CHAPTER 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).

[Illustration: FIG. 107.--Jaws of various Pulmonata: =A=, _Limax_ (_gagates_ Drap., Lancashire, × 15); =B=, _Helix_ (_acutissima_ Lam., Jamaica, × 15); =C=, _Bulimulus_ (_depictus_ Reeve, Venezuela, × 20); =D=, _Achatina_ (_fulica_ Fér., Mauritius, × 7); =E=, _Succinea_ (_elegans_ Riss., Aral District, × 30); =F=, _Limnaea_ (_stagnalis_ L., Cambridge, × 30).]

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

[Illustration: FIG. 108.--Jaws of =A=, _Triton australis_ Lam., Sydney; =B=, _Ampullaria fasciata_ Reeve, Demerara; =C=, _Calliostoma punctulatum_ Mart., New Zealand; =D=, _Cyclophorus atramentarius_ Sowb., Sanghir; all × 15.]

[Illustration: FIG. 109.--Jaws of =A=, _Chromodoris gracilis_ Iher., × 15; =B=, _Scyllaea pelagica_ L., × 7; =C=, _Pleurobranchus plumula_ Mont., × 10; =D=, _Pleurobranchaea Meckelii_ Lam., × 5/2.]

The carnivorous genera, whether marine (_e.g._ _Conus_, _Murex_, _Buccinum_, _Nassa_) or land (_e.g._ _Testacella_, _Glandina_, _Streptaxis_, _Ennea_), are entirely destitute of jaws, the reason probably being that in all these cases the teeth of the radula are sufficiently powerful to do the work of tearing up the food without the aid of a masticatory organ as well. Jaws are also wanting in the Heteropoda, and in many of the Nudibranchiata and Tectibranchiata.

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

[Illustration: FIG. 110.--Jaws of _Sepia_: =A=, _in situ_ within the buccal mass, several of the arms having been cut away; =B=, removed from the mouth and slightly enlarged.]

[Illustration: FIG. 111.--_Patella vulgata_ L., showing the normal position of the radula, which is doubled back in a bow; the shell has been removed, and the whole visceral mass is turned forward, exposing the dorsal surface of the muscular foot: _gr_, longitudinal groove on this surface; _i_, _i_, intestine; _l_, liver; _m_, _m_, mantle edge; _mu_, muscles (cut through) fastening the visceral mass to the upper sides of the foot; _ov_, ovary; _r_, radula; _u.f_, upper or dorsal surface of the foot.]

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

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

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

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

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

[Illustration: FIG. 112.--Example of a front portion of a radula (_Cantharus ringens_ Reeve, Panama), much worn by use. × 70.]

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

In _Chaetoderma_ there is but one tooth. In _Aeolis coronata_ there are about 17, in _A. papillosa_ and _Elysia viridis_ about 19, in _Glaucus atlanticus_ about 21, in _Fiona nobilis_ about 28. In the common whelk (_Buccinum undatum_) there are from 220 to 250, in the common periwinkle about 3500. As many as 8343 have been counted in _Limnaea stagnalis_, about 15,000 in _Helix aspersa_ (that is, about 400,000 to the square inch), about 30,000 in _Limax maximus_, and as many as 40,000 in _Helix Ghiesbreghti_, a large species from Mexico; they are very numerous also in _Nanina_, _Vitrina_, _Gadinia_, and _Actaeon_. But _Umbrella_ stands far and away the first, as far as number of teeth is concerned. In both _U. mediterranea_ and _U. indica_ they entirely baffle calculation, possibly 750,000 may be somewhere near the truth.

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

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

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

{ (_a_) _Toxoglossa_ { (_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.

[Illustration: FIG. 113.--Radula of _Bela turricula_ Mont. × 70.]

[Illustration: FIG. 114.--Portion of radula of _Spirotropis carinata_ Phil., Norway. × 70.]

[Illustration: FIG. 115.--Eight teeth from the radula of _Terebra caerulescens_ Lam. × 60.]

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

[Illustration: FIG. 116.--A tooth from the radula of _Conus imperialis_ L., S. Pacific, × 50, showing barb and poison duct.]

[Illustration: FIG. 117.--Portion of the radula of _Melongena vespertilio_ Lam., Ceylon. × 30.]

