168 



STRUCTURE OF THE VERTEBRATES 



fundamentally like other members of the class to which they 

 belong. All come to the surface for air, and the mammals feed 

 their young on milk. 



2. Bipedal. Bipedal animals are found in all the three amni- 

 ote classes. The pectoral appendages are small or highly modi- 

 fied, and the pelvic limbs are large. The weight of the body is 

 thrown backward for balance. All living birds are completely 

 bipedal. The same modification was found in many extinct rep- 

 tiles, although the bird was not descended from any known 



A. Plantigi-ade 



B. Digitigiade 



C. Unguligrade 



Fig. 100. "Walking and Running Modifications of the Appendages. The 



most marked specializations for running occur in the mammals. In this 



Class three major tj'pes are recognized, typical of mammals which walk 



(plantigrade), jump (digitigrade) or run (unguligrade). 



group of this type. Both herbivorous and carnivorous reptiles 

 were of this type, the latter being more frequent. Among the 

 mammals the specialization has occurred in several unrelated 

 groups. The marsupial kangaroos are highly modified for bi- 

 pedal life; and man has similar locomotion, although there is 

 less modification of the pectoral limb. The kangaroo has become 

 adapted for a hopping motion. ^Man descended from an arboreal 

 (tree living) group and has retained completely functional hands. 

 3. Cursorial. The running modifications have developed most 

 prominently in animals living on dry plains where a pre- 



