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]).
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.
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.