66 ANIMAL BIOLOGY 



these systems provide must be carried on by all animals, and the 

 chief differences in the structure of animals, from the lowest to 

 the highest, are the result of the means adopted to serve these 

 essential functions under different conditions imposed by the 

 environment and mode of life. It is on the basis of these funda- 

 mental structural characteristics that the Metazoa are classified. 



The Metazoa may be divided into two large groups known as 

 Invertebrates and Vertebrates. The former group, frequently 

 referred to as the lower animals, comprises nearly a million known 

 species and exhibits an enormous variety of form and complexity 

 of structure ranging from the lowly Sponges to the highly suc- 

 cessful Insects. On the other hand, the Vertebrates, or higher ani- 

 mals, form a relatively homogeneous group of about fifty thou- 

 sand species, including the Fishes, Amphibians, Reptiles, Birds, 

 and Mammals. It is to a survey of the Metazoa that we now turn. 

 (Fig. 297.) 



