84 Elementary Zoology. 



allow of a free flow of oxygen containing water over the gills. 

 When the gill-clefts and the associated gills appear, a fold of 

 skin, the opeixulutn, arises from the side of the head, and grows 

 over the gills, remaining open only at one point. This is on 

 the left side of the body, and its margin projects in a spout-like 

 fashion. It will be observed that in this stage of its existence 

 the young frog is practically a fish. If it grew no further, and 

 were to develop sexual organs, it vcould have to be classified 

 with the fishes. It has the gills of the fish, its circulatory 

 organs are constituted upon a similar plan, and the body is 

 fringed dorsally and ventrally with a continuous fin-fold as in 

 many fishes. But it has no lateral fins corresponding to the 

 paired fins of the higher fishes. These appear later in the form 

 of the limbs of the adult frog. The lungs also soon appear, 

 the tail gets less, respiration is effected entirely by the lungs, 

 and the tadpole leaves the water a frog. 



