156 ELEMENTARY BIOLOGY. [XIII. 



After leaving the egg, the young acquires three pairs of 

 external hy'anchim having the form of branched filaments, at- 

 tached to the sides of the hinder part of the head. Narrow 

 clefts in the skin at the roots of the branchise lead into the 

 back of the throat. Water taken in at the mouth passes out 

 by these branchial clefts. The animal crops the aquatic 

 plants on which it lives, by means of the horny plates with 

 which its jaws are provided. 



In the Tadpole, as the larval Frog is called, the intestine, 

 which is relatively longer than in the adult, is coiled up like 

 a watch-spring in the abdominal cavity. A membranous lip, 

 the surface of which is beset with numerous horny papillae, 

 surrounds the mouth, and the muscular tail acquires a large 

 relative size. The eyes, the nasal and auditory organs 

 become distinct, but no limbs are at first visible. 



A fold of the integument in the byoidean region, called 

 the opercular membrane, now grows back over the external 

 gills and unites with the integument covering the abdomen, 

 leaving only a small aperture on the left side, through which 

 the ends of the external gills of that side may, for some time, 

 be seen to protrude. The external gills atrophy and are suc- 

 ceeded functionally by short processes developed from the 

 opposing faces of the branchial clefts — the internal branchice. 

 The rudiments of the limbs appear, rapidly elongate and take 

 on their characteristic shape, the hind pair only being at first 

 visible on account of the anterior pair being hidden under the 

 opercular membrane. The lungs are developed and, for a time, 

 the tadpole breathes both by them and by its internal gills. 



As the legs grow the tail shortens and, at last, is re- 

 presented merely by the pointed end of the body ; the gape 

 elongates until the angle of the mouth lies behind the eye, 

 instead of a long way in front of it, as in the tadpole ; the 

 labial membrane and the horny armature of the mouth 

 disappear, while teeth are developed in the upper jaw and, on 



