gg OUR REPTILES. 



appears to be divided in two ou the under surface, each half 

 being prolonged as a sort of: process by which the animal 

 attaches itself to surrounding objects ; as yet there are no 

 traces of either eyes, nostrils, respiratory or auditory organs ; 

 and the belly, of an oblong form, is continued posteriorly as a 

 short tail bordered with a riband-like membrane. This primitive 

 condition, however, does not last long. About the fourth day 

 after birth, the head, which is now as long as the body, has 

 somewhat the appearance of a thimble ; the mouth is provided 

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

 tus 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, clothei with little 

 braaching 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 intes- 

 tine, which is nov 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 

 iatimately 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 cartila- 

 ginous 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 atro- 

 phied, and the lungs, which till now were solid and rudimentary, 



