AND THE LOWER ANIMALS. 89 



with a pair of soft lips ; the nostrils, eyes, and auditory 

 apparatus have made their appearance ; the . head is 

 separated by a deep groove from the belly, which has 

 assumed a spherical form, and from which spring a 

 pair of opercula, clothed with little branching gills ; 

 and the tail 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 arrangement ; 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 gradually 

 soldered to the abdomen, and the gills fade away and 

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

 complex branchia are developed, in chambers situate 

 on either side of the neck. The new gills are arranged 

 in tufts attached to a solid framework of four carti- 

 laginous 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 



