oF BIVALVE SHELLS: EULAMELLI- BRANCHIA (D. VENERACEA, E. CaARDIACEA, F, CHAMacgEaA, AND G, Myacza). XX. Various Forms or BIvaALvE SHELLS: EULAMELLI- BRANCHIA (H, ADESMACEA AND I, ANATINACEA) AND SEPTIBRANCHIA, XXI, CEPHALOPOD STRUCTURE AND Form oF ANIMAL, ETC, XXII. Various Forms oF THE SHELL 1m CEPHALOPODA TETRABRANCHIA (NAUTILUS AND AMMONITES). XXIII, REsTORATION OF ANIMAL AND SHELL OF BELEMNITES. XXIV. Forms oF THE SHELL IN THE BELEMNITE AND ITS DESCENDANTS, XXV. Various Forms oF THE ANIMALS AND INTERNAL SHELLS oF CEPHALOPODA Diprancuiata (CUTTLE- FISH, PEN-AND-INK-FISH, AND OcToPUS). XXVI, Various Forms oF Sea-BUTTERFLIES (PTEROPODA) AND HETEROPODA, WITH SHELLS OF PELAGIC Fry oF Mo.tyusca, aND SurF-DWELLING PUL- MONATES, ETC. LIST OF ILLUSTRATIONS xi PLATE XXVII, Feepinc TRACKS OF SNAIL AND SLUG. XXVIII. Lirz-History oF THE CorAL-INHABITING MaGILUS. EXAMPLE OF A Fiprous ForM oF OutTER COVERING, OR PERIOSTRACUM, AND OF A Mo.tousc (Xeno- phova) THAT BUILDS INTO ITS SHELL FRAGMENTS OF OTHER SHELLS, CORALS, AND STONES. XXIX. Typrcat Motiuscan Embryos. D1aGRAMMATIC SKETCHES OF THE STRUCTURE OF Mo.iuscan EYEs, ETC. XXX. Eaes anp EaG-CapstLes, ETC., OF SOME GASTROPODS, SEcTIONS OF SOME THICK BIVALVE SHELLS. VARIATIONS OF THE Foot IN BIVALVE MOoLLusca. XXXI. Common GARDEN SNAIL CRAWLING, AND SHOWING THE MOTION OF THE Foor. XXXII. Eccentric Forms of GasTROPOD SHELLS, AND EXAMPLES OF THE EXTERNAL RESEMBLANCE BETWEEN SHELLS OF WIDELY DIFFERENT Forms oF Motiosca. Map, All the Plates (except Plate I., frontispiece) and the Map are at the end of the book. THE LIFE OF THE MOLLUSCA CHAPTER I GENERAL INTRODUCTORY HELLS have ever been attractive objects owing to the beauty of form and coloration displayed by so many of them, and though they are no longer fashionable objects to collect, every one in his, or her, young days has listened with wonder to the supposed roar of the sea in their cavities, while there are some few who retain their predilection for them through- out life, 2-1 make more or less extensive collections. Fewer pet ons still, however, pay any attention to the animal that formed and built the shell, for it has never stood high in popular estimation; indeed, unless good to eat, it is usually an object of aver- sion, and yet there is no group of the invertebrate, or backboneless animals, that better repays study. The Mollusca (soft-bodied animals), of which the Slugs and Snails, the Oysters and the Cuttlefishes, I 2 THE LIFE OF THE MOLLUSCA are familiar examples, as now restricted, form a well- marked group (subkingdom or phylum) of the animal kingdom. Owing to their plasticity, they differ very much among themselves in external form ; indeed, some of the more aberrant members are at first scarcely recog- nizable as molluscs at all; nevertheless, apart from the protective shell, which is a leading feature of the group, a remarkable uniformity characterizes their internal organization in its main features, especially in the young forms. : Externally most possess a “head”; a ventral creeping organ, the “foot”; and a dorsal covering, the “mantle,” which bears and secretes the shell. This shell forms a protection to the more vital organs, and into it the animal can generally with- draw for security from attack. The mantle does not usually reach far beyond the shell-margin when the animal is extended, but in some cases it curls round over the shell (Plate IIL, Fig. 2), and even, as in the Cowry (Cypr@a), meets on the top (Plate II., Fig. 9). In the more special- ized Mollusca there is a tendency to the reduction, even to disappearance, of the shell, and in these there is a corresponding liability for the shell to become more and more enveloped permanently in the mantle as the animal becomes less and less able to use it as a place of retreat (see infra, p. 105). The muscular foot, which is generally an organ of GENERAL INTRODUCTORY 3 locomotion, takes various forms in the different groups of Mollusca; the Univalves (Snails and Whelks) creep along by its means; the Bivalves employ it to burrow with; in the Cuttlefish it is drawn out into the “arms”; in other Mollusca it is transformed into fins to swim with; whilst in some, like the Oyster, it has ceased to be used, and has degenerated into a mere rudiment. Most of the muscles of the body are concerned with the extension or retraction of the different organs of the body, and do not call for special enumeration. The shell is mainly composed of carbonate of lime, as much as 95 per cent., in the form of calcite or arragonite, being often present, with the admixture of a chitinous substance, “‘conchyolin”’’; a little phosphate of lime and a trace of carbonate of mag- nesium are also present. It originates in a shell- gland, or pit, in the embryo, and the successive layers of which it is built up (Plate VII., Figs. 11 and 12) are formed as the animal grows by additions to the margin, and are deposited in order from the outermost to the inner one by a series of special cells situated in the thickened margin of the mantle. The outermost layer, or “‘ periostracum,”* contains the greatest abundance of chitine-like material in its composition, and is the work of the cells at the very * The term “epidermis,” which has been extensively mis- applied to this layer, should be reserved exclusively for the outer layer of the skin of the animal itself. 4 THE LIFE OF THE MOLLUSCA edge of the mantle. Its function is to protect the underlying layers from the action of acid in the water, or from that of the weather on land. It varies greatly in appearance, being sometimes smooth and shiny, at others, rough and coarse; frequently it is fibrous (Plate XXVIII., Fig. 2a). In many forms it readily rubs off; in others it is firmly united to the true shell beneath. The second, or principal layer, usually forms the greater thickness of the shell proper, or “ostracum,” and is secreted by cells farther from the mantle margin ; it may be coloured, and is often made up of prisms of calcite, as in Pinna, though it frequently has a porcellaneous structure. The cells more remote from the mantle edge deposit the innermost layer of the ostracum, thus thickening and strengthening the shell. This layer (“nacreous layer”) is of arragonite, and frequently formed with overlapping plates, thus giving rise to . the iridescent appearance known as Mother-of-Pearl. The remaining surface of the mantle also secretes shelly matter on occasion, either for the purpose of _ further strengthening the shell, of repairing an injury remote from the edge, or of filling up unoccupied spaces (Plate VII., Fig. 13); and in most Mollusca this deposit differs in structure from that of the ‘other layers.* In the pearl-producing shells, how-- * The term “hypostracum ” has been applied to the shelly layers immediately under the points of attachment of the muscles that secure the animal to the shell, but they do not GENERAL INTRODUCTORY 5 ever, such as the Top Shells (Turbinide and Trochidz), the Pearl Oyster (Meleagrina), and many freshwater Mussels (Unio, Anodonta, etc.), as well as Nautilus, this last form of shelly secretion is not differentiated from the nacreous layer, and is very abundant. In the case of the Pearl Oyster and freshwater Mussels, foreign bodies introduced acci- dentally or intentionally between the. mantle of the animal and the shell become coated with pearl. In this way “blister pearls,” and occasionally detached pearls, are formed; but the true pearl of commerce, as will be shown later (infra, p. 79), is developed within the tissues of the animal. The successive additions along the growing edge of the shell generally leave ridges or marks parallel with it, that are known as “lines of growth.” The deposition of shell does not go on continuously ; every now and again there comes a period of rest, and these rest periods are frequently indicated by the occurrence of a stronger ridge or mark. The different details of sculpturing on the surface of the shell—striz, ribs, spines, etc.—are all the products of corresponding irregularities on the margin of the mantle, and were, when first formed, ‘situated on the growing edge. Certain molluscs, especially among the Gastropoda, further possess the power of dis- solving and removing portions of their shells, either appear to differ in microscopic structure from the shell im- mediately beneath them. = 6 THE LIFE OF THE MOLLUSCA from the exterior or from the interior, as occasion may require. The lime for the shell being probably obtained by the animal mainly from its food, it follows, especially in the case of the vegetable feeders, that where lime is abundant in the soil or water, and consequently in the plant tissues, the shell tends to become abnor- mally thick and heavy; while other members of the same species living where lime is scarce, will have exceedingly thin shells. The latter condition is very observable in the larger Land Snails of the Channel Islands, where, indeed, the living individuals will even resort to the shells of their dead comrades to obtain the requisite supply. Isolated cases of abnor- mally thick shells are probably due to physiological peculiarities of the individual, enabling it'to absorb lime more easily than its fellows. Internally the common trait in the Mollusca is the reduction of the ccelum, or body cavity, to a space around the heart, and the concentration of the prin- cipal nerve centres (“ganglia”) into a ring or collar surrounding the cesophagus. All sections of the group, except the_Biyalves, have a distinctive feeding organ called the “ radula,” and in their development almost always pass through what is known as the ‘‘veliger stage” (see infra, P- 93). The heart, which is well developed, is entirely arterial—that is to say, only receives the blood after GENERAL INTRODUCTORY 9 it has been aérated in the gills, and distributes it over the body through the circulatory system. The latter is peculiar in that its channels are distended in places into wide spaces, or sinuses, which are insinuated among the various organs, and so diminish the body cavity. The structure of thegills has been largely employed in the classification of the Mollusca. The gills, which are generally situated in a cavity under the mantle, vary in number from one in the majority of the Gastropoda to eighty pairs in some of the Chitons, or Coat-of-Mail Shells (Polyplacophora). Each gill (or ‘“‘ctenidium”) consists of an axis containing two blood-vessels, one to bring the blood to the gill, the other to convey it away after it has been aérated in the respiratory filaments. Of these last there is a row on either side of the axis, each filament being more or less flattened. In order to obtain the greatest possible amount of surface exposed to the water for aération, these filaments are either expanded into leaf-like plates (Aspido- | branchiate, Plate III., Fig. 1), as in all the earlier, representatives of each of the principal classes, or | lengthened out (Pectinibranchiate, Plate IIT., Fig. 2).