192 



THE INHABITANTS OF THE SEA. 



In most of the cartilaginous fishes, such as the sharks, rays, 

 and lampreys, the gills are differently formed, the water not 

 passing into a cavity closed by a cover, but flowing directly out- 

 wards through five (in the shark) or seven (in the lamprey) 

 vents or spiracles. In these species also the gills are fixed, their 

 margins being attached. Though the whole breathing apparatus 

 of a fish is comprised in a small compass, its surface, if fully 

 extended, would occupy a very considerable space ; that of the 

 common skate, for instance, being equal to the surface of the 

 human body. This single fact may convince us of the number- 

 less ramifications and convolutions of the gills, in which the 

 water is elaborated and attenuated in the course of giving out 

 its air ; and how wonderfully Nature has contrived to effect her 

 purpose with the greatest economy of space. 



Respiration is a species of combustion, and this must neces- 

 sarily be very slow in an element which contains so small a 

 portion of oxygen. No wonder that the circulation of the blood 



Smaller Circulation. 



Ventricle— — - 



/ 



Heart-— {• 



Auricle— --V 1 ;?:- 



Veins 



■"DorBal Artery. 



Arteries. 



Greater Circulation. 

 Theoretic representation of the Circulation in Fishes. 



in fishes is equally tardy. Their heart, in comparison with ours, 

 is but half a one, as it merely serves to force the venous blood 

 into the gills — whence the aerated blood does not flow back to 

 the heart as with us, to be rapidly and strongly propelled through 

 the body, but proceeds immediately to the arteries. Evidently 

 only a cold blood could be formed under such circumstances. It 

 may seem strange that, when fishes are taken outof the water, they 

 die from want of air ; such, however, is the case. Their delicate 

 breathing membranes collapse in the atmosphere, the blood can 



