BATRACHIA. 



303 



§ 2. Balrachia. 



The order of Balrachia, or Amphibious Reptiles, con- 

 stitutes the first step in the transition from aquatic to terres- 

 trial vertebrata. It is more particularly the function of res- 

 piration that requires to be modified, in consequence of the 

 change of element in which the animal is to reside; and as 

 if it had been necessary, conformably to the laws of animal 

 creation, that this change should not be abruptly made, we 

 find that Batrachian reptiles, with which this series com- 

 mences, are constructed, at first, on the model of fishes; 

 breathing the atmospheric air contained in the water by 

 means of gills, and moving through the fluid by the same 

 instruments of progression as fishes, which, indeed, they ex- 

 actly resemble in every part of their mechanical conforma- 

 tion. The tadpole, which is the young of the frog, is, at 

 first, not distinguishable in any circumstance of its internal 

 skeleton, or in the disposition of its vital organs from the 

 class of fishes. The head, indeed, is enlarged, but the body 



19; 



immediately tapers to form a lengthened tail, by the pro- 

 longation of the spinal column, which presents a numerous 

 series of coccygeal vertebrae, furnished with a vertical ex- 

 pansion of membrane to serve as a caudal fin, and with ap- 

 propriate muscles for executing all the motions required in # 

 swimming. The appearance of the tadpole, in its early 

 stage of development, is seen in Fig. 197 and 198, the 



