254 



GENERAL ZOOLOGY 



yolk rapidly disappears and the tadpoles gain sustenance from 

 food taken in at the mouth. The alimentary canal lengthens 

 and becomes spirally coiled. A peculiar larval feature of the 

 mouth is the development of horny sheaths on the jaws which 

 function as rasping organs. The lips are also provided with horny 

 papillae which aid in obtaining food (Figs. 156 and 157). 



Fig. 156. — A, B, and C, three stages in the development of mouth, suckers 

 and gills of Rana arvalis. D, side view of stage B. e, eye; g, gill; m, mouth; 

 n, nasal pit; s, sucker. (After Lieberkind.) 



Branchial Structures. — In addition to the notochord, another 

 important distinguishing feature of the phylum Chordata is the 

 presence of gill clefts and of gills (in aquatic forms) in the pharyn- 

 geal region. At the time of hatching the rudiments of the exter- 

 nal gills can be seen on either side of the body just behind the 

 head as a pair of small conical projections. Later these are 

 joined by a third rudiment on each side. All three rudiments 

 grow rapidly, though the third remains smaller than the other 

 two, until they form a tufted mass on either side of the body 

 (Fig. 156). Each gill develops finger-shaped lobes and is supplied 

 with blood vessels, much as in a fish. Meanwhile four gill clefts 

 develop on each side, one in front of the first pair of gills, one 

 behind the last pair, one between the first and second and 



