Mammalia. 



backbone. A comparatively recent grouping is into Pro- 

 tozoa, or one-celled animals, and Metazoa, or many-celled 

 animals. Each of these groupings simply characterizes 

 one group (one system taking the lower end and the other 

 the higher end of the animal series) and lumps off all the 

 rest in one negatively named group. Neither of these sys- 

 tems pretends to do anything more than call attention to 

 the presence or absence of a single feature of structure. 



Fish 



Toad Snake 



Sparrow 



Spinarl cord 



FIG. 167. DIAGRAM OF BRAINS OF VERTEBRATES. 



From Kellogg's Zoology. 



The Basis of Classification of Animals. The basis of classi- 

 fication of animals is structure, only so many branches being 

 recognized as there are distinct plans of structure. It is 

 generally understood that the structure considered is that of 

 the adult, since many animals change greatly during their 

 development. But in some cases, especially of degenerate 

 forms, the larval form shows, more clearly than the adult, 

 the true relationship. Consequently embryology must be 

 called in as an aid in classification. Fossils also clear up 

 many doubtful points in classification. 



