2P0 



THE VITAL FUNCTIONS. 



respiration. The section of the hmgs of the turtle 

 (Fig. 37''3), shows their interior structure, composed 

 of large cells, into which the trachea (t) opens. 



Few subjects in animal physiology are more de- 

 serving the attention of those whose object is to trace 

 the operations of nature in the progressive develope- 

 ment 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 

 perfect frog. We have already had occasion to 

 notice several of these transformations in the organs 

 of the mechanical functions, and also in those of 

 digestion and circulation : but the most remark- 

 able of all are the changes occurring in the re- 

 spiratory apparatus, corresponding with the oppo- 

 site nature of the elements which the same animal 

 is destined to inhabit in the difterent stages of its 

 existence. No less than three sets of organs are 

 provided for respiration ; the first two being 

 branchiae, adapted to the fish-like condition of the 

 tadpole ; and the last being pulmonary cavities, for 



