AMPHIBIANS. 



337 



In the larvae respiration takes place by gills, recalling 

 those of fishes ; and in a few forms these are retained during 

 life. Besides gills, all, in the adult condition, develop 

 lungs,* which grow out from the pharynx, and always re- 

 tain their connection with it by means of a windpipe (tra- 

 chea) .opening upon its floor (compare p. 310). The gills 

 are fewer in number than in any fish, and only three or 

 four gill-slits are formed. Between these slits are devel- 

 oped external gills (fig. 138). Later the slits are closed 



FIG. 138. Larval stage of a salamander with external gills. From Hertwig. 



in those salamanders which lose the gills, by the growing 

 together of the slits. In the frogs the process is preceded 

 by the formation of an opercular fold (compare fishe>) in 



FIQ. 139. Side view of tadpole. e, eye; g, gill-opening; I, hind leg; m, 



mouth: n, nostril; v, vent. 



front of the gill region on either side. These folds grow 

 back over the gill-slits, those of the two sides fusing below 

 the throat and uniting with the wall of the body above 



* It has recently been shown that some of the North American 

 salamanders never develop lungs, but respire solely through the 

 skin, 