\ Further modifications are dealt with later on. The nervous system of the Mollusca acquires peculiar importance in that, while every modification of an organ is reflected in it, it is the last feature to be influenced by these changes, and hence is of 8 THE LIFE OF THE MOLLUSCA extreme value in tracing relationships of the various parts. Its principal elements (Plate III., Fig. 3) comprise a series of paired nerve centres or ganglia ; of these one pair, the “‘ cerebral ganglia,” lying above the cesophagus, sends off nerves to (“ innervates”’) the head, eyes, and the special organs of sense ; another pair, the “pedal ganglia,” is situated below the cesophagus; whilst the “ pleural ganglia,” lying one on each side just above the pedal ganglia, form the third pair. These several ganglia are united by nerve cords, so that the whole forms a ring or collar round the throat. In the more primitive Mollusca the two last named are somewhat removed back from the other pair, but in the more specialized forms they are in proximity to it, and the resultant cesophageal ring is much more concentrated. Four nerve cords run back from either side of the ring, two in connection with the pedal ganglia serving the foot, and two in connection with the pleural ganglia innervating the viscera. These last, which are the more important, and are provided with minor nerve centres, are united towards their terminations, thus forming a continuous loop, known as the “ visceral loop.” Various sense organs are present. Many of these are situated on the integument, and are probably, like the tentacles, organs of touch. Eyes are found in most forms, sometimes very perfect organs, as in the higher Cuttlefishes ; in Snails generally they are GENERAL INTRODUCTORY 9 less well developed, and placed near the base of the “horns,” or on the summit of a special pair. The Bivalves, being headless, have usually no eyes; but some possess them during their larval existence, and they persist in the adult Mytilide and Piéeria (=Avicula). Secondary visual organs are developed in certain forms, and more usually occur in some part of the margin of the mantle or siphons (see infra, p. 133). In the case of one or two molluscs eyes are developed over the back. That molluscs can hear is inferred, rather than/ known, from the presence of “otocysts,” small, cavities filled with fluid in which grains of shelly| material float. These otocysts are situated close to’ the pedal ganglia, and supplied by nerves from the cephalic ganglia (Plate III., Fig. 3, 0). From their discrimination of food some Mollusca appear to be capable of tasting, and they certainly can smell. The seat of the olfactory sense is believed to vary, and in some to reside in a tentacle, while in others it can be traced to a special organ called the “osphradium,” which in marine snails is situated close to the gills, and resembles them somewhat in appearance. For procuring their food all classes of the Mollusca, except the Pelecypoda, are furnished with one or a pair of horny mandibles or jaws (Plate III., Figs. 4-8). and the special feeding organ, the radula (Plate III., Figs. 11-22). Io THE LIFE OF THE MOLLUSCA The mandible is single, and placed on the upper side of the mouth in the Limpets (Patellidz), one of the Sea-Slugs (42gives punctilucens), the Land and Freshwater Snails (Pulmonata, Plate III., Figs. 5 and 6), and the Elephant’s-Tusk Shells (Dentaliide). Two kinds of Pond Snails (Limnaa and Planorbis) have three mandibles, an upper and two side plates (Plate III. IIl., Fig. 7). The majority of the other gastropods—for the carnivorous species are generally without them—have two lateral plates. Many of these are ornamented with most elaborate patterns, and some have raised projections on them like a file (Plate III., Fig. 8). The Cephalopoda also have a pair of mandibles, upper and lower, resembling a Parrot’s beak, save that the upper fits within the lower one (Plate III., Fig. 4). These mandibles are formed apparently of dense chitine, strengthened in the case of Chatoderma and Nautilus with carbonate of lime. The radula is so important a feature as to call for more detailed mention.. With its supporting car- tilages and muscles it occupies the position in the mouth assumed by the tongue in the higher animals, and much resembles that organ in general appearance. The radula itself, however, consists of a series of recurved teeth formed of dense chitine, attached in transverse rows to a membrane of the same sub- stance. In the Limpets (Docoglossa) the teeth GENERAL INTRODUCTORY II further contain as much as 27 per cent. of silica hydrate or opal in their composition, while in the rest of the Gastropoda the chitine is hardened super- ficially by deposits containing calcium, iron, and phosphoric acid, arnounting in all from 2°4 to 6 per cent. The Chitons differ from this second group in . alone having ferric oxide as the most important mineral constituent. In use, not only is the whole tongue-like mass protruded and worked with a licking action, but the radula moves backwards and forwards on its surface over the cartilages ‘like a chain saw. The number of teeth in each transverse row varies from one, as in certain of the sea-slugs (4olis, Elysia, etc.) and species of Chetoderma, to upwards of 200 or 300, as in the Top Shells and their allies (Zvochus, Haliotis, etc.). When more than one is present the teeth generally vary in shape, ~ but those on either side of the centre correspond each to each. The number of rows also varies from only a few to very many, so that in some Mollusca, like the Limpet, the radula is nearly twice as long as the animal. In all, the teeth in front tend to become worn away by constant use, and to replace them fresh teeth are continually being formed at the other end, which is kept in a special sac or pouch under the gullet. The longest radule are generally found in those molluscs, like the Limpet, that feed on the microscopic plants growing on rocks, the teeth in 12 THE LIFE OF THE MOLLUSCA such case being more quickly worn away by use. The worn-out teeth usually fall off and are lost, but in one tribe (Ascoglossa= Elysioidea) theyare received and retained in a special sac. The total number of teeth in the radula conse- quently varies greatly in the different kinds. In some species of Chetoderma there is but a single tooth present, while the Whelk (Plate III., Figs. 16 and 17) has about 250, a big Limpet 2,000, the Periwinkle 3,500, the Common Garden Snail 15,000, and the big Grey Slug (Limax maximus) 26,800. In certain instances they almost baffle calculation, as many as 750,000 being computed for Umbra- culum. Since each succeeding row of teeth resembles its predecessor, a longitudinal striped pattern results when the teeth in the transverse rows vary in shape and colour. In each row there is generally a central, or “ rhachidian,” tooth present, flanked on either side by “laterals.” These laterals may be of one type, as in the Common Garden Snail and its allies (Helix), or they may be divisible into two or three sets, when those nearest the central tooth are styled “‘admedian”’ and the outer ones “ marginal,” these two series being generally divided by a conspicuous tooth, or “major lateral.” This is expressed by formule, as 1: 1:1 (Plate III., Figs. 16 and 17), 2:2:1:1:2 (Plate III., Fig. 15), and when the marginal teeth are very numerous, © :5:1:5:@ GENERAL INTRODUCTORY. 13 (Plate IIl., Fig. 13), © : 0: © (Plate III,, Fig. 14), etc. The shapes of the teeth are so constant in the several molluscs that they assist not only in the determination of families and genera, but, with few exceptions, in that of species also. The forms of the teeth are, further, some index to the diet of the animals, the purely herbivorous having short, broad-pointed teeth, the carnivorous sharp- pointed teeth, which in those feeding on living animals are barbed to retain their prey, while in the Cones the teeth are not only barbed, but perforated and connected with poison glands. In habit the Mollusca are far from active, only some of the Cuttlefishes being capable of spasmodic rapid motion, so much so that the term “ sluggish,” borrowed from them, best describes them. The exact relationship of the Mollusca is difficult to determine, though they belong to the same division of the animal kingdom as the Chetopoda, Gephyrea, Rotifera, Bryozoa, and Brachiopoda; but on the whole it is considered that the most archaic molluscs, the Chitons, come nearest to the free Polychetes, such as the gaudy Sea-Mouse (Aphrodite), so common on our coasts. CHAPTER II CLASSIFICATION HE Mollusca are divisible into five principal groups or classes: Crass I.: The AMPHINEURA, of which the Chitons (Coat-of-Mail Shells) are the type. In these the body is more or less elongate, without a distinct head, the foot forming the ventral surface. The extremities of the alimentary canal lie at either end of the body, and do not in any way approximate each other, while the various organs are situate in pairs on either side of the central axis, so that the two halves of the body correspond, and the animal is symmetrical. This Class is divided into two orders, POLYPLA- COPHORA and APLACOPHORA. Order I.: PoLyPLAcoPHORA, the Coat-of-Mail Shells, or Chitons (Plate IX., Figs. 1-3; Radula Plate III., Fig. 11) are readily distinguished by being alone amongst the Mollusca in possessing a shelly covering of many pieces, called. the “ lorica.” 