260 



SKELETONS OF THE LOWEB ANIMALS 



Other forms, there are a breastbone and well developed legs. 

 These legs, however, are poor organs -of locomotion and 

 support, for, like the frog's, the thighs point outward, and 

 instead of raising the body from the ground the legs are so 

 short that the beUy rests on the ground and must be dragged 



over the surface. The 

 elbows point backward 

 more than in the frog, 

 and the arm, or foreleg, 

 is, in this respect, an 

 improvement over the 

 similar structure in the 

 frog. 



Bird skeleton. — The 

 bird skeletons are in- 

 teresting as studies in 

 adaptation. In these 

 animals everytliing has 

 been sacrified to light- 

 ness and a flying habit. 

 The bones are hollow 

 and contain no marrow. 

 The arms are developed 

 as wings, the shoulder 

 girdle is sturdy and 

 rigid, to form a firm support for the wing joints, while the 

 greatly elongated breastbone, with its deep keel, is an excel- 

 lent adaptation for the attachment of the powerful flying 

 muscles. Both the birds and reptiles ha\'c a larger cranium 

 than the frogs or fishes. Another feature is the arrange- 

 ment of the legs. In the bird the toes are the only part of 

 the foot that is in contact with the ground, for the heel is 



Fig. 110— a bird skeleton (vulture). 