[Illustration: FIG. 118.--Portion of the radula of _Eburna japonica_ Sowb., China. × 30.]

[Illustration: FIG. 119.--Portion of the radula of _Murex regius_ Lam., Panama. × 60.]

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

[Illustration: FIG. 120.--Portion of the radula of _Imbricaria marmorata_ Swains. × 80.]

[Illustration: FIG. 121.--Three rows of teeth from the radula of _Fasciolaria trapezium_ Lam. × 40.]

[Illustration: FIG. 122.--Six teeth from the radula of _Cymbium diadema_ Lam., Torres Strait. × 25.]

[Illustration: FIG. 123.--Examples of degraded forms of radula: =A=, _Cantharus pagodus_ Reeve, Panama (nascent end), × 40; =A´=, same radula, central and front portion; =B=, _Columbella varia_ Sowb., Panama, × 50.]

[Illustration: FIG. 124.--Three rows of the radula of _Sistrum spectrum_ Reeve, Tonga, × 80. The laterals to the right are not drawn in.]

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

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

[Illustration: Fig. 125.--Portion of the radula of _Cassis sulcosa_ Born., × 40. The marginals to the right are not fully drawn.]

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

[Illustration: FIG. 126.--Four rows of teeth from the radula of _Vermetus grandis_ Gray, Andamans. × 40.]

In _Homalogyra_ the radula is much degraded, the central tooth is large and triangular on a broad base, the lateral is represented only by a thin oblong plate, and the uncini are absent. In some species of _Jeffreysia_ the uncini are said to be absent, while present in others. _Lamellaria_ has lost both its uncini, but the radula of the allied _Velutina_ is quite typical. A peculiar feature in this group is the tendency of the marginals to increase in number. A stage in this direction is perhaps seen in _Ovula_, _Pedicularia_, and the _Cyclostomatidae_. Here the outermost of the two marginals is by far the larger and broader, and is strongly pectinated on its upper edge; in the _Cyclostomatidae_ the pectinations are rather superficial; in _Ovula_ (where both marginals are pectinated) they are decidedly deeper; in _Pedicularia_ they are deeper still, and make long slits in the tooth, tending to subdivide it altogether. In _Turritella_ the number of marginals is said to vary from none (in _T. acicula_) to three (_T. triplicata_), but the fact wants confirmation. _Solarium_ is an aberrant form, possessing simply a number of long uncini, which recall those of _Conus_ or _Pleurotoma_, and is therefore hard to classify; the allied _Torinia_ has a radula which appears allied to _Ovula_ or _Pedicularia_. In _Triforis_ the teeth are identical throughout, very small, about 27 in a row, tricuspid on a square base, cusps short.

The normal formula of the _Taenioglossa_ is 2.1.1.1.2; in _Lamellaria_, 1.1.1; in _Triforis_, 13.1.13, or thereabouts.

[Illustration: FIG. 127.--Two rows of the radula of _Cypraea tigris_ L. × 30.]

[Illustration: FIG. 128.--Portion of the radula of _Ianthina communis_ Lam. × 40.]

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

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

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

[Illustration: FIG. 129.--Portion of the radula of _Margarita umbilicalis_ Brod., Labrador. × 75 and 300.]

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

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

[Illustration: FIG. 130.--Portion of the radula of _Nerita albicilla_ L., Andaman Is., with central tooth highly magnified: _c_, _c_, the capituliform tooth. × 40.]

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

(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.

[Illustration: FIG. 131.--Portion of the radula of _Patella cretacea_ Reeve, seen in half profile. × 40.]

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

[Illustration: FIG. 132.--Two rows of the radula of _Pterotrachea mutica_ Les., Naples. × 60.]

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

[Illustration: FIG. 133.--=A=, Portion of the radula of _Chiton_ (_Acanthopleura_) _spiniger_] Sowb., Andamans, × 30; =B=, portion of the radula of _Dentalium entalis_ L., Clyde, × 50.]

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

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

(_b_) _Aplacophora._--Of this rather obscure order, _Chaetoderma_ has a single strong central tooth, _Neomenia_ has no radula, _Proneomenia_ and _Lepidomenia_ have about twenty falciform teeth, much larger at one end of the radula than the other; formula, 0.1.0.

