16 THE STORY OF THE BIRDS 



skin which collectively form the covering of a 

 bird and enable it to fly. A feather, generally 

 speaking, consists of the following parts : A, the 

 barrel or quill, calamus ; B, the principal shaft, 

 rhachis ; C, the secondary or aftershaft, hypo- 

 rhachis ; D, the web composed of a regularly 

 arranged series of fibres or barbs, rami } attached 

 to which are a double series of barbules or radii, 

 which again in their turn give rise to barbicils 

 or cilia, which latter generally end in booklets, 

 hamuli. These latter it should be remarked are 

 of very great importance, because they serve to 

 interlock or join the barbules and barbs together, 

 and thus serve to render the surface of the flight 

 feathers almost impervious to air. Feathers may 

 be roughly divided into three classes ; viz. contour 

 feathers, down-plumes, and riloplumes. The first 

 are not only the largest but perhaps the most 

 important. They are those that appear nearest 

 the surface, and therefore form the outline or 

 contour of the bird's figure. They reach their 

 greatest development of course in the wings 

 (remiges) and tail (rectrices). In some of these 

 feathers the cilia, and consequently the hamuli 

 are wanting, thus forming the disconnected webs 

 and decomposed plumes which are displayed as 

 ornaments or decorative tufts in so many birds. 



