CHAPTER XIV. 

 BRANCH CHORDATA. 



CLASS AVES. 

 Example. The Common Pigeon. 



Adaptation for Flight. The pigeon is fitted for flying : 

 (i) by the wings, which, with their wide, strong, yet elastic 

 feathers, are air paddles of marvellous efficiency ; (2) by 

 the powerful breast muscles which move the wings; (3) 

 by the lightness of the skeleton ; (4) by the air sacs through- 

 out the body, which render it more buoyant; (5) and, not 

 least, by the shape, -being double pointed, so as to penetrate 

 the air with the least resistance. 



Structure of a Feather. Let us consider the structure of 

 one of the quill feathers. It consists primarily of two parts, 

 the shaft and the vane, or flattened part. The shaft is solid 

 so far as the vane extends, but the basal part is hollow, and 

 is called the quill. The two sides of the vane are unequal 

 in width, and the feathers overlap so that the wider side of 

 the vane is beneath. The vane is made up of side branches 

 called barbs, arranged in a close row. From each side of 

 each barb arise secondary branches, called barbules ; and 

 the barbules of adjacent barbs interlock by little hooked 

 ends, so that the whole vane is firm. 



Feathers are developed from papillae of the skin. In a 

 growing feather it can be seen that the quill is full of pulp 

 supplied with blood. In the fully developed feather the 



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