VIII. THE DIGESTIVE ORGANS, JAW, AND RADULA: EXCRETORY ORGANS…

VIII. THE DIGESTIVE ORGANS, JAW, AND RADULA: EXCRETORY ORGANS

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

{ (a) Toxoglossa { (b) Rachiglossa { Monotocardia { (c) Taenioglossa { { (d) Ptenoglossa =Prosobranchiata= { { (e) Gymnoglossa { { Diotocardia { (f) Rhipidonlossa { (g) Docoglossa[324]

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

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

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

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

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

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

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

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

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

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

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

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

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

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

(1) ∞.5.1.5.∞, i.e. one central, five laterals, including the ‘last lateral’ tooth.

(2) (∞.1).4.1.4.(1.∞), regarding the ‘last lateral’ as first uncinus, but specialising it by a number.

(3) ∞.1.(4.1.4).1.∞, regarding the ‘last lateral’ as the only lateral.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

=The Kidneys=

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

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

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

* * * * *

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

=D. Barfurth=, Ueber den Bau und die Thätigkeit der Gasteropodenleber: Arch. Mikr. Anat. xxii. (1883), pp. 473–524.

=Th. Behme=, Beiträge zur Anatomie und Entwickelungsgeschichte des Harnapparates der Lungenschnecken: Arch. Naturges. iv. (1889), pp. 1–28.

=R. Bergh=, Semper’s Reisen im Archipelago der Philippinen; Nudibranchiata: Theil ii. Band ii. (1870–78), Band iii. (1880–1892).

=W. G. Binney=, Terrestrial Air-breathing Mollusks of the United States: Bull. Mus. C. Z. Harv. iv. (1878), 450 pp.

„ On the Jaw and Lingual Membrane of North American Terrestrial Pulmonata: Proc. Ac. Nat. Sc. Philad. (1875), pp. 140–243.

=J. T. Cunningham=, The renal organs (Nephridia) of Patella: Quart. Journ. Micr. Sc. xxiii. (1883), pp. 369–375.

„ „ Note on the structure and relations of the kidney in Aplysia: Mitth. Zool. Stat. Neap. iv. (1883), pp. 420–428.

=R. von Erlanger=, On the paired Nephridia of Prosobranchs, etc.: Quart. Journ. Micr. Sc. xxxiii. (1892), pp. 587–623.

=H. Fischer=, Recherches sur la Morphologie du Foie des Gastéropodes: Bull. Scient. France Belg. xxiv. (1892), pp. 260–346.

=C. Grobben=, Morphologische Studien über den Harn- und Geschlechtsapparat, sowie die Leibeshöhle, der Cephalopoden: Arb. Zool. Inst. Wien, v. (1884), pp. 179–252.

„ Die Pericardialdrüse der Gasteropoden: ibid. ix. (1890), pp. 35–56.

=B. Haller=, Beiträge zur Kenntniss der Niere der Prosobranchier: Morph. Jahrb. xi. (1885), pp. 1–53.

=A. Hancock=, On the structure and homologies of the renal organ in the Nudibranchiate Mollusca: Trans. Linn. Soc. xxiv. (1864), pp. 511–530.

=A. Köhler=, Microchemische Untersuchung der Schneckenzungen: Zeits. Gesamm. Naturw. viii. (1856), pp. 106–112.

=Ad. Oswald=, Der Rüsselapparat der Prosobranchier: Jena. Zeits. Naturw. N.F. xxi. (1893), pp. 114–162.

=R. Perrier=, Recherches sur l’anatomie et l’histologie du rein des Gastéropodes prosobranches: Ann. Sc. Nat. Zool. (7), viii. (1889), pp. 61–315.

=C. Semper=, Reisen im Archipelago der Philippinen; Land Pulmonata: Theil ii. Band iii. (1870–77).

=C. Troschel=, Das Gebiss der Schnecken: Berlin, 1856–1892.

=W. G. Vigelius=, Ueber das Excretionssystem der Cephalopoden: Niederl. Arch. Zool. v. (1880), pp. 115–184.