=Opisthobranchiata.=--The radula of the Opisthobranchiata is exceedingly variable in shape, size, and number and character of teeth. Not only do allied families differ greatly from one another, but allied genera often possess radulae widely distinct in plan. Thus, among the Polyceridae, _Goniodoris_ has no central tooth, one large lateral and one marginal (form. 1.1.0.1.1); _Doridunculus_ the same, with five marginals (form. 5.1.0.1.5); _Lamellidoris_ one each of median, laterals, and marginals (1.1.1.1.1); _Idalia_, _Ancula_, and _Thecacera_ the same as _Goniodoris_; _Crimora_ several each of laterals and marginals. Even species of the same genus may differ; thus the formula for _Aeolis papillosa_ is 0.1.0, but for _Ae. Landsbergi_ 1.1.1; for _Philine aperta_ 1.0.1, but for _Philine pruinosa_ 6.0.6.

[Illustration: FIG. 134.--Two teeth from the radula of _Aeolis papillosa_ L. × 55.]

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

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

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

(_b_) Radula with the first lateral very strongly developed. This type may take the form of (1) a single lateral, no central or marginals, _e.g._ _Onchidoris_, _Scaphander_ (Fig. 137, A), _Philine_ (certain species), _Ringicula_, or (2) first lateral strongly developed, and repeated in succeeding laterals (2–6) on a smaller scale, _e.g._ _Philine_ (certain species). A few marginals are sometimes added, _e.g._ in _Polycera_, _Lamellidoris_ (where there is a degraded central tooth, Fig. 137, B), _Idalia_, and _Ancula_.

[Illustration: FIG. 135.--Radula of _Elysia viridis_ Mont. × 40. Type (_a_).]

[Illustration: FIG. 136.--Portion of the radula of _Gadinia peruviana_ Sowb., Chili. × 250. Type (_c_).]

(_c_) Radula with an indefinite number of marginals, laterals (if present) merging into marginals, central tooth present or absent, inconspicuous, teeth all very small. This type of radula, among the Nudibranchiata, is characteristic of certain sub-genera of _Doris_ (_e.g._ _Chromodoris_, _Aphelodoris_, _Casella_, _Centrodoris_), of _Hypobranchiaea_ and _Pleurophyllidia_; among the Tectibranchiata, of _Actaeon_, many of the Bullidae, _Aplustrum_, the Aplysiidae, _Pleurobranchus_, _Umbrella_ and _Gadinia_ (Figs. 136 and 137, C).

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

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

[Illustration: FIG. 137.--Portions of the radula of Opisthobranchiata, illustrating types (_b_) and (_c_); =A=, _Scaphander lignarius_ L.; =A´=, one of the teeth seen from the other side, × 40; =B=, _Lamellidoris bilamellata_ L., Torbay, × 60; =C=, _Hydatina physis_ L., E. Indies, × 75.]

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

[Illustration: FIG. 138.--Portion of the radula of _Glandina truncata_ Gmel. × 40.]

[Illustration: FIG. 139.--Portion of the radula of _Rhytida Kraussii_ Pfr., S. Africa. × 25.]

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

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

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

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

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

_Pupidae_: Radula long and narrow; teeth of the helicidan type, centrals and laterals tricuspid on a quadrate base, marginals very small, cusps irregular and evanescent. This type includes _Anostoma_, _Odontostomus_, _Buliminus_, _Vertigo_, _Strophia_, _Holospira_, _Clausilia_, and _Balea_.

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

_Achatinellidae_: Two types occur; (_a_) teeth in very oblique rows, central, laterals, and marginals all of the same type, base narrow, head rather broad, with numerous small denticles (_Achatinella_ proper, with _Auriculella_ and _Tornatellina_, Fig. 140, E); (_b_) central tooth small and narrow, laterals bicuspid, marginals as in _Helix_ (_Amastra_ and _Carelia_).

[Illustration: FIG. 140.--Portions of the radula of =A=, _Hyalinia nitidula_ Drap., Yorkshire, with central tooth, first lateral, and a marginal very highly magnified; =B=, _Helix pomatia_ L., Kent, showing central tooth, laterals, and one extreme marginal, the two former also highly magnified; =C=, _Orthalicus undatus_ Brug., Trinidad, with three laterals highly magnified; =D=, _Cylindrella rosea_ Pfr., Jamaica, central tooth and laterals, the same very highly magnified; =E=, _Achatinella vulpina_ Fér., Oahu, central tooth (_c_) and laterals, the same highly magnified.]

