50 ELEMENTS OF COMPARATIVE ZOOLOGY. 



nection of the nostrils with the cavity of the mouth; and 

 the presence of two auricles in the heart. 



In the larva 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. 35). The gills are 

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

 gill-slits are formed. Between these slits are developed 

 external gills. Later the slits are closed in most sala- 

 manders which lose the gills by the growing together of the 

 slits. In the frogs the process is preceded by the forma- 

 tion of an opercular fold (compare fishes) in front of the 

 gill region on either side. These folds grow back over the 



FIG. 19. Side View of Tadpole, e, ever g gill-opening; I, hind leg' 

 m, mouth ; n, nostril : v, vent. 



gill-slits, those of the two sides fusing below the throat and 

 uniting with the wall of the body above and behind the 

 gills, thus forming a large chamber outside the gills which 

 is connected with the exterior by a small opening on the 

 left side,f through which the water used in breathing 



In the larva the heart is two-chambered, and the blood, 



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

 salamanders never develop lungs _ but respire solely through the skin. 



f Right and left openings occur in two tropical toads (Aglossa). 

 A few forms have a median opening. 



