PIG. 52. I, CONDOR (Sarcoramphus eryplms) ; 2, OSTEICH 

 (Struthio camelus) ; 3, Ki\vi-Kl\Vl (Apteryx). 



33IEDS. 



The third class of the Yertebrata, comprises the " Birds," 

 or those whose young are not born alive, but in the form of 

 eggs ; who have warm blood, and breathe by lungs, the aux 

 cells of which communicate with the hollow shafts of the 

 bones, (these are full of air instead of marrow), this gives 

 them lightness, and serves as a reservoir of air while flying ; 

 they have two anterior extremities so modified as to form 

 wings, and a heart with four cavities ; most of them 

 have the power of flying through the air, and some to a 

 very great height, as the " Condor" (fig. 52), while in others 

 the wings are rudimentary and only assist them in running, as 



