578 



COLLEGE ZOOLOGY 



the barbules, along either side. The barbules on one side of the 

 barb bear hooklets which hold together the adjacent barbs. The 

 whole structure is thus a pliable, but nevertheless resistant, organ 

 wonderfully adapted for use in flight. 



The three principal kinds of feathers are: (i) the contour 

 feathers or pennae like that just described; these possess a stiff 

 shaft and firm vanes, and since they appear on the surface, 

 determine to a large degree the contour of the body. (2) The 



down feathers or plumulae 

 possess a soft shaft and a 

 vane without barbs; they 

 lie beneath the contour 

 feathers and form a cover- 

 ing for the retention of 

 heat. The barbs of some 

 down feathers arise 

 directly from the end of 

 the quill, and no shaft is 

 present (Fig. 472, C). 

 (3) Thefiloplumes (B) pos- 

 sess a slender, hair-like 



FIG. 473. Feather tracts of the pigeon. 

 A, ventral; B, dorsal, al.pt, alar pteryla or 



wing tract; c.pt, cephalic pteryla or head- shaft and few Qr 



tract; cd.pt, caudal pteryla or tail tract; 

 cr.pt, crural pteryla; cr apt, cervical apte- 

 rium. or neck-space; fm.pt, femoral pteryla; 

 hu.pt, humeral pteryla; lat.apt, lateral 

 apterium; sp.pt, spinal pteryla; v.apt, ven- 

 tral apterium; -o.pt, ventral pteryla. (From 

 Parker and Haswell, after Nitzsch.) 



barbs. 



Only certain portions of 

 the pigeon's body bear 

 feathers ; these feather 

 tracts are termed pteryla, 

 and the featherless spaces are known as apteria. The feather 

 tracts differ in different species of birds; those of the pigeon 

 are shown in Figure 473. 



Birds shed their old feathers, i.e. molt, usually in the fall, 

 and acquire a complete new set which are formed within the 

 follicles and from the papillae of those that are cast off. There 

 may be a partial molt in the spring, when the bird assumes its 

 breeding plumage. At this time the plumage often changes 



