CHAPTER LVIII 



CLASS AVES 



Although birds have many points in common with reptiles, living 

 birds are easily separated from living reptiles by certain pronounced 

 characteristics, the most prominent of which is the possession of feathers. 

 Living birds are all bipeds, the only reptiles sharing this characteristic 

 being the extinct dinosaurs from which the birds were probably derived. 

 Another characteristic of living birds is the absence of teeth, their jaws 

 being covered by a horny beak; this characteristic, however, is shared 

 with the turtles. The caudal vertebrae are greatly reduced in number, 

 and all but a few of the anterior ones, which remain free, are fused into 

 a single bone known as the pygostyle, or plowshare bone; this charac- 

 teristic is shared with extinct reptiles. Birds also possess forelimbs 

 modified to form wings; there were flying reptiles, now extinct, but their 

 wings were not constructed upon the same plan as those of birds, nor 

 are they regarded as the ancestors of birds. It appears, then, that in 

 spite of the similarities in fundamental structure and mode of develop- 

 ment between birds and reptiles, both living and extinct types can be 

 clearly distinguished. 



422. External Characteristics. — The body of a typical bird is spindle- 

 shaped, being fitted by form for rapid movement through the air. It 

 is divided into four regions — head, neck, trunk, and tail. The neck is 

 long and flexible, correlated with the modification of the forelimb for 

 flight. The latter fact makes it necessary for the bird to use its beak 

 in the carrying on of certain activities connected with the securing of 

 food and nest building which in other animals belong to the forelimbs. 



The forelimb is relatively slender, possesses muscles nearly to the 

 tip, and is covered by feathers. Along the posterior margin are attached 

 a series of long flight feathers which make up most of the surface of the 

 wing. When the bird is at rest the wing is folded against the side of 

 the body and occupies little space, but when it is outstretched the 

 distance from one wing tip to the other usually exceeds — in some cases 

 very greatly — the length of the body. 



The functions of the hind limb are mainly those of support and 

 locomotion on the ground or in the water, it being used in perching, walk- 

 ing, running, climbing, or swimming. It terminates in a foot typically 

 made up of four toes, three anterior and one posterior, these ending in 

 curved claws. The thigh and more or less of the next joint are muscular 



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