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.
Fig. 135.—Radula of Elysia viridis Mont. × 40. Type (a).
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.