THE ODONTOPHORA. 419 



face is not produced into a visceral sac (e. g., Chiton, Denta- 

 lium). But, in the great majority, such a visceral sac is 

 formed. In the Cephalopoda it coexists with bilateral sym- 

 metry, inasmuch as the mantle and the anus lie in the plane 

 which divides the body into two similar halves. But, in most 

 O<l<<t<>j>hora, the anus is twisted to one side (usually the 

 right), and in many it is situated, together with the pallial 

 chamber in which it is contained, on the anterior face of the 

 body. 



The mouth lies at the anterior end of the body, on the 

 haemal side of the anterior part of the foot (except in the 

 Cephalopoda). It may be provided with variously-disposed 

 jaws, or cutting-plates, of a chitinous or calcified substance. 

 But the structure which is most characteristic of the Odon- 

 tophora, and which is absent in only very few genera (e. g., 

 Tethys, Doridium, Rhodope), is a peculiar rasping and some- 

 times prehensile apparatus, the odontophore, or, as it is often 

 termed, the tongue, which is attached to the floor of the mouth 

 (Figs. 119, 120). 



This apparatus consists of a skeleton / of a subradular 

 membrane, which is continuous with the lining of the oral 

 cavity; of the radula / and of intrinsic and extrinsic mus- 

 cles. 



The skeleton is composed of two principal masses of par- 

 tially fibrous, or completely cartilaginous, tissue (odonto- 

 phoral cartilages), which may be more or less confluent, and 

 are further united together in the middle line by fibrous and 

 muscular tissue. Their anterior ends and oral faces are free 

 and smooth, and are usually excavated so as to present a 

 trough-like surface to the subradular membrane, which rests 

 upon them. Accessory cartilages may be added to these. 

 Behind, the subradular membrane is continued into a longer 

 or shorter sac, lined by a continuation of the buccal epithe- 

 lium. The radula is a cuticular chitinous product of the 

 epithelium of the subradular membrane. It is armed with 

 tooth-like processes arranged in one or many series ; and ad- 

 ditions are constantly being made to its posterior end, which 

 is lodged in the sac of the subradular membrane. Thus the 

 teeth are replaced from behind, as fast as they are worn away 

 by friction against the food which they rasp, at the anterior 

 end of the ribbon. 



The intrinsic muscles of the odontophore are attached, on 

 the one hand, to the posterior and under faces of the odonto- 

 phoral cartilages, and, on the other, to the subradular mem- 



