ii8 THE BIOLOGY OF THE FROG CHAP. 



pairs of internal gills, which are produced by foldings of the 

 membrane lining the gill slits. Both external and internal 

 gills receive an abundant blood supply from the vessels that 

 form the aortic or branchial arches. The disappearance of 

 the external gills is associated with the growth of a fold, 

 the operculum, which arises on either side of the head and 

 gradually extends backward. The free posterior edge of the 

 fold fuses with the body behind and below the gill region, 

 leaving only an open space on the left side of the body, 

 which is known as the spiracle. The water which passes 

 out of the gill slits comes into a chamber bounded exter- 

 nally by the opercular wall, and thence passes through the 

 spiracle to the outside. Soon after the completion of this 

 chamber the external gills disappear and the internal gills 

 function in their stead. 



The jaws of the tadpole are furnished with horny coatings 

 which function as teeth, but these are shed in later larval 

 life. In addition both upper and lower lips also contain 

 transverse rows of fine teeth, which vary in number and 

 arrangement in the different species. Around the outside 

 of the lip there are numerous small papillae, which also vary 

 considerably in tadpoles of different species of frogs. The 

 nasal pits do not break through into the mouth until some 

 time after hatching. The eyes are situated on the dorsal 

 side of the head, and look obliquely upward. There are 

 several rows of sense organs on the skin of the tadpole, 

 but these disappear when the animal assumes a terrestrial 

 mode of life. The ventral sucker in the recently hatched 

 larva is in the form of a horseshoe. The ectodermic cells 

 covering it are partly glandular, and they form a mucous 

 secretion by means of which the larvae adhere to various 

 objects. Later in larval life the sucker becomes divided in 

 two in the middle. The two parts become carried farther 