14 CLASSIFICATION 15 There are eight plates or valves in all on the back of the animal, fitting one over the other like the tiles ona roof. Generally the lorica is surrounded by a leathery “ girdle,” so called, which helps to unite the several plates, and which, though sometimes naked, is usually studded with scales or beset with bristly spines. In some forms the body is very long, and the shelly plates are placed at intervals. The muscular foot extends the whole length of the under surface of the body; the end of the snout is just visible; there are no tentacles. A row of small gills is seen along each side under the edge of the mantle. The valves are perforated for the passage of sense organs, which in the family Chitonidz are in part converted into eyes. When detached from the rocks to which they cling, the animals will coil up after the fashion of the well-known Wood-Louse. Order II.: The APLACOPHORA, or SOLENOGASTRA, are so modified that they scarcely resemble molluscs at all. The body is worm-like, and there is no trace of a shell, but the much thickened outer skin con- tains shelly spicules. The foot is extremely reduced, or altogether wanting. The gills are in a chamber at the latter end of the body, into which chamber also the excretory orifices open. Their blood is red. Two sub-orders are distinguished : Sub-Oider 1: NEOMENIINA (Plate IX., Figs. 4 and 5), in which the foot is sunk in a groove along the 16 THE LIFE OF THE MOLLUSCA ventral side of the body. The radula is of the many-toothed order or wanting. Sub-Order 2: CHZTODERMATINA, represented by a single genus Chatoderma (Plate IX., Fig. 6), in which the body is extremely long and cylindrical; the foot is wanting, its position being merely indi- cated by a groove; whilst the radula is represented by a single tooth. Crass II.: The GASTROPODA (Plate II.), typi- fied by the Snail and Whelk. In these there is a distinct head, furnished with one or two pairs of contractile sense organs, the “tentacles,” or “horns,” the form of which varies greatly in different groups. The two eyes are situated on the tentacles (the hinder pair when four are present), and are generally carried on tubercles near the base, but they sometimes. appear halfway up, while in the Common Snail and his kindred they are, as well known, borne aloft on the summits of the “horns.” The ventral foot forms a creeping disc, and sur- mounting it the visceral organs within the covering mantle are borne as a twisted hump, generally covered by a shell formed in a single piece. This shell is attached to the body by a powerful muscle, which serves to withdraw the animal into its house. The torsion of the body is brought about by a double process (Plate IV., Fig. 1, A-D). Firstly, as CLASSIFICATION 17 in the Cuttlefishes, the latter half of the alimentary canal is bent underneath the body till its termination approximates the mouth. Then the visceral mass is twisted round laterally, so that the terminal portion of the alimentary canal, with the two gills and other paired organs on either side of it, are brought round by the right side till they lie nearly over the head, and the original left gill and accompanying organs become in position the right-hand ones, while the visceral nerve loop is twisted into a figure of 8. Next, the original left (now right) gill and its accom- panying organs tend to wither and eventually dis- appear. In some cases the secondary twist takes place in the reverse direction, or round by the left side, when a left-handed animal results, and the originally right gills, etc., are suppressed. In this way the animal becomes asymmetrical, organs on one side of the body not being matched by corresponding ones on the other. A further result of the secondary, rotary twist is that the shell, which was beginning to coil forwards over the animal’s head, or exogastrically, as it does in Nautilus, is swung round so that it coils backwards, or endogastrically. The shell secreted by the mantle covering this twisted visceral hump, of course, reflects its form in every respect. Essentially the resultant shell is a longer or shorter hollow cone. In some, such as the adult Limpet, it is a simple cone, but in by far the greater number it is an elongated cone, coiled 2 18 THE LIFE OF THE MOLLUSCA round and round, each coil forming a “ whorl,” the last being the “ body whorl” (Plate V., Figs. 1 to 10). This coil may be a flat one, but most Gastropods have the visceral dome and shell twisted dextrally— that is to say, when the shell is placed with the mouth uppermost, and the apex directed away from the observer, the mouth lies to the right hand of the axis of the shell. Some are wound in the opposite direction and are sinistral. Reversed varieties of animals and shells normally dextral, or normally sinistral, are often met with, as well as species in which the coiling of the animal and shell is indiffer- ently dextral or sinistral in the individuals composing it. There are, moreover, Gastropods whose shells coiled in one direction enclose animals whose organ- ization is that usually associated with shells having the opposite twist. Thus among the Ampullaridez there are animals with dextral organization occupying sinistral shells (Lanistes), while in the Planorbide the common Flat-coiled Water Snail (Planorbis) is an example of a normally sinistral animal in an appar- ently dextral shell. This is brought about by what is known as “hyperstrophy,” and may be illustrated by taking a perfectly flat, coiled shell and placing it with the mouth downwards and directed away from the observer (Plate IV., Fig. 3, A); then in dextrally organized molluscs that assume a spiral growth, the spire would normally be exserted towards the right (Plate IV., Fig. 3, B, C), but may exceptionally be CLASSIFICATION 19 exserted towards the left side (Plate IV., Fig. 3, B’,C’). In the first case the shell will be dextral, in the latter sinistral. With sinistral animals these conditions are just reversed. In certain forms (Odostomia, Turbonilla, Tornatina, etc.) the young shell is sinistral, after which dextral growth suddenly ensues, and the completed shell is dextral (Plate IV., Fig. 2). The term “hetero- strophy ” has been applied to this condition. The axis, or ‘‘ columella,” of the shell is sometimes hollow, or “umbilicated” (the hollow itself being called the “ umbilicus” (Plate VII., Fig. 9; and VL, Fig. 3) ; sometimes the whorls are closely coiled, and a solid pillar of shell results (Plate VI., Fig. 2). In the genus Natica the umbilicus is open, but in many of the species the cavity is more or less filled by a shelly deposit known as “ callus” (Plate VI., Fig. 3, A-E). The “apex” or extreme top of the shell generally differs in markings and other features from the rest, and offers important characters. This “nucleus,” or “ protoconch,” is the portion formed in the egg, hence it is also known as the “ embryonic shell.” ” The spiral channel formed by the junction of the whorls is termed the “suture.” A spiral line traced along the whorls midway between the sutures would mark the “ periphery.” The “‘ mouth,” or “ aperture,” has sometimes quite a circular margin (peristome), when it is said to be “entire” or “continuous” ; 20 THE LIFE OF THE MOLLUSCA more often it is “interrupted,” the side next the columella being bounded by the wall of the body whorl only, when the exterior portion is known as the “outer lip” or “labrum,” and the other side as the “inner,” “ columella lip,” or ‘‘ labium.”” There is frequently a notch in the margin of the aperture near the junction of the outer lip with the body whorl, and another where it joins the anterior end of the columella; these are respectively termed the * posterior’ and “anterior canal,” and give passage to the siphons of the animal. (All these terms are illustrated on Plate VI.) This group has been called ‘‘siphonostomatous,” and those without any notch ‘“‘ holostomatous.”’ In some rhipidoglossate forms (Pleurotomaria, the fossil Belerophon, Emarginula, etc.) and the toxo- glossate Pleurotoma there is a slit near the periphery of the labrum that gives passage to the anal fold or siphon. In others—Haliotis, etc.—the slit gives place to a series of perforations (Plate VIL, Figs, 1-8. See also infra, pp. 94.and 95). The labrum is thin and sharp in most shells, and in some adult forms; but more frequently it is either thickened, or curved outwards. (reflected), or curled inwards (inflected), or expanded or fringed” with spines. When these thickenings or expansions occur periodically during the growth of the shell, . they form conspicuous transverse markings on the. whorl termed “ varices.” The exterior of the shell: CLASSIFICATION al is also more frequently than not ornamented with either spiral lines or ridges running in the direction of growth, or with transverse markings coinciding with the lines of growth or with both (Plate V., Fig. 11-17). The external spines that come in the way of the growth of the shell as whorl is added to whorl are dissolved in some way, and removed by the animal. In some cases also the internal walls of the whorls and the columella are similarly removed to make more room for the growing creature (¢g., Neriia, Theodoxis, Cypraa, Conys [Plate VIL., Figs. 12 and 13], Auricula, etc.). On the other hand, when the animal in the course of growth leaves a space in the upper whorls, this is either filled up with a shelly deposit (Plate XXXII., Fig. 6), or cut off by the formation of a wall or septum across the whorl (Plate VI., Fig. 2, s.m., and in Vermetus, etc.).