CHAPTER XX 



SUBPHYLUM VERTEBRATA: CLASS VI. AVES 



The class Aves contains the birds. Birds are easily dis- 

 tinguished from all other animals, since they alone possess 

 feathers. The ten thousand or more species of birds are grouped 

 into two subclasses: (i) Arch^eornithes, which contains the 

 fossil form Archceopteryx ; and (2) Neornithes, which contains 

 four orders of extinct forms and seventeen orders with living 



representatives. 



1. The Pigeon 



The common pigeons have been derived from the blue rock- 

 pigeon, Colwnba livia (Fig. 470), which ranges from Europe 

 through the Medi- 



terranean coun- 

 tries to central 

 Asia and China. 

 Since pigeons are 

 easily obtained 

 and of moderate 

 size, they are 

 usually selected 

 as a type of the 

 class Aves for 

 laboratory study. 

 External Fea- 

 tures. — The body 

 of the pigeon is - « ^ 



spindle-shaped, and .therefore adapted for movement through 

 the air. Three regions may be recognized, — head, neck, and 



575 



The blue rock pigeon, Col umbo, livia. 

 (From Brehm.) 