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

Chapter IX

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

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

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

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

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

The shell may be

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

=Special Points in the Growth of Certain Genera.=--In the young of Coecum the apex is at first spiral, but as growth proceeds and the long tube begins to form, a septum is produced at the base of the apex, which soon drops off. Soon afterwards, a second septum forms a little farther down, and a second piece drops off, leaving the shell in the normal cylindrical form of the adult (Fig. 170). The development of Fissurella is of extreme interest. In an early stage it possesses a spiral shell, with a slit on the margin of the outer lip of the last whorl. As growth advances, shelly matter is deposited on both margins, which results in the slit becoming a hole and the spire a mere callosity, until at last they appear to coalesce in the apex of the adult shell (Fig. 171). The singular formations of Magilus and Rhizochilus have already been described (pp. 75, 76). Cypraea, in the young stage, is a thin spiral shell with a conspicuous apex. As growth proceeds, the surface of the whorls, which are nearly enveloped by two large lobes of the mantle, becomes overlaid with new layers of shelly matter, until eventually the spire becomes embedded, and ultimately disappears from view (Fig. 172).

Patella, when young, has a nautiloid shell (see Fig. 45, p. 134), but it is a remarkable fact that we are entirely ignorant, in this commonest of molluscs, of the transition stages which convert the nautiloid into the familiar conical shell. The young shell of Pteroceras is deceptively unlike the adult, and is entirely devoid of the finger-like processes which are so characteristic of the genus (chap. xiv.).

Among the bivalve Mollusca, Anomia in a young stage is not to be distinguished from Ostrea. Soon a small sinus appears on the ventral margin, which gradually deepens and, as the shell grows round it, forms a hole for the byssus, eventually becoming fixed beneath the umbones (see Fig. 173). In Teredo the two valves of the shell proper, which is very small, become lodged in a long calcareous tube or cylinder, which is generally open at both ends (see chap. xvi.). In Aspergillum a somewhat similar cylinder is developed, but the valves are soldered to the tube, and form a part of it, the tube itself being furnished, at the anterior end, with a disc, which is perforated with holes like the rose of a watering-pot. In Clavagella the left valve alone becomes soldered to the tube, while the right valve is free within it (see chap. xvi.). Fistulana encloses the whole of its shell in a long tapering tube, which is not at any point adherent to the shell.

=Terms employed to denote various Parts of the Univalve Shell.=--The apex is the extreme top of the spire, and generally consists of the embryonic shell, which may often be recognised by its entire want of sculpture. When the embryonic shell happens to be large, the apex is often mammillated, e.g. in Fusus, Neptunea, and some Turbinella; in the Pyramidellidae it is sinistral.

The suture is the line of junction between any two successive whorls. It may be deep, and even channelled, or very shallow, as in Fig. 150 B (p. 246).

The spire is the whole series of whorls except the last or body whorl. A whorl is a single revolution of the spiral cone round the axis. The spire may be subulate (as in Terebra, Fig. 150 C), turreted (Scalaria), depressed (Polygyratia, Fig. 150 A), conical (Trochus), globose (Ampullaria, Natica, Fig. 150 B), with almost all conceivable gradations between these types. The number of whorls is best counted by placing the shell mouth downwards, and reckoning one for every suture that occurs between the extreme anterior point of the shell and the apex.

The mouth or aperture may be (a) entire, as in Helix, Natica, Ampullaria, when its peristome or margin is not interrupted by any notch or canal, or (b) prolonged at its anterior and sometimes also at its posterior end into a canal. The anterior canal serves as a protection to the siphon,[347] the posterior canal is mainly anal in function, and corresponds, in part, to the hole of Fissurella, the slit in Pleurotoma and Emarginula, and the row of holes in Haliotis. The mouth presents every variety of shape, from the perfect circle in Cyclostoma and Trochus, to the narrow and prolonged slit in Conus and Oliva.

The right margin of the mouth (the left, in sinistral shells) is termed the outer lip or labrum, the left margin the inner lip, labium, or columella lip.[348] In young shells the outer lip is usually thin and unfinished, while in the adult it is generally thickened into a rib, or furnished with more or less prominent teeth, or given an inward or outward curve. In some genera, especially the Strombidae, the outer lip of the adult develops long finger-like processes, which sometimes attain an extraordinary size (chap. xiv.). As growth proceeds, these marginal teeth and ribs are either dissolved and disappear, or are permanently incorporated, in the shape of varices, with the framework of the shell. Some shells, e.g. Natica, Turritella, Actaeon, have a permanently unfinished outer lip, even in the adult stage. The columella lip varies in shape with the mouth as a whole; thus it may be straight, as in Conus, or excavated, as in Sigaretus, Struthiolaria, and Bulla. Frequently it is continued by part of the body whorl, as in Ficula, Dolium, and Fasciolaria.

