70 



ZOOLOGY 



orgkns must be provided. This may be effected by thin out- 

 growths of the body surfaces, which are especially adapted 

 to water forms and are called gills or branchice (Fig. 32); or a 

 similar increase may be attained by pits or ingrowths of the 

 body surface, suited to get oxygen from the air. Such are 

 called lungs or trachea (Fig. 33). Carbon dioxid?- a gaseous 

 waste product resulting from the union of oxygen with carbon 



Fig. 32. 



Pig. 33. 



b. C, 



Fig. 32. Diagram illustrating gills or branchiae. b.c., cavity in which the body fluids circu- 

 late; hr., branchial filaments which are merely much thinned out-pocketings of the body wall (ai); 

 ex, the external medium — water — in which the oxygen is dissolved. 



Questions on the figure. — What are the essential features of gills as suggested 

 by this figure? Why are they better suited to water than to air? 



Fig. 33. Diagram illustrating lungs or traches. ft.c, the cavity in which the body fluids 

 circulate; 2, the walls of the lung, which are much thinned in-pocketings of the body wall (lo); ex., 

 the external medium — usually the atmosphere — in which the oxygen is found. 



Questions on the figure. — ^What are the essential features of lungs as suggested 

 by the figure? Why are such organs better suited to aerial than to aquatic life? 

 In what respects are gills and lungs better than the mere body- wall for the exchange 

 of the gases? 



which takes place in the tissues, is economically eliminated by 

 the same organ which admits the oxygen, inasmuch as the en- 

 trance of one gas is not retarded by the outward passage of the 

 other. This double process constitutes respiration, although 

 the latter half is also appropriately described as excretory. The 

 surface devoted to the exchange of the gases and the special 

 devices necessary to renew the air or water make up the respira- 

 tory system. The respiratory organs are frequently associated 

 with the anterior or posterior end of the digestive tract. As in 



