6 DIGESTIVE ORGANS. 



These plates are apparently wanting in all the Rachiglossata 

 (Murex, Fusus, Xassa, etc.). The linear hornj" plates described 

 in Buccinum undatum by Cuvier and Valenciennes, are probably 

 appendages of the tongue, and used as a handle in perforating 

 the shells on which they prey. 



Tongue. The odontophore or tongue (pi. 4, fig. 40) is attached 

 to the floor of the mouth. It contains two parallel cartilages, 

 which may be more or less confluent, and which are united to- 

 wards the middle by fibrous and muscular tissue. <c The in- 

 trinsic muscles of the odontophore are attached at one end to 

 the posterior and under faces of the subradular membrane, some 

 being inserted into its posterior and lateral portions, and others 

 into its anterior extremity, after it has turned over the anterior 

 extremities of the principal cartilages. Certain of the muscular 

 bundles, are also attached to the forepart of the odontophoral 

 cartilages themselves. The contraction of these muscles must 

 tend to cause the subradular membrane, and with it the radula, 

 to travel backwards and forwards over the ends of the cartilages 

 in the fashion of a chain-saw, and thus to rasp any body against 

 which the teeth may be applied. When undisturbed, the radula 

 is concave from side to side, and the teeth of the lateral series, 

 being perpendicular to the surface to which they are attached, 

 are inclined inwards to one another. But when the intrinsic 

 muscles come into action, the radula, as it passes over the ends 

 of the cartilages, becomes flattened, and the lateral teeth are 

 consequently erected or divaricated. The extrinsic nriscles pass 

 from the odontophore to the lateral walls of the head, and pro- 

 tract or retract the whole apparatus. They may give the pro- 

 truded extremity of the radula a licking motion, which is quite 

 independent of the chain-saw action due to the intrinsic mus- 

 cles." HUXLEY, Anat. Invert., 490.* 



The subradular membrane does not terminate behind with the 

 muscular mass of the tongue, but is continued and invaginated 

 into a pouch called the tongue-sheath. The under wall of the 



* Geddes has recently carefully investigated the mechanism of the 

 odontophore in Loligo Buccinum and Patella. He does not altogether 

 agree with Huxley as to the mode of action of this organ, but thinks 

 its movements depend on those of the cartilages, whilst Huxley regards 

 the cartilages as passive. Trans. Zool. Soc., London, x, 485, 1879, with 

 three plates. 