The folds or plaits on the columella, which are often characteristic of the genus or even family (e.g. Fasciolariidae, Mitridae, Turbinellidae) are not merely external, but continue down the whole spire (see Fig. 177, which also shows how successive fresh growths have thickened the columella).

The whorls may be wound in a spiral, which is either hollow, as in Solarium, or quite compact, as in Oliva, Terebra, Cypraea, with every possible intermediate grade. This concavity, which varies in depth and width, is known as the umbilicus, and shells are accordingly spoken of as deeply (e.g. most Trochidae and Naticidae), narrowly (e.g. Lacuna, Littorina), or widely (e.g. Solarium) umbilicated. When the spiral is quite flat, as in Planorbis and some Helix, the umbilicus vanishes entirely. Shells in which the whorls are so compactly coiled on an ascending spiral that there is no umbilicus, are termed imperforate.

The Slit.--Many shells are furnished with a slit in the last whorl, which opens, in most cases, on the outer lip, and is sometimes of considerable depth, at others a mere notch. In the patelliform shells it is always in front of the apex. The function of the slit appears to be mainly anal, the excretory products being thus allowed to escape by a passage of their own, without soiling the clean water taken in by the branchiae. The posterior canal of some Gasteropoda probably performs a similar function. In the adult Fissurella the slit becomes an apical hole (see Fig. 178 F), in the allied genera it is either immediately in front of the spire (Puncturella), or half-way between the spire and the anterior margin (Rimula), or on the margin and well marked (Emarginula), or a mere indentation of the margin (Hemitoma). In Pleurotomaria it is exceptionally long, and is well marked in Bellerophon, Schismope, Scissurella, Murchisonia, and Pleurotoma (where it is sutural). In Haliotis and Polytremaria it is replaced by a series of holes, which are closed up as the animal grows past them. Some of these holes (at least in Haliotis) certainly serve the purpose of admitting water to the branchiae, while others are anal. In Trochotoma there are only two holes, united by a narrow fissure.

The Tubed Land Operculates.--A group of the Cyclophoridae, which is restricted to Further India and the great Malay Islands, has developed a remarkable sutural tube on the exterior of the last whorl, near the aperture, A similar tube, but more obscure, exists in Alycaeus. Several stages in the development of this tube may be noticed, beginning with the elevation of part of the peristome into a simple irregular shelly plate, which is continued, first into a short, and then into a long tube, which becomes soldered to the shell; finally, the tube becomes free, and the anterior part of the last whorl is disconnected from the spire (Fig. 180 A-D).

It is singular that the tube does not appear to be of any use to the animal, since its internal extremity, in the complete form, is closed, and does not communicate with the interior of the whorl. It may be presumed, however, that in origin the tube served as a means of conveying air to the animal when the operculum was closed. The holes in the peristome of Pupina, Cataulus, and Anostoma (Fig. 154) may be compared.

=The Operculum.=--The operculum is a cuticular development of a group of cells situated on the dorsal side of the foot, exactly over the terminal point of the fibres of the columellar muscle. It is so situated that in crawling it is generally carried free of the shell, sometimes at the extreme upper end of the foot, more usually somewhat nearer to the shell (Fig. 181). In Pterocyclus it is pushed back into the umbilicus when the animal is in motion.

The operculum is present in nearly all land, fresh-water, and marine Prosobranchiata, absent in all Opisthobranchiata in the adult state, except Actaeon, and in all Pulmonata, except Amphibola. It has been lost in the following marine Prosobranchiata: many Cancellariidae and Conidae, Oliva (though present in Olivella and Ancilla), Harpidae, Marginellidae, Voluta proper (though present in V. musica), nearly all Mitridae, Cypraeidae, Doliidae, Ianthinidae; and, of land genera, in Proserpinidae. It is evident, therefore, that its presence or absence is of limited value in classification. In some species of Ampullaria and Natica it is horny, in others shelly. Dall found that in a number of specimens of Volutharpa ampullacea, 15 p.c. had opercula, 10 p.c. traces of the operculigenous area, but no operculum, the rest no trace of either. Monstrosities of Buccinum undatum sometimes occur, which have two, or in rare case three opercula.

