118 A COURSE ON ZOOLOGY. 



arities, due to the aquatic life. Like all animals, fishes 

 require air in order to live, and they find this air in 

 the water; their organs for renewing the air are so 

 constituted that they can only function usefully in 

 water. 



% It can easily be shown that water contains dissolved 

 air. This air separates and collects in little bubbles on 

 the walls of a glass in which water is allowed to stand 



FIG. 65. 



-r 



HEAD OF CARP WITH THE GILL-COVER REMOVED. 6, barbels ; i, iris ; p, pupil ; 

 a, branchial arches, having the comb-like teeth, d, on the internal border ; 

 r, rays of the gills. 



and become warm. If water be boiled in a glass vessel 

 the bubbles may be seen to rise and escape at the surface, 

 and with the proper apparatus the gas may be collected 

 and its chemical composition determined. It is then 

 found to be air. This air is expelled from the water by 

 the elevation of temperature, and all the air may be thus 



