CHAPTER XVII 



THE VERTEBRATES (continued): BIRDS 



Birds are readily and unmistakably distinguishable from 

 all other kinds of animals by their feathers. They are 

 further distinguished from the reptiles on one hand by their 

 possession of a complete double circulation and by their 

 warm blood (normally of a temperature of from 100-112° F.), 

 and from the mammals on the other by the absence of milk- 

 glands. There are about 10,000 known species of living 

 birds; they occur in all countries, being most numerous 

 and varied in the tropics. 



Body form and structure. — The general body form 

 and external appearance of a bird are too familiar to need 

 description. The covering of feathers, the modification 

 of the fore limbs into wings, and the toothless, beaked mouth 

 are characteristic and distinguishing external features. 

 The feathers, although covering the whole of the surface of 

 the body, are not uniformly distributed, but are grouped 

 in tracts called pterylcR, separated by bare or downy spaces 

 called apteria. They are of several kinds, the short soft 

 plumules or down-feathers, the large stiffer contour-feathers, 

 whose ends form the outermost covering of the body, the 

 quill-feathers of the wings and tail, and the fine bristles or 

 vibrissas about the eyes and nostrils called thread-feathers. 

 The fore limbs are modified to serve as wings, which are 

 well developed in almost all birds. However, the strange 

 Kiwi or Apteryx of New Zealand with hair-like feathers is 



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