PHYLUM CHORDATA 



291 



first two pairs increase greatly in size (^, g'') : the stomodseum joins 

 the archenteron, gill-slits (branchial clefts) are formed between the 

 branchial arches, and the eyes appear. The mouth is small, 

 bounded by lips beset with horny papillee and provided with a 

 pair of horny jaws. The enteric canal grows to a great length 

 and is coiled like a watch-spring, and the tadpole browses upon 

 the water-weeds which form its staple food. 



Soon the external gills show signs of shrivelling, and at the 

 same time internal gills, like those of Fishes, are developed in the 





Pig. 940.— Rana temporaria. Stages in the life-history, from the newly-hatched Tadpoles (1) 

 to the young Frog (8). 2a is a magnified view of 2. (From Mivart.) 



branchial clefts. A fold of skin, the operculum, appears on each 

 side, in front of the gills, growing from the region of the hyoid 

 arch, and extending backwards until the gill-slits and external gills 

 are covered and there is only a single small external branchial 

 aperture on each side, as in Holocephali (3, I^). On the right side 

 the operculum soon unites with the body-wall so as to close the 

 branchial aperture, but on the left side the opening remains for 

 a considerable time as the sole exit of the water. At this time 

 the tadpole is to all intents and purposes a Fish. 