* A similar partition is found when the apex of the spire becomes worn through or broken by accident, or attacked by boring Molluscs (Plate VII., Fig. 13), or is broken off as a regular successive phase of the animal’s existence (e.g. in Caecum [Plate VII., Fig. 14], Rumina decollata ’ [Plate VII., Fig. 15], etc.). Many of the Gastropods, like the Periwinkle, close the mouth of the shell, on retiring into it, with * One, and only one, instance of this in a helicoid has been recorded—viz., in a species of Glypiostoma. 22 THE LIFE OF THE MOLLUSCA a trap-door (Plate VIII., Fig. 2), the “ operculum ”; when extended, this lies on the animal’s back, and in many cases forms a sort of pillion on which the shell rests (Plate II., Fig.1, and Plate VIII., Fig. 1). It consists of a horny layer, sometimes strengthened by the addition of shelly matter, which differs in structure from that of the shell itself. Its inner side is marked by the scar of the muscle which is attached to it, while outside it exhibits lines of growth. The operculum, various forms of which are shown on Plate VIII., Figs. 4-13, grows by additions made to the original point of beginning, the “nucleus,” either all round, concentrically; or on one side, so that the nucleus remains at the end or at one side; or the growth may result in a spiral. This spiral is sinistral in dextral, dextral in sinistral shells. Nearly all the Gastropoda are furnished with an operculum in the young stage within the egg, and though some discard it on hatching out, the greater number retain it throughout life. Many of the spiral and concentric forms fit the mouth of the shell accurately, others only partly close the aperture, in yet others it becomes rudimentary, while in certain specialized forms, such as Stvombus and its allies, it is converted into a sort of claw at the end of the elongated foot, and is used to assist the animal in progression (Plate VIII., Fig. 3). A more peculiar means for blocking the entrance of the shell when the animal retires into it is possessed CLASSIFICATION 23 by the genus of land shells called Clausilia (Plate VIII, Fig. 17). The “ clausium (c),” as it is called, is a spoon or shoehorn shaped elastic plate, attached by its stalk to the columella within the shell, and closes automatically behind the retiring occupant. One remarkable little Land Snail (Thyrophorella), inhabiting the island of St. Thomas, in the Gulf of Guinea, is said to be able to close the mouth of its shell by bending down a portion of the peristome which projects beyond the upper half of the aperture, and is furnished with a hinge for the purpose (Plate VIII., Figs. 18 and 19). In many Gastropods, especially the Land Snails, the ingress of larger enemies is barred by projecting teeth placed round the aperture, or, less frequently, some way back in the shell. Sometimes these teeth are so long and numerous that it is a matter of wonder how the rightful owner itself gets in and out of its house (Plate VIII., Figs. 14-16). The Gastropoda are divided into two sub-classes : I. STREPTONEURA, in which the torsion of the visceral loop is well marked. g. EUTHYNEURA, in which, in the adults, owing to partial detorsion of the visceral hump, the visceral loop nearly becomes once more simple. Sub-Class 1: The STREPTONEURA (also known as PROSOBRANCHIA, because the gills are in advance of the heart), primarily by the gill structure and secondarily by the radula, are divided into— 24 THE LIFE OF THE MOLLUSCA say % Order I.: AsprpopRANcHIA, having the gill filaments flattened out into leaf-like expansions. Sub-Order 1: DocoGLossa (Plate IX., Figs. 7-9; Radula, Plate III., Fig. 12), comprising the Limpet family. Sub-Order 2: RuHIpIpoGLossa (Plate IX., Figs. 10-24; Radula, Plate III., Fig. 13), comprising the Ormers (Haliotidz), Keyhole Limpets (Fissurel- lidze), etc., in which the older members still retain the right (originally left) gill, while the more specialized have only one, the left (originally right) gill. Order II.: PECTINIBRANCHIA, having elongated gill filaments. Sub-Order 1: T#NioGLossa (Plate X.), which includes the greater part of the sub-class, and in which the radula formula is generally 2:1:1:1:2 (Plate III., Fig. 15). Sub-Order 2: STENOGLOSSA. Tribe 1: RuacHIGLossa (Plate XI.), including the Whelks, Dog-Whelks, and their kind, and having a radula formula of 1:1: 1 (Plate III., Fig. 17), although in higher members of the tribe this is reduced to 1 (Plate III., Fig. 18). Tribe 2: ToxocLossa (Plate XII., Figs. 1-5), of which the Cones and Auger Shells (Tevebra) are the best-known examples. The radula formula is 1: 0:1 (Plate ITI., Fig. 19), or exceptionally 1: 1: 1. Sub-Class 2: The EuTHyNeurA fall into two orders and four sub-orders : CLASSIFICATION 25 Order I.: OpISTHOBRANCHIA, So called because the heart is in front of the gills. Sub-Order 1: TECTIBRANCHIA (Plate XII., Figs. 6-19), or those having shells, which are mostly fragile or rudimentary, and often concealed in the folds of the mantle. To this belong the Sea-Hares (Aplysiidz), the Bullas, etc. Sub-Order 2: NuDIBRANCHIA (Plate XII., Figs. 20-28), or the shell-less Sea-Slugs, are externally symmetrical animals, without true gills, respiration being effected by secondary gills, or by the general surface, aided in some cases by the “ cerata,” which are appendages of the dorsal integuments. Order II.: PutmMonaTa, or the majority of land and freshwater shells, in which the true gill has disappeared, and its function is taken up by the wall of the mantle cavity. Sub-Order 1: BASOMMATOPHORA (Plate XIII, Figs. 1-10), or those having eyes at the bases of the tentacles, embraces the freshwater pulmonates and a few land shells. Sub-Order 2: STYLOMMATOPHORA (Plate XIII, Figs. 11-35, and Plate XIV.; Radula, Plate III., Figs. 21 and 22), or those having the eyes at the end of the “horns,” takes in nearly all the land shells. Cxiass III.: The SCAPHOPODA, or SOLENO- CONCHA (Plate XV., Figs. 1 and 2), of which the 26 THE LIFE OF THE MOLLUSCA Elephant’s-Tusk Shell (Dentalium) is the type, form a single small group by themselves. The animal is symmetrical, with a rudimentary head, and a long cylindrical foot used for burrowing in the mud in which these creatures live. The borders of the mantle are united beneath, forming a tube, open at both ends, enclosing the rest of the body, and encased in the tubular shell, which is likewise open at both ends, there being no approximation of the two ends of the alimentary canal. There is generally a notch in the margin of the shell at the smaller end (or posterior opening) on the ventral or convex side. In one genus, Schizo- dentalium, there are, besides, in a line with and close to this notch, a series of openings. Sometimes the shell is finely striated lengthwise. Crass IV.: The PELECYPODA, or LAMELLI- BRANCHIA (Plate XV., Figs. 3-9), commonly known as Bivalves, of which the Oyster, Mussel, etc., are familiar examples. In these the head is rudimen- tary, hence they were called ACEPHALA by Cuvier; while, because a head is present in the embryo but does not develop, Lankester proposed to term them LipocepHaLa. The characteristic radula is, of course, wanting in this group. The stone-axe- shaped foot is usually well developed, and serves as a burrowing and, rarely, as a creeping organ. The symmetrically disposed organs of the body CLASSIFICATION 27 are enclosed between the two, right and left, lobes of the mantle, which in their turn are covered by the two shelly plates, or valves. In a few instances the mantle is extended and reflected over the valves, and partly (Galeommide) or completely (Ephippodonta, etc.) envelops them. The mantle lobes are attached along the back and extend out to the margins of the valves. In the more primitive forms the mantle margins are quite simple, and open from the front round the ventral edge to the back; but many different modifications take place in the higher forms. Instead of the margins being simple, there may be folds, thicken- ings, protuberances, tentacles (Plate XV., Fig. 5), various glands, and even eyes (Plate XV., Fig. 8). Nor are the margins always free; indeed, in extreme cases they are united nearly the whole way round. There are various stages (Plate IV., Fig. 4, A-F): reckoning the open one as the first (A), then in the second (B), instead of the water being admitted all the way round, the edges of the mantle are kept closely applied to each other except where the foot protrudes and at two points at the hinder end, one of which serves to admit the fresh water to the gills, and is called the ‘“‘inhalent aperture,” whilst the other allows the fouled water to escape, and is termed the “exhalent aperture.” In the next stage (C), the edges of the mantle are united permanently at the point between these two openings; and in the 28 THE LIFE OF THE MOLLUSCA fourth stage (D), at a second point below the inhalent aperture. In the fifth stage (E), the margins of these apertures have grown out into tubes (“siphons,” Plate XV., Fig. 7), and the remaining portions of the mantle margins have united all round, except where the foot is protruded. Finally (F), the two siphons become united externally (Plate XV., Fig. 4). In these cases there is frequently a fourth small aperture left in the ventral margin. The siphons, the ends of which are often fringed, can be wholly or partly withdrawn. The gills lie underneath the mantle, one on either side (Plate XV., Fig. 3, portion shaded with straight lines ; and Fig. 8, bv), in the space between the latter and the body of the animal. These organs vary progressively from a very simple structure to a very complicated one, and since their structure has been made the basis of the classification of the group, a brief description of it is necessary. In the more primitive Bivalves the gill is of the aspidobranch type (ante, p. 7, Plate IV., Fig. 5, a); but in the higher Pelecypods they are filibranch, only the fila- ments of each of the two rows, instead of remaining separate, have an arrangement whereby they inter- lock and form a continuous membrane, like the web of a feather. The mechanism by which this is brought about is extremely simple. At regular intervals on either