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]).
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.
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.