RESPIRATION IN REPTILES. 229 



of the parts which surround the cavity of lungs; for even 

 the ribs of serpents contribute but little, by their motion, 

 to this effect, being chiefly useful as organs of progressive 

 motion. ' 



The Chelonia have lungs of great extent, passing back- 

 wards under the carapace, and reaching to the posterior 

 part of the abdomen. Turtles, which are aquatic, derive 

 great advantages from this structure, which enables them 

 to give buoyancy to their body, (encumbered as it is with a 

 heavy shell,) by introducing into it a large volume of air; so 

 that the lungs, in fact, serve the purposes of a large swim- 

 ming bladder. That this use was contemplated in their 

 structure is evident from the volume of air received into 

 the lungs, being much greater than is required for the sole 

 purpose of respiration. The section of the lungs of the tur- 

 tle (Fig. 375,) shows their interior structure, composed of 

 large cells, into which the trachea (T) opens. 



375 





Few subjects in animal physiology are more deserving 

 the attention of those whose object is to trace the operations 

 of nature in the progressive development of the organs, than 

 the changes which occur in the evolution of the tadpole from 

 the time it leaves the egg till it has attained the form of the 



