CLASS AMPHIBIA 



373 



now begins to swim, feeds on algae and other vegetable matter, and is 

 known as a tadpole. At this time the external gills begin to shorten, 

 and internal gills, of which there are four pairs, are being formed. A 

 fold grows around the body just behind the head, covering the gill 

 slits on both sides and producing a chamber known as a branchial pouch. 

 The branchial pouch opens on the left side by a circular opening called 

 the spiracle. By this time the external gills have ceased to function 

 and the internal gills serve in respiration, water being passed through 

 Ora/ sucker 



Fig. 254. — Later stages in the development of the frog. A, embryo at time of hatching. 

 5, tadpoles clinging to vegetation after hatching. C, stage showing external gills. D, 

 gills covered by an operculum, the branchial chamber opening to the outside by the spiracle; 

 hind legs appearing. E, hind legs well developed. F, late stage in metamorphosis; legs 

 all present, and tail nearly gone. G, the adult leopard frog, Rana pipieris Schreber. From 

 models, and preserved and living (Fig. G) specimens. 



the mouth, on through the gill slits, into the branchial pouch, and out 

 through the spiracle. 



Of the two pairs of limbs the hind pair appear first. Later the fore- 

 limb on the left side emerges through the spiracle, while the one on the 

 right side breaks through the wall of the branchial pouch. The tail 

 diminishes in size, being in part absorbed by cells in the body and in 

 part inclosed by the body, until it is no longer apparent from the outside. 

 The internal gills are also absorbed and lungs develop to function as 

 respiratory organs, after which the gill slits close and the branchial 

 pouch disappears. During the time between the giving up of branchial 