side of each filament are little patches of stiff hairs which interlock with the corre- CLASSIFICATION 29 sponding ones on the two neighbouring filaments, just as two brushes may be made to do; these are known as the “ciliated junctions” (Plate IV., Fig.5,£). Moreover, the two membranes, “lamellz,”’ thus arising do not simply depend from the axis, but their edges are folded upwards so as to keep them within the margin of the shell. The outer lamella is folded outwards, and the inner inwards, so that they form a W in section (Plate IV., Fig. 5, B). A further complication of structure ensues when junctions, called “interlamellar junctions,” are formed between the dependent part of each filament and its reflected portion. In a more advanced stage still both the ciliated and interlamellar junctions become solid connections, so that the whole structure presents a sponge-like appearance, while the reflected ends of the filaments, uniting with the walls of the mantle and foot, subdivide the pallial chamber (Plate IV., Fig. 5, C). One further development there is, in which the gills have become converted into a sort of party-wall, separating the pallial chamber into a dorsal and ventral portion (Plate IV., Fig. 5, D). At certain points all over the gill filaments there are powerful cilia, which by their action keep a brisk current of water flowing over the gills in a constant stream from behind forwards. This not only insures a fresh supply of oxygen, but conveys the micro- organisms on which the animal feeds towards the mouth. 30 THE LIFE OF THE MOLLUSCA The muscular foot is a prominent feature in most Bivalves, lying in the middle line towards the front of the body (Plate XV., Figs. 3, 6-8, f). Towards the back of it is situated the gland for spinning the horny threads by which many Bivalves anchor them- selves to stones, etc. The Common Mussel and the Zebra Mussel are familiar examples, and the bunch of threads is known as the “byssus” (Plate XV., Fig. 9, 6). The animal generally has the power of rejecting its byssus at any time and spinning a fresh one. The mouth is situated a little behind and beneath the front adductor muscle; it is unarmed, neither jaw nor radula being present. It is flanked on either side by a pair of twin, triangular lobes, the “ labial palps” (Plate XV., Fig. 3, ¢), which are in a line with, and in front of, the gills. Their function is apparently to collect, and possibly to taste, the food before it passes into the mouth. Each valve of the shell is a hollow, irregular, shallow cone, the apex of which, termed the “ beak,” or “umbo,” is the point at which growth began, and is, in fact, the young shell, or ‘ prodissoconch’”’: it generally differs in shape and markings from the later growth. The umbo is usually curved more or less to one side (Plate XVI., Figs. 1 and 2, u), and generally points towards the head, or anterior end, when the shell is said to be “ prosogyre,” in contradistinction CLASSIFICATION 31 to those in which the umbones are straight, ‘“ ortho- gyre,” or are directed backwards, “ opisthogyre.” The last-named condition may be the pelecypod equivalent of hypertrophy in the Gastropod shell. More often than not the two valves are of equal size and shape, and the shell is “equivalve,” as in the Cockle ; sometimes, however, as in the Oyster, one valve is smaller than the other, and the shell is “inequivalve.”’ The Bivalves are all more or less “inequilateral”’—that is to say, if a line be drawn from the umbo to the ventral margin of the valve, the portion on one side of the line, usually the front one, will be found to be smaller than that on the other. When the shell is shut it is said to be “close” if the valves fit accurately, and to “ gape” if openings be left (Plate XVII., Fig. 5). Near the umbones the two valves are united by a chitinous “ligament ”’ (Plate XVI., Fig. 2, /), formerly known under the misleading name of “ cartilage,” which is made up of an outer non-elastic layer and an inner, fibrous, elastic layer. The whole forms a sort of C spring which tends to open the valves, the act of closing being effected by two powerful “adductor” muscles that pass from one valve to the other, and are situated at either end of the axis of the body. To prevent the opposed valves from shifting when closed, a series of projections, or “teeth,” fitting into each other, are developed near the ligament. 32 THE LIFE OF THE MOLLUSCA They are sometimes numerous and all alike (Taxo- dont), especially in the more primitive Bivalves Nucula, Arca, etc. (Plate XVIL., Figs. 1, 3, 6, etc.). More usually they are divisible into a central, more or less transverse, group of “ cardinal teeth,” flanked on either side by others running with the shell margin, or “lateral teeth” (Teleodont). This shell margin bearing the ligament and teeth is known as the “ hinge line,” and is sometimes extended inwards so as to form a sort of platform, the “ hinge plate” (Plate XVI., Figs. 1 and 2). The number of teeth in the two valves differs, and occasionally, as in Chama, an individual will have the normal dentition of the two valves reversed, a condition apparently corresponding as near as such may be to sinistrosity in the Gastropod. The teeth become exceptionally strong in shells that live in situations exposing them to strain, and dwindle and disappear in such as dwell in protected localities. The possibility of the valves being laterally dis- placed is further guarded against in very many by tooth-like crenulations of the ventral margins of the valves that interlock, as in the Scallops, Cockles, etc. (Plate XV., Fig. 8; Plate XVIII., Fig. 3). The shelly tubes secreted by the Ship-worm (Teredo, Plate XX., Fig. 5), Gastrochena (Plate XIX., Fig. 16), the Waterpot Shell (Brechites, Plate XX.., Fig. 19), and some others to line their burrows are produced, it is true, by extensions of the mantle or CLASSIFICATION 33 cuticular surface of the animal, but are distinct from the real shells, which in the Tevedo and Gastrochena are within the tube and free, while in Brechites they are built into its wall. The points of attachment of the adductor muscles to the shell are marked by well-defined scars (Plate XVI., Figs. 1-3, a.a. and p~.a.). Close to the posterior adductor scar, that of the muscle for with- drawing the foot (Plate XVI., Figs. 1-3, p.7.) is found, and minor scars occur nearer the umbones of the valves to which other small muscles are attached. Running from one adductor scar to the other, at a short distance from the ventral margin of the shell, is a shallow groove, the “ pallial line” (Plate XVI., Figs. 1-3, £./.), which marks the attachment of the mantle edge. In those molluscs that have large retractile siphons, room has to be made for these, and the pallial line, instead of forming a continuous curve, takes, shortly before it reaches the posterior scar, a sharp bend backwards towards the centre of the valve, forming a bay or sinus, known as the pallial sinus” (Plate XVI., Fig. 2). The size of this sinus naturally corresponds to that of the siphons. In some Bivalves, such as the Common Mussel of the seashore, the posterior portion of the body is more developed than the anterior. This develop- ment tends to bring the two adductor muscles and the hinge into a line, and in proportion to render 3 34 THE LIFE OF THE MOLLUSCA the anterior adductor superfluous, with the result -that it tends proportionately to dwindle and dis- appear, leaving the enlarged posterior muscle to do the work of both, as in the Oyster and Scallop (Plate XVI., Fig. 3, A-D). Bivalves with two adductor muscles are termed “ Dimyaria”; and if the latter are of equal size, are said to be “Isomyarian”; or, if unequal, “ Heteromyarian”; whilst those having only one are called ‘‘ Monomyaria.” The Pelecypoda are subdivided, according to the structure of the gills, into four orders: Order I.: PROTOBRANCHIA (Plate XVIL., Figs. 1-3), in which the gill-filaments take the form of flattened, leaf-like expansions (Aspidobranch, Plate IV., Fig. 5,4). To this order the Nut Shells (Nucula) and their. allies belong, and probably also the earliest known Bivalves, forms now quite extinct, that have been termed “‘ Palzoconcha.” With few exceptions they have a taxodont hinge. Order II.: FILIBRANCHIA (Plate XVIL., Figs. 5-17), having long, parallel gill-filaments, the ends of which are folded up, forming a W in section, and locked together by ciliated and sometimes interlamellar junctions (Plate IV., Fig. 5, B). To this order the Ark Shells, Mussels, and Pectens belong. Order III.: EULAMELLIBRANCHIA (Plates XVIII, XIX., and XX., Figs. 1-19), in which the elongated filaments are yet longer, more folded up, and per- manently united at intervals till they form a com- CLASSIFICATION 35 plete spongy network (Plate IV., Fig. 5, C). The Oysters, freshwater Mussels, Cockles, Venus Shells, Myas, and Ship-worms are examples of this order. Order IV.: SEPTIBRANCHIA (Plate XX., Figs. ; : pay 20-23), in which the gill-filaments are completely fused and transformed into a continuous muscular septum, with several perforations to admit of the circulation of the water (Plate IV., Fig. 5,D). A few deep-water forms (Poromyiide, Cetoconchide, and Cuspidariidz) constitute this order. Crass V.: The CEPHALOPODA (Plate XXI.), of which the Nautilus, the Cuttlefishes, and Octopods are examples, includes some of the most highly organized of the Mollusca, as well as the largest, for certain of the Cuttlefishes, it is calcu- lated, exceed 50 feet in length. The Cephalopoda are symmetrical animals, the two halves of the body corresponding in structure. The head, on either side of which there is a large and well-developed eye, is more or less distinct, and is surrounded by the foot, which has, so to speak, grown round it (Plate XXI., Fig. 1). Inthe Nautilus the margins of the foot are divided into lobes, each bearing a group of tentacles furnished with suctorial ridges, that are retractile within special sheaths (Plate XXI., Fig. 2). In the other groups the pedal appendages take the form of four or five pairs of elongate “arms,” these arms being furnished with rows of suckers or of hooks. 