88 OUE REPTILES. 



has grown so much that it is now quite as large as the body. 

 The mouth is very soon armed with a horny beak, capable of 

 dividing the vegetable food ; the intestine, which is now very 

 long, becomes more fully formed, and assumes a spiral ar- 

 rangement ; the tail is elongated and widened, and the little 

 creature is then called a tadpole. 



At this period, one of those alterations occurs which are 

 so intimately associated with the ideas we are endeavouring 

 to convey, that we must not pass them by in silence. Our 

 larva first breathed by its skin alone, and afterwards by a 

 pair of little branching gills attached to the opercula. About 

 the seventh or eighth day, however, the opercula are gra- 

 dually soldered to the abdomen, and the gills fade away and 

 disappear. At the same time a set of new and more com- 

 plex branchia are developed, in chambers situate on either 

 side of the neck. The new gills are arranged hi tufts at- 

 tached to a solid framework of four cartilaginous arches, and 

 are about a hundred and twelve in number for each side of 

 the body. Here we see a rapid substitution of one organ for 

 another, though both discharge their functions in the same 

 manner, inasmuch as the respiration is just as aquatic in 

 character after the alteration as it was before it. 



But the modifications of the respiratory apparatus do not 

 cease here. Before the tadpole can become a frog, it must 

 do away with these second gills and replace them by lungs ; 

 and at the necessary time, a set of changes takes place 

 analogous to those we have already described. The vascular 

 tufts are atrophied, and the lungs, which till now were solid 

 and rudimentary, open up and increase in size. The circu- 

 latory organs are correspondingly modified. The calibre of the 

 large branchial vessels is diminished, and the pulmonary trunks 

 increase in number and diameter. Later on, the solid parts 

 of the branchial apparatus disappear also, the bones and 

 cartilages being gradually reabsorbed. Eventually the 

 alteration is fully accomplished, and there remains not the 

 slightest trace of the former branchial apparatus. In this 



