160 FIRST BOOK OP ZOOLOGY. 



CHAPTER XXI. 



CONCEENIXG NATURAL GEOUPS. 



145. The pupils have seen, tlius far, that the various creat- 

 ures studied not only differ greatly in their structure, but that 

 some are complex or elaborate in their characters, "while 

 others are quite simple. But, while these anhnals 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 had the body 

 divided into transverse segments, and the appendages were 

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

 bliges of animals, are the essential characters by which they 

 are brought together into great groups or divisions. ThiTg, 

 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 ap- 

 plied the name ArticuJata to an assemblage of animals which 

 included the worms, crustaceans, and insects. Since then 

 other naturalists have had reason to separate the worms from 

 the crustaceans and insects, and to classify them in a great 

 branch called Vermes, for worms do not have a continuous 

 hardened and limy skin covering the body and appendages, 

 and, generally speaking, the segments cannot be grouped 

 in regions, as is possible with the crustaceans and insects ; the 



