HOW ANIMALS BREATHE. 



117 



Like the alimentary canal, the lungs of an animal are 

 really an inflected portion of the outer surface; so that 

 breathing by the skin and breathing by lungs are one in 

 principle. Indeed, in many animals, especially Frogs, res- 

 piration is carried on by both lungs and skin. 



The lungs of Vertebrates are deriveoLfrom the front 

 part of the alimentary canal. In some Fishes, air is swal- 

 lowed, which passes the whole length of the digestive 

 tract, and is expelled from the anus. Here 

 the whole canal serves for respiration. In 

 Reptiles, Birds, 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. 

 170, 171). 



All Vertebrates have two kinds of respir- 

 atory organs in the course of their life. 

 Fishes have gills; their lung (the air-blad- 

 der) rarely serves as a functional respiratory 

 organ, and is sometimes wanting. Amphibi- 

 ans 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 allantois 

 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 mem- 



. FIG. 84. Lungs 



branous sacs, divided more or less into cells of a snake: a, 



to increase the surface. Upon the walls of 



the cells are spread the capillary blood-ves- 



sels. The smaller the cells, the greater the nary veiu ; the 



i i xi 1.1 j lung, B,i8 nidi- 



extent of surface upon which the blood is mentnry. 



