RESPIRATION IN REPTILES. 323 



No less than three sets of organs are provided 

 for respiration ; the two first being branchioe, 

 adapted to the fish-like condition of the tadpole, 

 and the last being pulmonary cavities, for re- 

 ceiving air, to be employed when the animal 

 exchanges its aquatic for its terrestrial life. It 

 is exceedingly interesting to observe that this 

 animal at first breathes by gills, whicli project 

 in an arborescent form from the sides of the 

 neck, and float in the water ; but that these 

 structures are merely temporary, being provided 

 only to meet the immediate exigency of the 

 occasion, and being raised at a period when 

 none of the internal organs are as yet perfected. 

 As soon as another set of gills, situated inter- 

 nally, can be constructed, and are ready to 

 admit the circulating blood, the external gills 

 are superseded in their office ; they now shrivel, 

 and are removed, and the tadpole performs its 

 respiration by means of branchiae, formed on the 

 model of those of fishes, and acting by a similar 

 mechanism. By the time that the system has 

 undergone the changes necessary for its conver- 

 sion into the frog, a new apparatus has become 

 evolved for the respiration of air. These are the 

 lungs, which gradually coming into play, direct 

 the current of blood from the branchi^, and take 

 upon themselves the whole of the office of res- 

 piration. The branchiae, in their turn, become 

 useless, are soon obliterated, and leave no other 



