322 



THE VITAL FUNCTIONS. 



shows their interior structure, composed of large 

 cells, into which the trachea (r) opens. 



Few subjects in animal physiology are more 

 deserving the attention of those whose object is 

 to trace the operations of nature in the progres- 

 sive developement 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 transforma- 

 tions in the organs of the mechanical functions, 

 and also in those of digestion and circulation : 

 but the most remarkable of all are the changes 

 occurring in the respiratory apparatus, corres- 

 ponding with the opposite nature of the elements 

 which the same animal is destined to inhabit in 

 the different stages of its existence. No less 



