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CLASS IV. PISCES* 



IN all the preceding Classes, we have found many 

 animals which are in a greater or less degree deni- 

 zens of the water ; but all these have inhabited it 

 merely as a supporting medium, the grand vital func- 

 tion of respiration being performed in the air. We 

 now arrive at a very numerous and widely dispersed 

 Class, which are in every sense aquatic, never coming 

 to the surface to breathe, and, in fact, utterly in- 

 capable of respiring air. They breathe water ; and 

 for this vital action they are provided with peculiar 

 organs, instead of lungs ; viz. an apparatus called 

 gills on each side of the neck, composed of a vast 

 number of thin plates placed side by side in rows, 

 suspended on bony arches ; these plates consisting of 

 exceedingly minute but innumerable blood-vessels. 

 The water taken in at the mouth, is ejected through 

 the gills, and penetrating through these plates, and 

 bathing every part of their surface, parts with a 

 portion of its oxygen, which is thus communicated 

 to the blood, as that of the air is communicated to 

 the lungs of land animals. Thus the difference be- 

 tween aerial and aquatic respiration is not in its es- 

 sential nature, but only in the medium by which the 

 vivifying fluid is applied. 



* Piscis, a Fish. 



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