ORGANS OF iTASTICATIOX. 



189 



Fig. 201. 



340. Though all the vertebrata possess jaws, it must not 

 be inferred that they all chew their food. Many swallow their 



hole ; as most 

 . tortoises, and 

 whales. Even many 

 of those which are 

 furnished with teeth 

 do not masticate their 

 food ; some using 

 them merely for seiz- 

 ing and securing their 

 as the lizards, 

 frogs, crocodiles and 

 the great majority of fishes. In such animals, the teeth are 

 nearly all alike in form and structure, as, for instance, in 



Fig. 202. 



Fig. 204. 



the alligator (fig. 203) ; the porpoises and many fishes. A 

 few of the latter, some of the rays, for example, have a sort 

 of bony pavement (fig. 204), composed of a peculiar kind of 

 teeth, with which they crush the shells of the mollusca and 

 crabs on which they feed. 



341. The mammals, however, are almost the only verte- 

 wliich can be properly said to masticate their food. Their 

 teeth are well developed, and present great diversity in form, 

 arrangement, and mode of insertion. Three kinds of teeth are 

 usually distinguished in most of these animals, whatever may 

 be their mode of life ; namely, the cutting teeth, incisors ; the 



