PEIMARY DIVISIONS AND CLASSES. 



209 



well-known animals the dog, the crawfish or lobster, the 

 snail, the asterias or sea-star (Fig. 159.) 



In order that the zoological classification be a faithful 

 representation of the more or less important modifications 

 introduced into the structure of animals, it was necessary to 

 distribute these beings into four principal groups or divisions 

 and this is, in fact, what Cuvier did. 



Fig. 161. Skeleton of the African Ostrich. 



The animal kingdom is divided into vertebrate animals, 

 articulated or annulated animals, molluscs, and zoophytes. 



373. The fundamental differences distinguishing these 

 four primary divisions depend cbiefly on the mode of arrange- 

 ment of the different parts of the body and on the conformation 

 of the nervous system. It is easy to understand the im- 

 portance of these two dominant characters : to feel and to 



