508 



Comparative Morphology of Chordates 



defined areas, pterylae, of the body. The intervening areas, apteria, 

 are bare, or occupied only by sparsely distributed feathers of the 

 smaller varieties (Fig. 398). However, the length and slope of the 

 contour feathers are such that the bird is externally completely covered 

 by them. The feather pattern, or pterylosis, is characteristic for a 

 species. In some birds (penguins, screamers, toucans, and ostriches), 

 pterylae are not distinguishable, the contour feathers being uniformly 

 distributed over the body, or the apteria are, at most, restricted to 

 very small areas or merely temporarily evident in the very young bird 

 (ostriches) . 



The color of feathers is due partly to pigments and partly to 

 physical action of the horny substance. There are commonly yellow, 

 orange, red, black, and, rarely, green pigments; the bright blue and 

 purple, and usually green, and the iridescence of some feathers, are 

 spectral effects resulting from the disruption of white light as it en- 

 counters surfaces of the more or less transparent horny substance — 

 "structural colors." White results from absence of pigment. 



Feathers, like most horny structures, are not permanent organs. 

 While in birds there is no periodic shedding of the stratum corneum, 

 there is usually a seasonal molting during which extensive loss of 

 feathers occurs. The attachment of the quill of the old feather at the 



Fig. 398. Feather tracts of a cuckoo, Geococcyx calif ornianus. (After Shufeldt. 

 Courtesy, Kingsley: "Comparative Anatomy of Vertebrates," Philadelphia, The 

 Blakiston Company.) 



