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.—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: 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.:—

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.