432 



ZOOLOGY 



life, and are not formed, once for all, like our teeth. In addition 

 to plant materials, rats and mice eat a certain amount of ani- 

 mal food. 



Rats and mice belong to the class Mammalia ' characterized 

 by having imlk glands for the nourisluiient of the j'oung. 

 Most manmials have also hair, although in the case of por- 

 poises and whales the hair is nearlj' or cjuite absent. 



Fig. 



394. — The raVjbit. Lateral view of skeleton enelosed in outline of 

 body. From Parker and Ha.sweU. 



The structure of the mammalia may be illustrated by a 

 stud}' of the rabbit. 



General Form of the Body. — The bodj' consists of head, 

 trunk, and tail, the latter in a rudimentary condition (Fig. 394). 

 The head is connected to the trunJv Ijy a relatively narrow neck 

 to insure mobility. Its large size results from the fact that it 

 bears the strong teeth, brain, sense organs, and their supporting 

 and protecting skull. Nearly the entire l)ody is covered with 

 hairs which develop in the skin. At the base of each hair is 

 usually a gland that secretes an oily substance — sebaceous 



' From mammals, l)elonging to the breast. 



