OUTLINES OF ICHTHYOLOGY. 153 



great majority of fishes, however, cannot continue 

 to breathe in atmospheric air alone beyond a very 

 limited period, as, without the aid of fluid, the 

 leaves of the gills, or gill filaments, shortly become 

 stuck together, when, of course, the air cannot 

 pass between them, and they lose the power of 

 imbibing oxygen. From this results the gasping 

 of fishes out of water, which is the effort of nature 

 to separate the gill filaments. 



The blood is propelled to the gills by the heart, 

 which thus answers to the right ventricle of warm- 

 blooded animals ; and from the gills it is sent to 

 an arterial trunk, lying immediately under the 

 backbone, forming the left or systemic ventricle 

 of the heart, which sends the blood throughout 

 the body. 



In intimate connection with the organs of res- 

 piration occurs the question of the blood tem- 

 perature. 



The consumption of oxygen in all fishes is 

 comparatively small, which may be the cause of the 

 cold nature of their blood. The temperature of 

 the bodies of fish that swim near the bottom is 

 seldom more than two or three degrees higher than 

 that of the water at its surface. In surface-swimmers 

 a temperature of 10 Fahr. above that of the water 

 has been occasionally found ; and it may be re- 

 ceived as a general law, that those fish which swim 

 near the surface have a high standard of respira- 



