IV. VERTET3RATA: AVES. 603 



Class II. Aves. 



While structurally the birds stand very near the reptiles, yet by 

 the development of wings and the feathering of the body the group 

 is one strictly circumscribed. The skin is in some places, as the 

 lower part of the legs, covered with horny scales and shields, on 

 the toes are claws, but as a rule the fingers are feathered. On 

 most places the skin is soft and thin, since the derma and stratum 

 corneum are poorly developed. Periodic molts of the integument 

 do not occur, since the horny layer, as in mammals, undergoes a 

 constant renewal. These peculiarities of the skin are correlated 

 with the appearance of the protecting plumage. 



The feather, like the hair of mammals, is exclusively epithelial 

 in character, but of a much more complicated structure. The cor- 

 nified epithelium forms a firm axis, the scape, from which, right 

 and left, arise branches, or barbs. The scape is solid as far as the 

 barbs extend (rachis, or shaft), while below it is hollow (quill, or 

 calamus). The quill is inserted deep in the derma, in a follicle, 

 and is provided with muscles for its movement. Its hollow in 

 most fully developed feathers is empty save for the ' pith/ a small 

 amount of dried tissue. In young growing feathers it is occupied 

 by a richly vascular connective tissue, the feather papilla, which, 

 for purposes of nourishment, extends inwards from the derma. 

 The feather may therefore be regarded as a cornified outgrowth 

 from the skin which has arisen on a papilla of the derma, a view 

 which corresponds well with its development and shows its 

 homology with the scales. In many birds (cassowaries) two well- 

 developed feathers arise from the same follicle a fact which 

 explains the existence of a rudimentary feather, the hyporachis, 

 or after-shaft, attached to the scape below. 



In contour feathers the barbs are, to a great extent, united into a 

 vane. Right and left of the shaft they lie close together and parallel, 

 each repeating in miniature the entire feather, the barb having branches 

 or barbules, which, overlapping the barbules of adjacent barbs, give the 

 vane its close texture. The vane is held together by minute hooks on the 

 barbules of one barb interlocking with those of the next. Down feathers 

 (plumes) differ from contour feathers in the absence of hooks and the 

 loose arrangement of the barbs. Since feathers consist of cornified epithe- 

 lium and these cells are held firmly (only in powder down is there a 

 gradual loss), they, like the scaly coat of the snakes and lizards, must be 

 molted yearly and replaced by new. 



Young birds or embryos have only down feathers. Later the contour 

 feathers arise in regular order in the feather tracts, or pterylae, between 



