STRUCTURE OF FISH. 175 



wanted or subsist alone. If the accomplishment of 

 any purpose demands a change of power, or even of 

 structure, there is ample provision for the effecting of 

 that. When young frogs, and naked larvae of insects, 

 continue habitually in the water, they have the fins and 

 the habits offish ; but, when they change their abodes, 

 they change also their forms and habits. 



The organs of respiration in fishes are very curious, 

 more so, perhaps, than those of land animals, because 

 they have a double function to perform, first, to sepa- 

 rate the air from the water, and then, to decompose 

 it. The system of circulation in fishes is, however, less 

 complicated than that of the warm-blooded land animals. 

 In these, the heart is double ; and every time that it is 

 compressed, that which has been aerated in the lungs, 

 is poured, by the aorta and its ramifications, over the 

 whole system ; while that which has passed through the 

 system, and in its course supplied new materials, and 

 washed away such as were unfit for life, is sent by the 

 pulmonary artery to the lungs, in order that it may be 

 there washed, renovated, and made fit for the purposes 

 of life, by contact with the air. It may be that this 

 double circulation is necessary for keeping up the heat 

 of the animal ; and this is rendered as probable as any 

 thing of a similar kind can be, by the fact of its being 

 peculiar to the warm-blooded animals, and by their 

 being always the animals which are most exposed to 

 the atmosphere, and liable to be affected by its changes 

 of temperature. 



In fishes, the heart is single, and the whole of the 

 blood which returns from the circulation by the veins 

 is sent directly to the organs of respiration. For this 



