HKPTILIA. 



565 



other, not only in their outward form, but in their anatomical con- 

 struction ; and, in considering this portion of our subject, we shall 

 have another most striking illustration of Fig. 254. 



this great law in zoology. 



The perfect and typical Reptile, as the 

 Lizard, the Tortoise, and the Serpent, 

 breathes air and air only, and is there- 

 fore only provided with lungs adapted to 

 this kind of respiration : but the Peren- 

 nibranchiate Amphibia, possessing both 

 lungs and gills, participate to a greater 

 or less degree in the characters of Fishes, 

 so that in some, as, for example, in the 

 Lepidosireu (Jig. 238), so near is the 

 approximation, that it becomes almost 

 impracticable for the most accomplished 

 anatomist precisely to determine whether 

 the animal ought rather to be called a 

 Reptile or a Fish ; and lastly, in the 

 Batrachian Amphibia, as we have al- 

 ready seen, we have the same animal 

 gradually changed from a Fish into a 

 complete and perfect Reptile. 



In considering the apparatus provided 

 for circulation and respiration in the ani- 

 mals comprised in the class before us, we 

 shall therefore first describe the organiza- 

 tion of these viscera in Reptiles furnished 

 with lungs only ; secondly, of those hav- 

 ing permanent gills as well as lungs ; and 

 thirdly, the metamorphoses that take 

 place in the construction of the breath- 

 ing organs during the developement of 

 the lungs, and the obliteration of the 

 branchiae in those forms in which the 

 branchiae are not persistent. 



(627.) The lungs of Reptiles are two 

 capacious membranous sacs occupying 

 a considerable portion of the visceral 

 cavity which, as there is no diaphragm as 

 yet developed, cannot properly be di- 





