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—
| Prosobranchiata | Monotocardia | (a) Toxoglossa |
| (b) Rachiglossa | ||
| (c) Taenioglossa | ||
| (d) Ptenoglossa | ||
| (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.