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

_Janellidae_: Central tooth very small, laterals and marginals like _Achatinellidae_ (_a_).

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

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

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

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

[Illustration: FIG. 141.--Portions of the radula of =A=, _Limnaea stagnalis_ L., with the central tooth and two first laterals, and two of the marginals, very highly, magnified; =B=, _Ancylus fluviatilis_ Müll., with two of the marginals very highly magnified; =C=, _Physa fontinalis_ L., with central tooth and two of the marginals very highly magnified.]

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

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

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

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

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

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

[Illustration: FIG. 142.--Portion of the radula of _Octopus tetracirrhus_ D. Ch., Naples, × 20.]

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

[Illustration: FIG. 143.--Alimentary canal of _Helix aspersa_ L.: _a_, anus; _b.d_, _b.d´_, right and left biliary ducts; _b.m_, buccal mass; _c_, crop; _h.g_, hermaphrodite gland; _i_, intestine; _i.o_, opening of same from stomach (pyloric orifice); _l_, _l´_, right and left lobes of liver; _m_, mouth; _oe_, oesophagus; _r_, rectum; _s.d_, salivary duct; _s.g_, salivary gland; _st_, stomach; _t_, left tentacle. (After Howes and Marshall, slightly modified.)]

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

[Illustration: FIG. 144.--Alimentary canal, etc., of _Sepia officinalis_ L.: _a_, anus; _b.d_, one of the biliary ducts; _b.m_, buccal mass; _c_, coecum; _i_, ink-sac; _i.d_, duct of same; _j_, jaws; _l.l_, lobes of the liver; _oe_, oesophagus; _p_, pancreatic coeca; _r_, rectum; _s.g_, salivary glands; _st_, stomach. (From a specimen in the British Museum.)]

[Illustration: FIG. 145.--Gizzard of _Scaphander lignarius_ L.: =A=, showing position with regard to oesophagus (_oe_) and intestine (_i_), the latter being full of comminuted fragments of food; _p_, left plate; _p´_, right plate; _p.ac_, accessory plate; =B=, the plates as seen from the front, with the enveloping membranes removed, lettering as in =A=. Natural size.]

[Illustration: FIG. 146.--Section of the stomach of _Melongena_, showing the gastric plates (_g.p_, _g.p_,) for the trituration of food; _b.d_, biliary duct; _g.g_, genital gland; _i_, intestine; _l_, liver; _oe_, oesophagus; _st_, stomach. (After Vanstone.)]

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

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

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

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

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

The actual function performed by the hyaline stylet is at present a matter of conjecture. Haseloff’s experiments on _Mytilus edulis_ tend to confirm the suggestion of Möbius, that the structure represents a reserve of food material, not specially secreted, but a chemical modification of surplus food. He found that under natural conditions it was constantly present, but that specimens which were starved lost it in a few days, the more complete the starvation the more thorough being the loss; it reappeared when they were fed again. Schulze, on the other hand, believes that it serves, in combination with mucus secreted by the stomach, to protect the intestine against laceration by sharp

## particles introduced with the food. W. Clark found that in _Pholas_ the

stylet is connected with a light yellow corneous plate, and imagined therefore that it acts as a sort of spring to work the plate in order to comminute the food, the two together performing somewhat the function of a gizzard.[330]

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

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

[Illustration: FIG. 147.--Ink-sac of _Sepia_, showing its relation to the rectum: _a_, anus; _d_, duct of sac; _i.g_, ink-gland; _i.r_, portion of the sac which serves as a reservoir for the ink; _o_, orifice of ink-gland; _r_, rectum; _sp_, double set of sphincter muscles controlling upper end of duct. (Modified from Girod.)]

In nearly all Pelecypoda the intestine is very long and convoluted, being sometimes doubled forward over the mouth. Towards its terminal

## part it traverses the ventricle of the heart, except in _Ostrea_,

_Anomia_, _Teredo_, and a few more. The anus is always at the posterior end of the animal, adjacent to and slightly above the adductor muscle.

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

=The Kidneys=

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.

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=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.

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=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.

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