_Amphibolidae_: Central tooth five-cusped on a broad base, central cusp very large; two laterals only, the first very small, thorn-like, the second like the central tooth, but three-cusped; laterals simple, sabre-shaped. =Scaphopoda.=--In the single family (_Dentaliidae_) the radula is large, and quite unlike that of any other group. The central tooth is a simple broad plate; the single lateral is strong, arched, and slightly cusped; the marginal a very large quadrangular plate, quite simple; formula, 1.1.1.1.1 (Fig. 133, B). =Cephalopoda.=--The radula of the Cephalopoda presents no special feature of interest. Perhaps the most remarkable fact about it is its singular uniformity of structure throughout a large number of genera. It is always very small, as compared with the size of the animal, most of the work being done by the powerful jaws, while the digestive powers of the stomach are very considerable. The general type of structure is a central tooth, a very few laterals, and an occasional marginal or two; teeth of very uniform size and shape throughout. In the Dibranchiata, marginals are entirely absent, their place being always taken, in the Octopoda, by an accessory plate of varying shape and size. This plate is generally absent in the Decapoda. The central tooth is, in the Octopoda, very strong and characteristic; in _Eledone_ and _Octopus_ it is five-cusped, central cusp strong; in _Argonauta_ unicuspid, in _Tremoctopus_ tricuspid. The laterals are always three in number, the innermost lateral having a tendency to assume the form of the central. In _Sepia_ the two inner laterals are exact reproductions of the central tooth; in _Eledone_, _Sepiola_, _Loligo_, and _Sepia_, the third lateral is falciform and much the largest. [Illustration: FIG. 142.--Portion of the radula of _Octopus tetracirrhus_ D. Ch., Naples, × 20.] In _Nautilus_, the only living representative of the Tetrabranchiata, there are two sickle-shaped marginals on each side, each of which has a small accessory plate at the base. The two laterals and the central tooth are small, very similar to one another, unicuspid on a square base. [Illustration: FIG. 143.--Alimentary canal of _Helix aspersa_ L.: _a_, anus; _b.d_, _b.d´_, right and left biliary ducts; _b.m_, buccal mass; _c_, crop; _h.g_, hermaphrodite gland; _i_, intestine; _i.o_, opening of same from stomach (pyloric orifice); _l_, _l´_, right and left lobes of liver; _m_, mouth; _oe_, oesophagus; _r_, rectum; _s.d_, salivary duct; _s.g_, salivary gland; _st_, stomach; _t_, left tentacle. (After Howes and Marshall, slightly modified.)]