288 BIOLOGY 



elongates, and a slight constriction appears behind the anterior 

 end resembling a neck. The front portion is, however, not 

 the head alone, but the head and body fused together, while the 

 back portion soon grows out into an elongated tail. From the 

 side of the two branchial openings feather-like external gills 

 or branchiae develop, which, projecting laterally from the head, 

 serve as respiratory organs; Fig. D. The free larva is now known 

 as a tadpole, and from this time it is obliged to depend upon 

 itself. Its digestive organs have become developed enough to 

 perform their functions, and the larva begins to feed upon vege- 

 table food, eating the delicate green plants that are found grow- 

 ing on the bottom of the pool where the larva attaches itself. 

 The rapidity with which the animal goes through the subsequent 

 changes is dependent chiefly upon the amount of food it obtains, 

 and the temperature; but it soon begins to pass through the 

 stages represented in Figures C to G. The front end of the body, 

 which is the head and body fused together, increases in size 

 and becomes rounded, while the tail elongates and becomes 

 flatter, serving as a swimming fin. The external gills disappear; 

 but the gill slits remain, the animal still breathing by the use 

 of internal gills, not visible from the outside. The size of the 

 tadpole varies with the different species of the frog; in some of 

 the ordinary frogs it may become two or even three inches in 

 length, while in other species it is not more than half an inch. 



The next change is the appearance of a pair of small pro- 

 tuberances, or buds, on the posterior end of the body on either 

 side; Fig. 133 F. These grow rapidly in length and develop 

 into the hind legs. A similar pair of buds appears at the an- 

 terior end of the body, a little behind the gill slits, which later 

 grow into the fore legs or arms. As these legs and arms grow, 

 the whole shape of the body changes; the eyes appear on the 

 sides of the head; the mouth, which is at first a round sucking 

 slit, elongates into a large slit surrounded by the jaws; the 

 head assumes more of its final form; the shape of the body 

 changes from the rounded tadpole to a more elongated structure. 