As a rule, the operculum exactly fits the mouth of the shell. But in cases where the mouth is very large (e.g. Conus, Strombus, Concholepas, some Bullia), it only covers a very small portion and is quite inadequate as a protection (Fig. 62, p. 155). Again, when the shell has assumed a more or less limpet-shaped form, and habitually adheres to flat surfaces without much occasion for locomotion, the operculum becomes degraded and is probably on the way to being lost altogether. This is the case with Navicella (a modified Nerita, see Fig. 13, p. 17), Concholepas (a modified Purpura), Sigaretus (a modified Natica). Probably the more completely patelliform shells of Crepidula, Haliotis, Fissurella, and Patella have reached the stage at which the operculum has been lost entirely. In Navicella, besides becoming degraded, the operculum has actually become partly internal, and apparently serves the purpose of separating the viscera from the upper part of the foot, something like the shelly plate in Crepidula. This explains why the operculum in this genus is polished on both sides.[349]

Some authors have imagined that the operculum is homologous (a) to the second valve in Pelecypoda, (b) to the byssus. It differs, however, morphologically from the former in the essential point of not being produced by the mantle, and from the latter in not being produced by a special gland.

As regards shape and formation, the operculum has usually a more or less well-marked nucleus which may be central (e.g. Livona), sub-central (Ampullaria), lateral (Purpura), or terminal (Pyrula). As a rule, both the inner and outer surfaces are fairly flat, but in Torinia, Cyathopoma, and Pterocyclus the outer surface is elevated and conically spiral, in some Turbo (e.g. Sarmaticus) it is covered with raised tubercles resembling coral, while in others (e.g. Callopoma) it is scored with a deep trench. Aulopoma, a land genus peculiar to Ceylon, has a paucispiral operculum with hollow whorls, deceptively like a Planorbis; it fits over the aperture instead of into it. In Livona and most Trochidae the operculum is cartilaginous and multispiral. In Strombus it is narrow, curved, and often serrated like a leaf on one of the edges; in Conus it is narrowly oblong and rather featureless; in Littorina, paucispiral and always cartilaginous. In many cases (e.g. Paludina) there is no true spiral form, but the striae are concentric to a nearly central nucleus, and thus give the appearance of a spiral. The evolution of the operculum in Navicella from Nerita has already been illustrated (p. 10). Neritopsis has a very remarkable operculum, the striated appendage of which locks behind the columella of the shell, like the tooth in the opercula of the Neritidae.

=Terms employed to denote various parts of the Bivalve Shell.=--The umbo, or beak, is the apex of the hollow cone, of which each valve may be regarded as consisting. This apex is usually more or less twisted: it is markedly spiral in Isocardia, Diceras, some Chama, and especially Requienia, while in Pecten, Lepton, and others the spiral is altogether absent. As a rule the umbones point forward, i.e. towards the anterior end of the shell. In Donax, Nucula, and Trigonia, however, they point backward. The umbones are generally more or less approximated, but in Arca they are widely separated.

An equilateral shell is one in which the umbones are more or less central with regard to its anterior and posterior portion, while in an inequilateral shell the umbones are much nearer one end than the other. On the other hand, equivalve and inequivalve are terms used to express the relation of the two valves to one another as a whole. Thus nearly all bivalve shells are more or less inequilateral, but a comparatively small proportion are inequivalve.

The dorsal margin is adjacent to, the ventral margin opposite to, the umbones. The anterior and posterior margins are respectively the front and hinder edges of the shell.

The muscles which serve to close the valves leave impressions on the inner surface of each valve. These, when both muscles are present, are known as the anterior and posterior adductor impressions. The impression produced by the muscular edge of the mantle, which curves downwards and backwards from the anterior adductor impression, is known as the pallial line. In shells with only one muscle it is represented by an irregular row of small marks, or disappears altogether (Ostrea). The pallial sinus is produced by the muscles which retract the siphons, and is most marked in those genera in which the muscles are powerful and the siphons large (e.g. Tellina, Mya). It is entirely absent in genera possessing no retractile siphons.

Right and Left Valve.--The simplest way of distinguishing the valves as right and left is to hold the shell in such a way that the siphons point towards the observer, and the mouth away from him; in this position the valve to the right is called the right valve, and the valve to the left the left valve. If, however, the animal is not present, it may be remembered that the ligament is nearly always behind the beaks, and that the beaks, as a rule, point forward, thus the right and left valves can generally be named by observation of the beaks and ligament. When the ligament is median to the valves (e.g. Ostrea, Pecten), and the beaks are not curved, the valves may be recognised by noting the fact that the impression of the adductor muscle (in these cases always single) is nearer to the posterior than to the anterior side. In a similar way the pallial impression, which only forms a sinus on the posterior side, furnishes a guide to the valves of Donax, in which the beaks point backward, and of Tellina, in which the beaks are frequently central.

