GENERAL OUTLINES OF ICHTHYOLOGY. 9 



ments, the water imparts to the latter the oxygen of the 

 air which it contains, and receives carhon in return, as in 

 the lungs of an air-breathing animal : the gUls do not 

 decompose the water, so as to derive oxygen from it, but 

 merely separate the oxygen from the common air contained 

 in the water ; hence, if water is deprived of this air, or im- 

 pregnated with unwholesome gases, fish cannot exist in it. 



The blood is propelled to the gills by the heart, which 

 thus answers to the right ventricle of warm-blooded ani- 

 mals ; and from the gills it is sent to an arterial trunk, 

 lying immediately under the back-bone, which is the left, 

 or systemic, ventricle of the heart, and which sends the 

 blood throughout the body of the fish. 



The gills of fish possess the power of imbibing oxygen 

 not only from that portion of air which is mixed with the 

 water, but also directly from the atmospheric air itself; and 

 this process may frequently be observed in a vivarium, or 

 other receptacle for fish, where the water is foul, or not 

 changed sufficiently often. I recently witnessed a re- 

 markable example in the case of a stock-pond newly con- 

 structed, into which the fish had been put before the water 

 had properly cleared. In this instance the whole shoal, 

 amounting in all to some hundreds, consisting of various 

 species, remained for long periods together with their 

 mouths partially out of water, for the purpose of breathing. 

 It has also been observed that when fish confined in a 

 limited space are prevented by any means from taking in 

 air at the surface, they die much sooner than others which 



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