Chap, iv.] ODONTOPHORE OF MOLLUSC A. 135 



The rest of the Mollusca, or Cephalophora, are 

 provided with a rasping organ, which lies on the floor 

 of the pharynx, the odoiitophore. But, anteriorly 

 to this, and at the edge of the mouth, there are one 

 or more horny plates, with a sharp cutting edge ; these 

 are best developed in the cuttle-fishes, where they 

 have the appearance of a parrot's beak turned upside 

 down ; in the nautilus these beak-like plates are 

 calcified. 



The characteristic organ of the digestive system of 

 such Mollusca as have not suffered degeneration of 

 the head is the just-mentioned odontophore. This 

 consists essentially of an overlying chitinous sheet, 

 the surface of which is produced into a variable 

 number of more or less sharp processes, the so-called 

 teeth ; this, then, is a rasping organ, or radaila. 

 Underlying are connective and muscular fibres, and 

 supports are afforded for it by the development 

 beneath of masses of cartilage ; as muscles are inserted 

 into the anterior and posterior faces of these cartila- 

 ginous supports, it is clear that, by their alternate 

 contraction and relaxation, they will draw the radula 

 backwards and forwards. The whole apparatus is 

 developed in a blind diverticulum, lying 011 the 

 ventral surface of the cavity of the mouth. The 

 teeth, which may be very variously arranged, are 

 greatly strengthened by the deposit of silica ; and as 

 they and the chitinous sheet are worn away they are 

 replaced by the hinder part of the radula, which 

 passes forwards on its bed ; the replacement of the 

 effete parts being effected, in other words, in a way 

 comparable to that of the human nail. The radula is 

 ordinarily divisible into a central piece, with a lateral 

 piece on either side ; the teeth on the former are 

 spoken of as the racliidiaii teeth, and those on the 

 lateral pieces as the uiiciiii. The arrangement of 

 these teeth varies very greatly, and, for the purpose 