36 THE LIFE OF THE MOLLUSCA The mouth lies in the centre of these arms, and is furnished with a pair of powerful chitinous jaws that resemble a parrot’s beak (Plate III., Fig. 4); while the radula, though proportionately small, is well developed (Plate III., Fig. 20). The symmetrical body is surrounded by the bell- shaped mantle, covered in some cases by a shell, while those without an external shell frequently have lateral fins. On the ventral side a cavity is left between the mantle and the body (Plate XXI., Fig. 3, m.c.). In this “mantle cavity” the gills (g) are placed, and into it the termination of the reflexed alimentary canal opens, as well as the ink-sac (é), with which most members of the class are provided. Just at the mouth of the mantle cavity two lobes of the skin above the foot, sometimes free and some- times united at their margins, form a “funnel” or “siphon” (f). Through this siphon the water is discharged from the mantle cavity; quietly during ordinary respiration, or with great force when the creature propels itself backwards through the sea by means of the ejected stream. Through the same orifice the animal can at will eject the inky fluid from the ink-sac, with which it clouds the water on emergency when seeking to escape its foes. This power is possessed by all the living Cephalopoda save Nautilus, Cirroteuthis, and two species of Polypus. An ink-sac was present in the fossil Belemnites, and CLASSIFICATION 37 there is a picture in the possession of the Geological Society of London representing the fossil head of a reptile (Ichthyosaurus) executed with the fossil sepia from a Belemnite preserved in the same strata. The gills of the Cephalopoda are aspidobranch in type, and either four or two in number: the class is consequently divided into Tetrabranchia and Dibranchia. Among the Cephalopoda we meet, for the first time in the Mollusca, with internal structures of great import—namely, cartilages—which are espe- cially developed in the head. In Nawtilus there is an H-shaped cartilage, which supports the ventral portion of the nerve centres, two of its branches extending to the base of the funnel. In the Dibran- chia a cartilage completely invests the central nervous system, the cesophagus passing through it. Different Cephalopoda have additional cartilaginous pieces in other parts of the body, such as the bases of the fins and arms, at the base of the neck (when the mantle is not fused to the head), at the internal extremities of the retractor muscles of the head and funnel, and even in the two branchial lamelle. The shell, as seen in its fullest (external) develop- ment in the Nautilus and its fossil relations, as well as in the Ammonites, resembles that of the Gastro- poda in consisting of a single conical tube. In the earliest kinds of Nautilus (Orthocervas) the shell is quite straight, in some others it is curved; but in 38 THE LIFE OF THE MOLLUSCA the majority of Nautiloids and Ammonites it is more or less coiled discoidally, and generally the coil is in a forward direction over the animal’s back, or “ exo- gastric.” A few, however, are coiled in the reverse direction, or are ‘‘ endogastric” ; while in some cases the coil is produced into a helicoid, or even a turricu- lated spire. In yet other instances the shell may be coiled in the young state, and become more or less uncoiled (Scaphites, Crioceras, Hamites, etc.) or even straight (Baculites) in the adult (Plate XXII.). The striking feature of the Cephalopod shell, how- ever, lies in its internal structure (Plate I.). As the animal grows, it builds on to the open end of the shell to obtain increased accommodation, just as the Gastropod does; but since it grows in girth more than in length, it has, in order to obtain the requisite space, to shift bodily forward in its shell. This takes place gradually by the forward growth of the shell muscles on either side and the intervening pallial attach- ments (“annulus”’) till an unoccupied space is thus left behind, which is then partitioned off by a shelly wall (“‘septum”’); in this way the series of chambers, so familiar in sections of the Nautilus shell, are formed, each marking the completion of a period of growth. The septa are, however, not entire, a perforation in each is connected with that in its neighbour by a pipe, in part calcareous, in part chitinous, the whole forming a continuous tube which passes from the outermost chamber, or that occupied by the animal, CLASSIFICATION 39 through all the preceding ones to the apex of the shell. This tube, or “shell siphuncle,” covers a prolongation of the mantle, the function of which is not quite clear. The margins of the perforation in each septum are produced on one side into a short neck: these necks in the Nautiloidea and the most primitive of the Ammonoidea all point backwards; in the other Ammonoidea they point forwards. Further, the margins of the septa, where they join the outer shell, form a simple curve in the Nauti- loidea, whereas the “‘suture-line”’ in the Ammonoidea becomes highly folded (Plate XXI., Fig. 5). Another feature of interest distinguishes the shells of these two sub-orders ; in the Nautiloidea the protoconch is not calcareous, and the only trace left of its existence is a scar on the exterior termination of the first chamber; in the Ammonoidea the protoconch is calcareous and preserved. The chambers of the shell of Nautilus (and pre- sumably the samé was true of the Ammonites) during the life of the animal are filled with air containing more nitrogen than is found in atmospheric air. This gives great buoyancy to the shell, and so permits of the animal swimming rapidly. In Spirula alone of living Dibranchia the shell is partly internal (Plate XXV., Fig. 1), and the same was probably the case with some fossil forms; but in all other members of the order the shell is completely internal, and frequently rudimentary, while in the 40 THE LIFE OF THE MOLLUSCA Octopods there is no longer any true shell, but only some simple chitinous rudiments, to which the retractor muscles of the head and funnel are attached. In certain fossil Dibranchia (Belemnites, etc., Plate XXIII.) the internal chambered shell, known as the “phragmocone,” is enclosed in a pointed calcareous sheath, or “ guard,” at the end opposite to the head, while from the dorsal margin of the anterior end of the phragmacone there arises a broad, thin, chitinous plate, called the “ pro- ostracum.” To modifications in form of these three constituent parts, or to the partial or total suppres- sion of one or more of them, the resultant differences between the internal shells of the other members of the order may be traced. This, however, will be better gathered from the diagrammatic figures on Plate XXIV. than from any lengthy description. The shell of the Paper Nautilus, or Argonaut (Plate XXI., Fig. 4; Plate XXV., Fig. 12), stands on a different footing to the ordinary shell, and does not originate in the shell-gland; it is only formed some days after the creature is hatched, and is peculiar to the female, being chiefly used as a vehicle for carrying and protecting the eggs, which, when deposited, are agglutinated to it. It is exo- gastric, and composed of three layers, of which the outer and inner are alike and prismatic, while the middle one is fibrous: there is no pearly layer within. The animal is not attached to it in any CLASSIFICATION 41 way, and the shell is principally held in place and protected on the exterior by the anterior pair of arms, which are furnished with web-like expansions for the purpose. The ends of the remaining arms, which are carried folded back over the animal’s body, are tucked into the shell, and seemingly also assist in retaining it by the aid of the suckers. It is commonly stated that the shell is entirely the pro- duct of the pair of webbed arms, and that once the animal] quits its tenement it cannot re-occupy it; but both statements are incorrect. The latter has been shown by actual observation to be the reverse of fact, while the occurrence of fractured shells, showing evidences of repair from the inner side, upsets the former statement, which is otherwise wanting in probability. The more reasonable supposition is that the shell is mainly secreted by the mantle cover- ing the visceral dome, and that the webbed arms, which are also furnished with secretory cells, con- tribute to the exterior and assist in moulding it, during formation, to the animal’s body. The Cephalopoda are divided into— Order I.: TETRABRANCHIA (Plate XXII.), or those having four gills and an external shell. The funnel is in two parts, and the eyes are open, having no crystalline lens. Sub-Order 1: NAUTILOIDEA, or the Nautilus and its allies. Sub-Order 2: AMMONOIDEA, or the extinct group 42 THE LIFE OF THE MOLLUSCA of Ammonites and their kindred, that from the close parallelism of their shells, by which alone they are known, were probably similar in structure to the Nautiloidea, and are therefore better classed with them than in a group apart. Order IJ.: Drprancuia (Plate XXV.), or those having only two gills, and the shell generally more or less internal, The funnel is a complete tube, and the eyes have a crystalline lens. Sub-Order 1: Decapopa, or those with ten arms. Two of these, the “tentacular arms,” situated on each side between the third and the last pair, are more or less retractile into special pouches, and as a rule only bear suckers at their free extremities. The eight ordinary arms are shorter than the body. There is generally a fairly well-developed internal shell, and usually lateral fins. Two tribes are distinguishable: (2) Those (Oigop- sida) in which the cornea covering the eye is incom- plete—z.e., has an aperture left. This includes Sfirula, many of the Pen-and-Ink Fishes, and probably such fossil forms as the Belemnites, Belemnoteuths, etc. (6) Those (Myopsida) in which the cornea is complete, like the Common Cuttle (Sepia), the Common