CH. VIIl] FROGS, TOADS, AND NEWTS 287 



axis; the blood may be seen coursing up one side and 

 down the other in each filament, travelling in a jerky 

 manner at each beat of the heart. Very soon four slits 

 appear on each side of the throat, opening up communi- 

 cation between the pharynx and the outer water. The 

 sides of these gill-slits become folded and give rise to 

 the internal gills, and at the same time the external gills 

 begin to decrease. Meanwhile a fold of skin, the oper- 

 culum, appears in front of the gills and grows back over 

 them so as to enclose them in a chamber. The right 

 opercular fold grows more rapidly than the left, so that 

 for a few days the external gills of the left side alone 

 are visible, those of the right having disappeared beneath 

 the operculum. At the end of about a month after 

 hatching the opercular folds, which have united with each 

 other ventrally, fuse also at their hinder edges with the 

 body wall, except upon the left side where a spout-like 

 opening is left. In jts circulatory and respiratory arrange- 

 ments the tadpole is essentially a fish. The blood is sent 

 from the heart to the gills to be aerated and thence is 

 carried all over the body before returning to the heart 

 again. The water of respiration is taken in at the mouth, 

 passed through the pharynx and gill-slits, where it gives 

 up its dissolved oxygen and receives carbonic acid, and is 

 ultimately discharged through the spout upon the left 

 side. Further piscine characters are to be found in the 

 cartilaginous brain-case, the muscular tail with its dorsal 

 and ventral fins and " lateral line " sense organs, and the 

 rod-like, unconstricted notochord which forms the axial 

 skeleton. 



