160 FIRST BOOK OF ZOOLOGY. 



CHAPTEK XXI. 



CONCERNING NATURAL GROUPS. 



145. THE pupils have seen, thus far, not only that the 

 various creatures studied differ greatly in their structure, but 

 that some are complex or elaborate in their characters, while 

 others are quite simple. But, while these animals differ 

 so much among themselves, there are certain characteristics 

 which many of them have in common, as in the crustaceans 

 and insects, for example, where all of them have the body 

 divided into transverse segments, and the appendages are all 

 jointed. These features, which are common in large assem- 

 blages of animals, are the essential characters by which they 

 are brought together into great groups or divisions. Thus, 

 all those animals which have the body jointed, that is to say, 

 divided into a series of segments, as in the worms, crustaceans, 

 and insects, form the great branch of Articulates of Cuvier, 

 because Cuvier, the celebrated French naturalist, first applied 

 the name Articulata to an assemblage of animals which in- 

 cluded the worms, crustaceans, and insects. Since then natu- 

 ralists have separated the worms from the crustaceans and 

 insects, and have made a great branch of them called Vermes. 



The worms differ from the other two classes with which 

 they were associated by Cuvier in not having jointed legs, 

 and, generally speaking, in not having the segments grouped 

 together into regions. 



