FEATHEES. 33 



These patches of contour feathers are termed pterylm and their interspaces 

 apteria. This general arrangement of the contour feathers, which prevails (with 

 many modifications in detail) in nearly all birds, is related to the facility of 

 movement of the body and limbs. The pterylosis, as the arrangement of the 

 feather tracts is called, is of importance in determining the natural affinities of 

 birds. 



II. Down feathers (plumulce). In these the stem is little developed, and the 

 soft barbs have long, slender, thready barbules, with little knotty dilatations in 

 place of barbicels, and no booklets. They more or less completely invest the 

 body, but are almost always hidden away beneath the contour feathers. If the 

 contour feathers are removed from the breast of a Wild Duck, the thick covering 

 of brown down feathers next the skin will be clearly seen. In a Swan the down 

 is white, the downy layer is very thick, and is composed partly of true down 

 feathers and partly of the plumulaceous or downy part of the contour feathers. 



Down feathers of the young. The down of a duckling is replaced by the true 

 feathers (pennce). The rudiment of the new feather is formed at the base of the 

 down plumule. As the new feather grows it pushes the down plumule out from 

 the skin. The plumule remains attached to the apex till the new feather is about 

 an inch long, when it is shed. This process is analogous to the shedding of the 

 "milk" teeth in Mammals. The down feathers of a young Albatross are on a 

 far more magnificent scale. 



III. Semiplumes (semiplumce). Many feathers are intermediate in character 

 between the pennce and the plumulce, possessing the large and more rigid stem of 

 the former and the downy barbs and barbules of the latter. The contour feathers 

 from the breast of a Swan (C olor) show the downy or plumulaceous character 

 of the basal portion of the vanes and the plumous character of the terminal part. 

 The feathers of the Common Heron show various transitions from the plumous 

 to the plumulaceous or downy form. The under tail coverts of the Adjutant 

 {L. argala) are semiplumous, and usually called Marabou feathers. 



IV. Filoplumes {filoplumce), or thread feathers, have a very slender stem, and 

 barbs either rudimentary or wanting, so that they closely resemble hair. In the 

 Golden Eagle these feathers have a slender stem with a small terminal tuft of 

 barbs, and grow out from the edge of the folds of skin forming the sacks of the 

 larger feathers. 



V. Powder down (pulviplumce). Certain down feathers are peculiarly modified, 

 growing indefinitely from the follicle, and continually breaking down at the ends 

 of the barbs. From the white dust-like material with which they are constantly 

 covered, they are called "powder down," or "pulviplumes." They occur in some 

 Hawks, Parrots, and other birds, and are especially developed in the Herons and 

 their allies. Sometimes they are scattered over the surface, but more often are 

 collected in definite tracts or patches. The breast of the Common Heron (A. cinerea) 

 shows large "powder-down patches," which reach to the middle line in front, and 

 there are a pair of such patches on the lower part of its back. 



VI. After-shafts. In very many birds each quill bears two vexilla; the second 

 is called the after-shaft. The after-shaft {hyporhachis), or accessory plume, when 