Pen-and-Ink Fish (Loligo), and their fossil kindred. Sub-Order 2: OcTopopa, or those with eight similar arms all longer than the body. These again fall into two tribes: (a) LEIOGLossA, CLASSIFICATION 43 in which the radula is wanting and the arms are united by a membrane, while fins are developed on the body. Some pelagic (Cirroteuthis) and deep-sea forms (Opisthoteuthis) belong here, and possibly also the fossil Palgoctopus. (b) TRAcHYGLossa, the members of which have a radula, but no true fins. The Common Octopus (Polypus vulgaris) and the Argonaut are familiar examples of this tribe. Of the interrelationships of these five classes it is impossible to say much in the present state of our knowledge, beyond that the first four are the more closely related. CHAPTER III GEOLOGICAL HISTORY we Mollusca made their appearance very early in the world’s history, and, as might be expected, on the whole the more generalized forms preceded the more specialized. It has, of course, to be borne in mind that many of the conclusions here epitomized have been drawn of necessity from shell characters alone, and are consequently liable to modification with possible advance of knowledge. In the oldest fossiliferous beds, belonging to the Lower Cambrian epoch, only a few representatives have been found. These consist of some Limpet- like shells that have been referred in part (Scenella) to the Docoglossa, and in part (Stenotheca, Platyceras) to the Tzenioglossa. These last, however, it has been suggested are, instead, descendants of the primi- tive Mollusca (Prostreptoneura) that gave rise to the Gastropod branch. With these is a turreted, con- volute shell (Rhaphistoma) representing the older, or two-gilled, section of the Rhipidoglossa, and the 44 GEOLOGICAL HISTORY 45 remains of two Bivalves. One of these Bivalves (Modioloides priscus) is known only by the internal cast, 2 millimetres long, and appears to belong to the Protobranchia, or oldest order, while the other (Fordilla) may prove to be a Bivalve crustacean and not a mollusc at all. ; In the Upper Cambrian further examples (Murcht- sonia, Cyrtolites, Owenella and Straparollina) of the early Rhipidoglossa are found, as well as one (Tvochonema) supposed to belong to the higher one- gilled section of that sub-order. Another Gastropod (Subulites) of doubtful affinities, but almost certainly belonging to the Pectinibranchs, and having a sipho- nostomatous shell, shows that three out of the four principal divisions of the Streptoneura were already represented at this early stage. Already, too, seven species of Cephalopoda had made their appearance. They all belong to the more primitive Nautiloidea, and, with one exception, the straight-shelled section of that group. During the succeeding Ordovician epoch the Aspidobranch Gastropods predominated, their ranks reinforced by representatives (Cyclonema) of the Top- Shells (Turbinidz) that belong to the more specialized one-gilled section, while Holopea and Scalites were added to the Tzenioglossates. Pelecypods were still rare, the Protobranchs were represented by Citeno- donta, and the more specialized Filibranchs by Cyrto- donta and Eopteria. The epoch also produced the 46 THE LIFE OF THE MOLLUSCA first known Chiton (Priscochiton). ‘The Cephalopoda ‘had increased in number to sixty-five species of Nautiloidea, the majority still being straight-shelled, but some curved and a few coiled forms (the first) are included. With the Silurian epoch a considerable increase in the number of Mollusca becomes evident. To the Gastropods are added members of the families Trochidz, Epitoniide, and Xenophoride. Among Pelecypods, Palzeoconchs were most abundant, but all orders save the Septibranchs were represented. Two more Chitons (Helminthochiton and Chelodes) made their appearance, and shells that have been referred, though doubtfully, to Scaphopoda. The most marked feature of the epoch, however, was the abundance of the Nautiloidea, which then attained their zenith with about 230 species, among which coiled were almost as abundant as the other shell forms. From that day the group has steadily diminished in numbers, only five species now exist- ing, or as some reckon them, fewer still. The Devonian strata have yielded evidence of the further increase in the Pelecypods, representatives of the Filibranch families—Trigoniide, Pectinide, and Mytilida — appearing with the Eulamellibranch families—Pinnide, Cardiniide, Megalodontide—and such specialized forms as Pholadella and Allorisma, the latter being the earliest example of a Pelecypod, showing clear evidence of retractile siphons. Of GEOLOGICAL HISTORY 47 most note, however, is the advent on the scene of the first freshwater Mussel, Avchanodon Jukesii, which is closely allied to, and greatly resembles, the modern Anodonta (Swan Mussel) of our ponds and lakes. A true Scaphopod (Dentalium) and representatives of the more primitive Ammonoidea (Clymenia) and the Goniatites likewise came into existence in the Devonian epoch. Just at the close of the Devonian epoch the first evidences are met with of the existence of Land Snails (Strophites, Dendropupa, etc.) allied to the Chrysalis Shells (Pupillidz); these were found in the plant beds at St. John, New Brunswick. The Coal-Measures of the succeeding carboniferous period have yielded some other interesting air-breathing Snails, including the oldest known terrestrial Rhipi- doglossate (Dawsonella); the first brackish-water Snail (Ampullaria); the earliest freshwater Snail, (Physa), both these last belonging to genera well known at the present day, besides the oldest known species (Zaptychius) of the most primitive of Pulmon- ates, the Auriculide, as well as other species of Dendropupa and a small Land Shell,* closely allied to the genus Pyvamidula, which is a common form to-day. Further examples of freshwater Bivalves belonging to the family Cardiniidz were plentiful in the Carboniferous, from which, too, the oldest example of a Tectibranch (Cylindrobullina) has been * Still miscalled Zonites in textbooks. 48 THE LIFE OF THE MOLLUSCA obtained, and the first evidence of the highly specialized Bivalve (Lithophagus) that burrows into rock, shell, or coral. At the close of the Palzozoic period many of the older genera of Bivalves disappeared, but at the opening of the Mesozoic period in the Trias, a number of others came in. Among them further representatives of the freshwater Mussels (Unio), of the Thorny Oyster family (Spondylidz), and the Cockles (Cardiidz). The oldest examples (Campylo- sepia, Aulacoceras, and Atractites) of the group of ten-armed Cuttlefish made their appearance also at this time. The Jurassic strata are rich in Molluscan remains, which sometimes form whole masses of rock. The Rhipidoglossate Gastropods attained their acme of development at that period, while Pectinibranchs multiplied in great variety, and representatives of the higher forms began to appear. Further remains of Land Shells (Helix, etc.) and the earliest examples of freshwater Gastropods (Planorbis, Valvata, and Melania) have been recorded from the lowest Jurassic beds, but there is some doubt as to the exact determination of these genera in most cases. In the uppermost beds, however, the well-known Purbeck marble is composed of masses of Valvata, Vivi- para, etc. Among Bivalves genuine Ark Shells (Arca), Anomia, and various families of Eulamellibranchs, GEOLOGICAL HISTORY 49 including the freshwater Cyrenide, made their appearance; while if Corburella be admitted as a member of the Septibranchia, that order must be added for the first time. The “pens” and ink-sacs of earlier members of the Sepia tribe (Beloteuthis and Geoteuthis) are first found in the Lias, with the remarkable Belemnites whose “ guards,” often called “ thunder-bolts,” are familiar fossils. In the succeeding Cretaceous period further de- velopment took place. Among the Gastropods there was a decided increase in the higher Pectinibranchs, including representatives of most of the families of Rhachiglossa. Amongst Bivalves two most remark- able aberrant families (Radiolitidz and Hippuritidz) were confined exclusively to this period. Externally these look not unlike simple corals with a lid, while internally they display highly peculiar modifications. Numerous other Eulamellibranchs, including some boring forms such as Petricolide and Saxicavide, as well as the Razor-fish (Eustis), arose, with an undoubted representative of the Septibranchs (Leio- pistha). An Octopus (Palgoctopus Newboldi) standing for the highest Cephalopods was revealed for the first time in the Cretaceous of Mount Lebanon, but, on the other hand, the Ammonites and Belemnites died out. During the Tertiary epoch the Rhachiglossa and Toxoglossa became the dominant Gastropods, while the Bivalves showed an approximation to present 4 50 THE LIFE OF THE MOLLUSCA conditions. A great majority of the Lower Tertiary (Eocene and Oligocene) genera still exist, but none of the species. During the succeeding Miocene a few species, which are still in existence, made their appearance, while of the Pliocene species 80 or go per cent. are represented in the recent fauna. At the close of the Eocene the wide distribution of many types now characteristic of warm, temperate, or tropical waters, began to be restricted, and during the Miocene the faunal boundaries of the Mollusca were mapped out nearly on existing lines. This was more true of the non-marine forms; but not till the Pliocene did each geographical province come to assume its present distinctive features. CHAPTER IV PRESENT HISTORY AND DISTRIBUTION W ITH such a past history as just recorded, it is little wonder that the Mollusca both have been and are abundant. In 1866 a rough estimate gave the number of The list has been considerably extended since that time, and though no further estimate of the fossil forms appears to have been attempted, a recent cal- culation puts the number of living species known at the end of last century at upwards of 50,000. This total may be distributed among the five classes as follows: Amphineura, 600; Gastropoda, 40,100 ; Scaphopoda, 230; Pelecypoda, 8,600 ; Cephalopoda, 470. Naturally, too, so ancient and so numerous a race is widely distributed over the surface of the globe to-day, and its members have become adapted to very varied conditions of life. The majority are marine, and mostly confined to the littoral and 51 52 THE LIFE OF THE MOLLUSCA laminarian zones (.