THE FROG AS A REPRESENTATIVE VERTEBRATE 11 



illustrations. This may be done either by following a larger group 

 of animals through its lesser subdivisions until the individuals 

 comprising the smallest groups, or species, are reached ; or by pro- 

 ceeding in the reverse order. The existence of the group known as 



the Vertehrata (Figs. 1, 2, 3) was 

 recognized only after it had been 

 discovered that a great array of 

 animals all possessed a backbone 

 composed of vertebrae and hence 

 could be called " vertebrates." 



Fig. 2. — A representative of 

 the Class Aves of the Sub- 

 phylum Vertehrata: the 

 ostrich in silhouette show- 

 ing endoskeleton of bones 

 and exoskeleton of feathers 

 (c/. Figs. 1 and 3). 



(After Pander and d'.\Iton.) 



Fig. 3. — A representative of the Class 

 Mammalia of the Sub-phylum Vertehrata: 

 the lion in silhouette showing endoskele- 

 ton of bones and exoskeleton of hair 

 {cf. Figs. 1 and 2) 



(After Pander and d' Alton.) 



These vertebrates have been found, in turn, to possess features 

 that lead one to include them, along with certain other simpler 

 animals (Fig. 4), in one of the major divisions, or -phyla (singular, 

 phylum), of the Animal Kingdom, the Chordata {cf. Table, p. 36), 

 which are so called l^ecause they all possess, at some stage in their 

 development, a notochord, or primitive skeletal axis. In the verte- 

 brates the notochord appears in the embryo, but is later replaced 

 by the segmented vertebral column, or backbone (cf. Fig. 40, 

 p. 69, and Fig. 213, p. 405.) 



The frog, as shown by the table of classification (p. 36), is a 

 member of the Class Amphibia, a subdivision of the Vertehrata, 

 which includes also the toads and salamanders. The classification 

 of Amphibia is as follows : 



