310 



BRAXCH CHORDATA 



all climates, and their mode of locomotion makes them less re- 

 stricted by barriers. 



Important Biologic Facts. — Birds have, in common with 

 reptiles, the quadrate bone, and but one occipital condyle. 

 They are wonderfully adapted to their aerial mode of life by 

 their feathers, by the modifications of the fore limbs into wings, 

 and by the air-filled cavit>' of the bones. Birds are warm- 

 blooded (homoiothermal) , the heart being completely divided 

 into right and left halves. The fusion of the bones of the hand 

 and the tibiotarsus and the tarsometatarsus are peculiar to 

 birds. Nest building, as well as incubation, is peculiar to this 

 class, though, in rare instances, fishes build nests and reptiles 

 practice incubation. 



(Perching birds), birds. 



