324 



COMPARATIVE ZOOLOGY 



and mammals the hinder part of the intestine develops 

 an outgrowth (the allantois} during embryo life which 

 serves as the embryo's breathing organ 

 (Figs. 365, 366). 



All vertebrates have two kinds of re- 

 spiratory organs in the course of their 

 life. Fishes have gills ; their lung (the air 

 bladder) rarely serves as a functional re- 

 spiratory organ, and is sometimes wanting. 

 Amphibians have gills while in the larval 

 state. Some keep them throughout life ; 

 but all develop functional lungs, and also 

 breathe by means of the skin. 



In the remaining vertebrates, the allan- 

 tois is the breathing organ of the embryo, 

 and the lung is the breathing organ of 

 the adult. The skin is of small or no 

 importance in respiration. 



The lungs of vertebrates are elastic, 

 membranous sacs, divided more or less 

 into cavities (the air cells] to increase the 

 surface. Upon the walls of the air cells 

 are spread the capillary blood vessels. 

 The smaller the cells, the greater the 

 extent of surface upon which the blood 

 l S a 8 s'nake" n ^ * s exposed to the influence of the air, and, 

 trachea; t, its therefore, the more active the respiration 



bifurcation; c, 



pulmonary ar- and the purer the blood. The lungs are 

 naj veinTThe relatively largest in reptiles, and smallest 



lung, B, is rudi- 

 mentary. 



in mammals. But in the cold-blooded 

 amphibians and reptiles, the air cells are 

 few and large ; in the warm-blooded birds and mammals, 

 they are exceedingly numerous and minute. 120 Respira- 

 tion is most perfect in birds ; they require, relatively 

 to their weights, more air than mammals or reptiles, and 