¢., between tide-marks; and so far as the seaweeds grow, or to about fifteen fathoms) ; a smaller number inhabit the deeper nullipore or coralline zone; whilst a few stragglers are met with at great depths. In all cases the nature of the sea bottom governs their individual distribution. Certain forms frequent the rocks, others sandy or muddy sea- floors. Some, on the other hand, spend their lives in the surface waters of the open sea. The brackish waters of estuaries and lagoons are tenanted by a few kinds, including the strange pul- monate Amphibola (Plate XXVI., Fig. 23); while rivers, streams, and lakes are the dwelling-places of a considerable number. On the land, every spot capable of supporting life yields its quota of Mollusca, and the total number of known terrestrial species is consequently very great, and yearly being added to. As will presently be mentioned, some forms will normally trespass out of their regular habitat, but cer- tain more exceptional cases of interchange between marine and freshwater haunts may be appropriately alluded to here. Freshwater Snails sometimes become accustomed to salt-water conditions; thus, at Bornholm, in the Baltic, specimens of Limnaa and Theodoxis have been found living in company with marine molluscs in water containing as much as I to 1°5 per cent. of salt. In Southern Algeria Melania and Melanopsis inhabit waters surcharged with salt, where the marine Cockles failed to survive. PRESENT HISTORY AND DISTRIBUTION 53 Experiments conducted many years ago show that species of the pulmonate genera—Limnaa, Physa, Planorbis, and Ancylus—can be habituated by the gradual addition of salt to as much as 4 per cent. in the water ; so, too, but less easily, can the Proso- branchs Vivipara, Bithynia, and Theodoxis ; while the Pelecypods Anodonta, Unio, and Spharium die before that degree of salinity can be attained. Conversely, marine forms can be gradually accustomed to fresh- water existence: the Mussel (Mytilus) very easily ; the Cockle (Cardium edule), the Oyster, the Common Limpet (Patella vulgata), Turbo neritoides, and others, less successfully. In these experimental cases, how- ever, propagation would not take place. On the rocks by the margin of the sea, within reach only of the splash of the waves, or of the water at the highest tides, will be found certain of the Periwinkles (Littorinidz)—a situation in which they are joined by such of the Pulmonates as several of the Auriculide and the slug-like Oncidium, that dwell close down by the sea margin. A little lower down, just below high-water mark, those strange Limpet-like Pulmonates, Siphonaria, Gadinia (Plate XXVI., Figs. 24 and 25), and the recently discovered Apovemodon, that have partially reverted to marine life, are to be found. Between tide-marks the Chitons, Limpets (Patel- lide), Keyhole Limpets (Fisurellidz), Ormers (Halio- tidz), and similar molluscs, cling to the rocks or the 54 THE LIFE OF THE MOLLUSCA under surfaces of big stones. The Mussels, as well known, attach themselves to the rocks by their stout byssus threads, their clustered masses affording secure shelter to many lesser animals; while in mangrove swamps the Oysters will attach them- selves to the branches of the trees that dip in the water at high tide—a fact which was observed and recorded by W. Smith at Sierra Leone in 1726. Periwinkles, Top Shells (Trochidz and Turbinide), and other holostomes, haunt the tangled masses of seaweeds ; while among the siphonostomes are the Dog-Periwinkles (Purpura), Dog-Whelks (Nassa), etc., all stout-shelled forms capable of withstanding con- siderable buffeting amid the waves. The Piddock, or Pholas, Saxicava, Lithodomus, and other boring molluscs, excavate burrows in various rocks, in coral, and even the shells of their bigger confréres. Others, like Tapes and Coralliophaga, too lazy to make their own retreats, take possession of the deserted burrows of others. The latter generally selects the crypt of a dead Lithodomus, which it closely resembles in shape (Plate XXXII., Figs. 17 and 18), and the shells of successive generations of Coralliophaga, packed one within the other, will be found lying between the valves of ‘the original architect of the home. In the cracks and crevices of the rocks the Octopods hide. The majority of the Pelecypods, such’as the Cockles, Ark-Shells, 7 apes, etc., remain more or less buried in the sand or silt, the Razor-fish PRESENT HISTORY AND DISTRIBUTION 55 (Ensis) sinking itself well below the surface. Some species of Lima and Volsella, on the other hand, construct a sort of nest out of all kinds of marine refuse, held together by the threads of the byssus. The Dentalium, again, buries in the sand, leaving only the apex of the shell protruding. The quieter waters of the laminarian zones are also tenanted by many of the foregoing, but they are especially the haunt of the Sea-Slugs (Nudibranchia, Plate XII., Figs. 20-28), as well as certain Opistho- branchs, like the Sea-Hare (Aplysia), and the smaller Gastropods, with the Oyster and other Bivalves. The yet deeper regions of the coralline and nulli- pore zones are the special resort of the large Whelks (Buccinum undatum, Neptunea antiqua, etc.), Naticas, and other carnivorous Gastropods, of the Scallops (Pecten maximus, P. opercularis, etc.), Pinna, many of the Venus Shells, and other Bivalves. Here, too, the strange Aplacophora frequent the oozy areas. Down to a depth of 300 fathoms there is a con- siderable number of deep-sea representatives of the Mollusca, mostly of the smaller and more delicate kinds, while a few abyssal examples are found at very great depths. So far the extreme depths for each class that have been obtained were all recorded on the Challenger Expedition. Thus, a Chiton (Leptochiton benthus) was dredged at a depth of 2,300 fathoms in the Pacific Ocean; of Aplacophora, two immature 56 THE LIFE OF THE MOLLUSCA examples of Chétoderma off Nova Scotia at 1,250 fathoms. A Gastropod (Stylifer brychius) was brought up in the South Atlantic from a depth of 2,650 fathoms; a Nudibranch (Bathydoris abyssorum) in the Pacific Ocean from 2,425 fathoms; and some Pelécypods also in the Pacific from 2,g00 fathoms; while a Scaphopod (Dentalium leptoskeles) was ob- tained from 2,600 fathoms off the south coast of Australia. Cuttlefishes have been taken from between 2,000 and 3,000 fathoms, but some uncertainty attaches to the records in their case, since none of the forms obtained at these depths were distinctly dwellers on the sea-floor. No hard and fast line can be drawn between the several zones above enumerated, and some species range over more than one of them; nevertheless, the prevailing forms in each serve to distinguish them. Conspicuous and most abundant among the ocean- swimming molluscs are the Sea-Butterflies (Plate XXVIL., Figs. 1-16), formerly grouped in a class, as Pteropoda, but now recognized as highly specialized Tectibranchs—those with shells branching off from the Bulla-like section, while the shell-less ones are more nearly related to the Sea-Hares (Aplysiidz). Less abundant are the Heteropoda (Plate XXVII., Figs. 17-22), now known to be free-swimming Tzenio- glossa, although they, too, were once classed apart. A Nudibranch (Phyllivrhoé) similarly specialized, and the Tzenioglossate (Ianthina), with countless fry PRESENT HISTORY AND DISTRIBUTION 57 of many species, also pass their lives far from land, with the larger number of the Cuttlefishes. A more peculiar pelagic assemblage is afforded by «those molluscs that dwell among the Gulfweed (Saragassum). They comprise a series of Nudibranchs, some of which are peculiar to the locality, a Taenio- glossate (Litiopa), and two or three Limpets (Helcion and Lepeta). Just as some representatives of the race have crept down into the depths, so others have ascended the rivers, and there become accustomed to a freshwater existence. In the brackish water near the mouths of rivers, and occasionally for long distances up the rivers themselves, some stragglers from the marine host are to be met with, such as Cerithium, Littorina, Nerita, Mytilus, Cardium, Macoma, Mya, etc., where they may be found dwelling side by side with fresh- water kinds. The genus Theodoxis, of course, has both marine and freshwater species, while there are freshwater representatives of the Ark-Shells, such as Scaphula pinna, of Volsella, and of the Piddocks, Pholas rivicola, and others. There is, however, a distinct brackish-water group, including Gastropods belonging to the Tzenioglossate genera Potamides. Certain of the Paludestrinide, Assemania, Truncatella, the Pulmonate Chilinia, and Pelecypods such as Iphigenia and Scrobicularia. The true freshwater forms consist solely of Gastro- 58 THE LIFE OF THE MOLLUSCA poda and Pelecypoda. The former include some species of the Rhipidoglossate Theodoxis and its relative Septaria; Teenioglossate families such as the Vivip- aride, Ampullariide (a completely amphibious clan), Valvatide, most of the Paludestrinidz, Melaniide, Typhobiide (in Lake Tanganyika), and Pleuroceride ; Rhachiglossate forms as Canidia, Clea,and Nassodonta, and Pulmonate families as Limnzidz, Planorbide, Ancylide, Physide, commonly known as Pond Snails (Plate XIII., Figs. 1-7). The Pelecypoda belong mostly to the Sub-Mytilacea, and include the big family Unionide, or freshwater Mussels, of which the United States boasts no less than about 530 species, or more than half of those known; with their kindred the Mutelide; the freshwater Oysters, Etheriidz, of the African and American rivers; the Rangiide, Cyrenide, and the interesting Zebra Mussel (Dreissensia, Plate XV., Fig. 9).