Ki:i^TlL!A, 



389 



abruptly made, we find that Batrachian Reptiles, 

 with which this series commences, are constructed 

 at first on the model of fishes ; breathing the atmo- 

 spheric air contained in the water by means of 

 gills, and moving through the fluid by the same 

 instruments of progression as fishes, wliich indeed 

 they exactly resemble in every part of their me- 

 chanical conformation. The tadpole, which is the 

 young of the frog, is at first scarcely distinguish- 

 able 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 immediately tapers to form a lengthened tail 

 by the prolongation of the spinal column, which 

 presents a numerous series of coccygeal vertebrae, 

 furnished with a vertical expansion of membrane 

 to serve as a caudal fin, and with appropriate 

 muscles for executing all the motions required in 

 swimming. The appearance of the tadpole in its 

 early stage of developement is seen in Fig. 197 and 

 198, the former being a side, and the latter an 

 upper view of that animal. 



Yet with all this apparent conformity to the 

 structure of a strictly aquatic animal, the tadpole 

 contains within its organization the germs of a 



