INTEGUMENT 



35 



Feathers. — There are several kinds of feathers but all may be 

 grouped under three heads: hair feathers (filoplumes), down feathers 

 (pliunulae), and contour feathers (plmnae). The latter have all of 

 the feather features (fig. 25) and in the typical form consist of shaft 

 and vane. The basal part of the shaft is the hollow quill, in which 

 is a small amount of loose pith. In the region of the vane the shaft, 

 here called rhachis, is solid, and running the length of its lower sur- 

 face is a groove, the umbilicus. The vane consists of lateral branches 

 (barbs) on either side, which have, in turn, smaller side branches 

 (barbules), these with small hooks at their sides and tips (A). In- 



FiG. 26, — Feather tracts of Geococcyx californianus, after Shufeldt. 



terlocking of these hooks gives firmness and continuity to the whole 

 vane. In dowii feathersjhe barbs arise_dir^tly_frpm the end ^' the 

 quill, and, as hooks are lacking, the b^-rbs ^fl_not interlock and no 

 vane,is forcLCd. (Hair feathers are merely long and slender shafts 

 with no barbs, the-simpJesT, if not the mo st pr imitive kin d of feather .^ 

 It is still a question as to the primitive type. The oldest known fos- 

 sil bird, ArchcBopleryx, had well-developed contour feathers. 



Except in the ostriches, penguins, and toucans, feathers are not 

 distributed everywhere on the surface of the body, but are gathered 

 in feather tracts (pterylae), separated by apteria in which no contour 



