Chap. I. 



RESriRATION. 



283 



the Cistudo, the process of respiration is no doubt performed entirely by the lungs. 

 This remarkable difference is not only owing to the greater or less thicknes.s of 

 the epidermis, but particularly to this circumstance, that air does not penetrate a 

 horny epidermis so easily as water. Thus, aquatic animals probably absorb the 

 water through the whole surface of their body, and that water, being impregnated 

 with oxygen, is made subservient to respiration ; while, on the contrary, animals 

 living on land are much less capable of breathing through their skin, the air 

 penetrating the epidennis with greater difficulty. This seems to be rendered 

 evident by our table, if we compare Testudo with Trionyx. AVe suppose the 

 same law may have its application in regard to the respiration of all animals; 

 and that animals living in Avater generally require a much smaller development 

 of the breathing organs proper than animals living in the atmospheric air. 



TABLE, 



SHOWING THE CAPACITY OF THE LUNGS COMPARED WITH THE WEIGHT OF THE BODY. 



But there is another interesting circumstance, to which I would allude in this 

 connection. Dr. A. Sagcr says, that, "arranged along the surfiice of the tongue 

 of Trionyx and somewhat in rows, as well as on the fauces and about the rima 



