CARNIVOROUS MOLLUSCS 



95 



species. It consists of (vol. i, fig. 199) a cushion rising up from 

 the floor of the mouth, and supported by hard parts of a gristly 

 nature. Stretched over this swelling from front to back is a 

 horny ribbon-shaped structure, the radula (L. radula, a scraper), 

 the surface of which is beset with regular rows of small, sharp, 

 horny teeth. By means of appropriate muscles the odontophore 

 can be somewhat protruded, so as to press against the object to 



Fig. 376. — Beaks of Cuttle- Fish {Sepia officinalis) 

 A, From below (the radula on its cushion is seen within), ri. From the side, c, Lower beak, d, Upper beak. 



be rasped, and there are also muscles by which it can be drawn 

 back again and put out of action when required. Within the 

 cushion of the odontophore are other small muscles for working 

 the radula (which rests loosely upon its support) backwards and 

 forwards, after the manner of a chain-saw, so as to rasp very 

 effectively anything to which it may be applied. The gristly 

 supports of the odontophore give points of attachment to these 

 little muscles. After a certain amount of work the teeth of the 

 radula become blunt and worn, but this contingency is provided 

 for, as the ribbon is continually growing forwards, much like a 

 finger-nail. And just as by gently moving a finger-nail up and 

 down you can see that it runs back under a fold of skin to a 

 " root " where new nail is constantly being formed, so can the 

 radula be followed back into a projection, the radula sac, out of 

 which it is continually growing. This toothed ribbon, which con- 

 stitutes the actual scraping part of the odontophore, is often called 



