RESPIRATION IN REPTILES. 323 



than three sets of organs are provided for respi- 

 ration ; the first two being branchiae, adapted to 

 the fish-like condition of the tadpole ; and the 

 last being pulmonary cavities, for receiving 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, which project in an arbo- 

 rescent form from the sides of the neck, and 

 float in the water ; but 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 in- 

 ternal organs are as yet perfected. As soon as 

 another set of gills, situated internally, can be 

 constructed, and are ready to admit the circu- 

 lating blood, the external gills are superseded in 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 mecha- 

 nism. By the time that the system has under- 

 gone the changes necessary for its conversion 

 into the frog, a new and very different apparatus 

 has been evolved for the respiration of air. 

 These are the lungs, which gradually coming 

 into play, direct the current of blood from the 

 branchiae, and take upon themselves the whole 

 office of respiration. The branchiae, in their 

 turn, become useless, are soon obliterated, and 



