COMPARATIVE ANATOMY OF ORGANS. 633 



purpose effected by the serrated edges of the mandibles of 

 the genuine Crustacea and insects. 



Among vertebrates, the lancelet is no better off than the 

 majority of the Ccelenterates and worms, having no solid 

 parts for mastication ; and we have seen that the jaws and 

 teeth of the hag-fish and even the lamprey eel form a very 

 different apparatus from the jaws and its skeleton in the 

 higher vertebrates ; and that, even in the latter, the bony 

 elements differ essentially in form in the different classes, 

 though originating in the same manner in embryonic life. 

 In the birds we have seen that the mandible and maxilla are 

 encased in horny plates, that true teeth are remarkably ex- 

 ceptionable, the gizzard being, however, provided with two 

 hard grinding surfaces ; on the other hand, mammals with- 

 out teeth are exceptionable. 



The teeth of fishes are developed, not only in the jaws, 

 but on the different bones projecting from the sides and 

 roof of the mouth, and extend into the throat. In many 

 cases, in the bony fishes, these sharp recurved teeth serve to 

 prevent the prey, such as smaller fish, from slipping out of 

 the mouth. On the other hand, the upper and lower sides 

 of the mouth of certain rays {Myliobatis) are like the solid 

 pavement of a street, and act as an upper and nether mill- 

 stone to crush solid shells. 



In the toothless ant-eaters the food consists of insects, 

 which are swallowed without being crushed in the mouth ; 

 true teeth in the duckbill are wanting, their place being 

 taken by the horny processes of the jaws, while in Steller's 

 manatee the toothless jaws are provided with horny solid 

 plates for crushing the leaves of aquatic succulent plants. 

 Examples of the most highly differentiated teeth in verte- 

 brates are seen in those animals, like the bear, whose food is 

 omnivorous, consisting of flesh, insects, and berries, where 

 the crown of the molars are tuberculate; while the canines are 

 adapted for holding the prey firmly as well as for tearing the 

 flesh, and the incisors, for both cutting and tearing the food. 



The simplest form of a genuine digestive or enteric canal 

 is to be found in the Hydra, and in a more advanced stage 

 in the marine Hydroids. For the technical name of tlie 



