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them. A fish was destined to be always in the 

 water, and a bird or quadruped always in the air ; 

 and hence the structure of their respiratory, as 

 well as circulating system, is such as to incapaci- 

 tate them each for the other element ; but a rep- 

 tile was intended to be sometimes in the one, 

 and sometimes in the other, and both systems 

 are accordingly so constructed as to allow of this. 

 Further, some serpents, the hydrus bicolor, for 

 example, are enabled to continue under water for 

 an indefinite time by a provision similar to that 

 which I have already described, as found in the 

 water-scorpion among insects, being capable of 

 bringing the upper part of their wind-pipe al- 

 most to a line with the point of their lower jaw, 

 so that they can breathe, although scarcely any 

 portion of them is out of the water. 



Before quitting reptiles, it is proper to observe, 

 that there are individuals of this class, for in- 

 stance, the proteus and siren, which, with re- 

 spect to their respiratoiy organs, may be consi- 

 dered as composed of fishes and reptiles, being 

 furnished at once with gills and lungs ; or per- 

 haps, with more propriety, as fishes, with a more 

 developed air-bladder. The tadpole, a larva of 

 the frog, during its metamorphosis presents a 

 somewhat similar structure, the gills only gra- 

 dually disappearing, in proportion as the lungs 

 become developed. 



In birds, the lungs instead of being membran- 

 ous are fleshy, and instead of being loose and 

 floating, as in reptiles, are tightly bound down to 