In the fixed inequivalves (e.g. Chama) it is sometimes the right, sometimes the left valve which is undermost, but the fixed valve, whether right or left, is always deep, and the free valve flat. Ostrea and Anomia are always fixed by the left valve.

The lunule is a well-marked area in front of and close to the umbones, usually more or less heart-shaped, and limited by a ridge. Generally it is shallow, but sometimes, as in Dosinia, Opis, and some Cardium, deeply impressed. A corresponding area behind the umbones, enclosing the ligament, is called the escutcheon (Fig. 186), but it seldom occurs.

The ligament is a more or less elastic band, which unites the two valves along a line adjacent to the umbones. As a rule, the greater part of the ligament is external to the shell, but it may be entirely internal. It is placed, normally, behind the umbones, but in a few cases, when the hinge line is very long (Arca, Pectunculus), it extends in front of them as well. The edges of the valves, when the ligament is mainly external, are more or less excavated for its reception. When internal it is generally contained in a groove or spoon-shaped pit, known as the fossette (compare Fig. 187).

The ligament consists of two distinct parts, which may occur together or separately, the external, or ligament proper, and the internal, or cartilage. Only the external portion can be seen when the valves are closed. As a rule, the two portions are intimately connected with one another, the ligament folding over the cartilage, but in some cases, e.g. Mya, Mactra, where the cartilage is lodged within the hinge, they are completely disconnected (Fig. 187).

In Pecten the external ligament is very thin, and runs along the dorsal margin, while the internal ligament is large, solid, and situated in a shallow pit. In Perna, where the hinge is toothless, the ligament is folded into a number of transverse ridges, which fit into corresponding grooves in the shell.

The ligament proper is inelastic and insoluble in caustic potash. The cartilage is very elastic, composed of parallel fibres, slightly iridescent, and soluble in caustic potash.

The operation of the ligament--using the word as including the whole ligamental process--is in opposition to that of the adductor muscles. When the latter close the valves, they compress the ligament, an action which its elasticity resists: thus its operation tends in part towards keeping the valves open. But when ligament and cartilage are both fully developed, they work in opposition to one another, the ligament, by its resistance to compression, preventing any straining of the adductor muscles when the valves are open, and the cartilage, for the same reason, preventing the ventral margins of the shell from closing too rapidly upon one another when the valves are being shut.

The Hinge.--The valves of Pelecypoda are generally articulated, below the umbones, by a hinge which is furnished, in the majority of cases, with interlocking teeth, small pits or depressions in each valve corresponding to the teeth in the other. The teeth are distinguished as cardinal, or those immediately below the umbo, and lateral, or those to either side of the cardinals, the latter being also distinguished as anterior and posterior laterals, according as they are before or behind the umbo (Fig. 184). In shells which are tolerably equilateral there is no difficulty in distinguishing between cardinal and lateral teeth. But when they are very inequilateral, the whole hinge may share in the inequality of growth, and an anterior lateral may be thrown backward and simulate a cardinal, or a cardinal may be thrown backward and simulate a posterior lateral (e.g. Cardita, Unio, Fig. 188). In many Chama the cardinals are pushed up into the umbo and become a mere ridge, while the strong anterior lateral becomes nearly central and simulates a cardinal.

Some bivalves, e.g. Anodonta, Ostrea, Pedum, many Mytilus, have no hinge teeth at all, in others the laterals are wanting (Psammobia, Diplodonta). In the Arcadae the hinge consists of a number of very similar denticles, which are often serrated like the teeth of a comb (Fig. 189).

Hinge-teeth are probably, in origin, derived from the crenulations or ribbings of the surface of the shell, the upper ends of which impinge upon the dorsal margin and mark it in a way which is quite recognisable when the shell is thin. Similar crenulations, resulting in interlocking of the valves, are not uncommon upon the ventral margin in certain genera (Fig. 190). The mechanical effect of these continued riblets, when fitted together on the opposing valves, would be to prevent the valves sliding upon one another while closing, or after being closed. Thus there would be a probability of their surviving, even after the ribbing had disappeared from the surface of the shell, the increased strength given by the hinge compensating for, and making it possible to do without, the extra strength supplied by the ribs. It is therefore possible that the teeth of the Nuculidae and Arcadae, which have no distinction between cardinals and laterals, represent a very ancient type, from which have been evolved the various forms of hinge in which cardinals and laterals are distinguished. Even in some forms of Arcadae (comp. Pectunculus) we get a hint how the transverse teeth of the typical Arca may have become transformed into the longitudinal tooth of the normal lateral.[350]

