PBIMAEY DIVISIONS AND CLASSES. 197 



which its organization offers of most importance, since it is 

 in fact these very peculiarities which serve to characterize 

 these successive divisions. Now, we repeat, the functions and 

 manners of an animal are always dependent on the mode of 

 conformation of its organs, or at least in harmony with its 

 structure, and that consequently we may deduce from this 

 knowledge all the most important points in the history of the 

 species submitted to our investigations. 



Such are the bases on which rest the zoological classifi- 

 cations called natural. Let us now see what have been the 

 results of the application of these principles to the methodical 

 distribution of animals, and let us study the principal groups 

 formed by these beings. 



BASES OF THE DIVISION OF THE ANIMAL KINGDOM INTO 

 PBIMAEY DIVISIONS (EMBEANCHEMENTS) AND CLASSES. 



372. Primary Divisions. Four general plans of struc- 

 ture, modified in a thousand ways, seem to have served as 

 guides for the creation of the animal kingdom. These four 

 principal forms may be understood by a reference to four 

 well-known animals the dog, the craw-fish or lobster, the 

 snail, the asteria or sea- star (Fig. 136). 



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. 



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 chiefly 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 

 move is the especial character of animal life, and these two 

 functions belong to the nervous system. It might readily, 

 then, be anticipated that the mode of conformation of this 

 system would exert a powerful influence over the nature of 

 animals, and would furnish characters of primary importance 

 in classification. 



The general disposition or mode of reunion of the different 

 parts of the body exercises an equally important influence, as 



