EXTERNAL PARTS OF BIRDS. — FEATHERS. 



85 



like processes, while hamuli are hooked at the end ; they are not found on all feathers, nor on 

 all parts of some feathers. Barbicels occur on both anterior and posterior rows of barbules, 

 though rarely on the latter ; booklets are confined to any an- 

 terior series of barbules, which, as we have seen, overlie the 

 posterior rows, furming a diagonal mesh-work. The design 

 of this beautiful structure is evident; the barbules are inter- 

 locked, and the whole made a web ; for each booklet of one 

 barbule catches hold of a barbule from the next barb in front, 

 any barbule thus holding on to as many of the barbules of the 

 next barb as it has booklets ; while, to facilitate this interlock- 

 ing, the barbules have a thickened upper edge of the right size 

 fur the booklets to grasp. The arrangement is shown in fig. 

 22, where a, a, a, a, are four barbs in transverse section, viewed 

 from the cut surfaces, with their anterior, b, b, b, b, and pos- 

 terior, c, c, c, c, barbules, the former bearing the booklets 

 which catch over the edge of the latter. 



Fig. 21. — 

 A single bar- 

 bule, bearing 

 barbicels and 

 booklets ; mag- 

 nified ; after 

 Nitzscb. 



Fig. 

 Four barbs in 

 cross section, a, 



a, a, a, bearing 

 anterior, b, b, b, 



b, and posterior 



c, c, c, c, bar- 

 bules, the form- 

 er bearing hook - 

 lets which catch 

 over the latter; 

 magnified ; after 

 Nitzsch. 



Types of Feathery Structure. — But all feathers do not 

 answer the above description. The after-shaft, may be wanting, 

 as we have seen. Hooklets may not be developed, as frequently 

 happens. Barbicels may be few or entirely wanting. Barb- 

 ules may be similarly deficient, or so defective as to be only 

 recognized by their position and relations. Even barbs them- 

 selves may be few or lacking on one side of the shaft, or on 

 both sides, as in certain bristly or haii'-like styles of feathers. Consideration of 

 these and other modifications of feather-structure has led to the recognition of 

 three types or plans : 1. The perfectly feathery, p^MmoMS, or pennaceous (Lat. pluma, a plume, 

 or pemm, a feather fit for writing with; fig. 23), as above described. 2. The downy or 

 plumulaceous (Lat. plumula, a little plume, a down-feather), when the stem is short and 

 weak, with soft rhachis and barbs, with long slender thready barbules, little knotty dilata- 

 tions in place of barbi- ^_^ ^_ _ -'f-^~-"^^^^^bit 



eels, and no hooklets. 



3. The hairy, bristly, or 



filo - plumaceous (Lat. 



filum, a thread), with 



a very long, slender stem, 



and rudimentary or very 



smaU vanes composed of fine cylindrical barbs and barbules, if any, and uo barbicels, knots, 



or hooklets. There is no abrupt definition between these types of structure ; in fact, the same 



feather may be constructed on moi-e than one of these plans, as in fig. 19, partly pennaceous, 



partly plumulaceous. All feathers are built upon one or another, or some combination, or 



modification, of these types ; and, in all their endless diversity, may be reduced to four or five 



v^^^p" 



Fi(, I J A li^iiLt. Uoiu the tail of a kingbird, Tyrannus carohnensls, 

 almost entiiely peimaceous, no aftei -shaft From nature, by Coues. 



Different Kinds of Feathers. — 1. Contour -feathers, pennce or plumce proper, have a 

 perfect stem composed of calamus and rhachis, with vanes of pennaceous structure, at least 

 in part, usually plumulaceous toward the base. These fonn the great bulk of the surface- 

 plumage exposed to light ; their beautiful tints give the bird's colors ; they are the most 

 modified in detail of all, from the fish -like scales of a penguin's wings to the glittering jewels of 

 the humming-bird, and all the endless array of the tufts, crests, rufi"s, and other ornaments of the 

 feathered tribes ; even the imperfect bristle-like feathers above mentioned may belong among 