The developed hinge-teeth, then, ensure the opening of the valves in one direction; they also secure their accurate closure upon one another in exactly the same plane. Exposed shells and gaping siphons matter little to animals which are protected by their burrowing propensities, but to those which live in material which can be easily penetrated by their foes, it must be of advantage to be able to buckle their armour absolutely tight. The edentulous hinge of Anodonta is a degeneration from a dentate type, which retains its teeth (in Unio, etc.) when subject to the jar of rapid streams, but tends to lose them in the stiller waters of canals, lakes, and ponds.

Other processes in the bivalve shell.--In Anatina each umbo is fissured and strengthened on the inside by a kind of umbonal plate which carries the ligament. Some forms of Liligna develop a strong internal umbonal rib, which serves as a buttress to strengthen the shell. In Pholas, the so-called falciform process serves as a point of attachment for the muscles of the foot and viscera. There is no ligament or hinge-teeth, the place of the latter being taken by the anterior adductor muscle, which is attached to the hinge-plate, the latter being reflected back into the shell.

In Septifer the anterior adductor muscle is carried on a sort of shelf or myophore, and in Cucullaea the posterior adductor is partly raised on a similar and very prominent formation.

Length and breadth of bivalve shells is variously measured. Most authorities measure length, or ‘antero-posterior diameter,’ by a straight line drawn from the extreme anterior to the extreme posterior margin, and breadth by a similar line, drawn from the umbones to a point, not very clearly marked, on the opposite ventral margin (see Figs. 184 and 185). Others, less correctly, reverse these terms. Thickness is measured by the extreme distance of the opposite faces of the closed valves. As a rule, the length exceeds, and often greatly exceeds, the thickness, but in a few cases--e.g. the Cardissa section of Cardium--this is reversed.

The periostracum.--Nearly all shells are covered, at some period of their growth, by a periostracum,[351] or surface skin, which serves the purpose of protecting the shell against the destructive effects of the chemical action set up by water or air. It also, in some cases (see p. 258), acts as a kind of base upon which the shell is deposited. In old shells it is commonly worn away, especially at those parts which are likely to become abraded.

The form and composition of the periostracum varies greatly. Sometimes (e.g. Oliva) it is a mere transparent film, at others (Zonites) it is transparent, but stout and solid. It is corneous in Solenomya, covered with fine hairs in many Helicidae, in Conus, Velutina, and Cantharus it is thick, fibrous, and persistent; in Trichotropis and some Triton it is furnished with long bristles on a thick ground (Fig. 191). In fresh-water shells it is usually rather thick, in order to protect the shell from the erosive powers of certain kinds of water. In some cases (Mya, Anatina) the periostracum is continued over the siphons, so as to form a protection throughout their whole length.

Erosion.--The fresh-water Mollusca generally, and marine mollusca in a few rare cases (Purpura, Littorina) are subject to erosion, or decay in the shell substance. In univalves erosion usually sets in near the apex (Fig. 192), where the life of the shell may be regarded as weakest, and in bivalves near the umbones. It is commonest in old shells, and rarely occurs in the very young. So long as the periostracum is present to protect the shell, erosion cannot set in, but when once it has been removed the shell is liable to the chemical changes set up in its substance by water. There is abundant evidence to show that erosion is caused by pollution of water. Out of many instances one must suffice. In a certain stream near Boston, U.S., numbers of Mollusca occurred, the shells of which were very perfect and free from disease. Some little way down stream an alkaline manufactory drained its refuse into the water. At and below this point for some distance every shell was more or less eroded, most of them seriously. Farther down, when the alkali refuse became diluted away, the shells retained their normal condition.[352]

A small percentage of lime in the water appears to produce erosion. The result of some experiments by G. W. Shrubsole, in the investigation of this point, may be tabulated as follows:[353]--

Water from Lime present Result per gall.

River Dee, near Chester 3·00 grs. acted strongly on shells Wrexham 4·00 grs. „ „ „ River Dee, near Llanderyel 0·53 grs. „ „ „ Trent Canal 8·33 grs. no action „

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