Xiii.] THE FROG. 163 



of a minute fish, devoid of limbs and with only rudiments of 

 gills, but provided with two adhesive discs on the ventral 

 side of the head behind the mouth. 



After leaving the egg, the young acquires three pairs of 

 external branchitz 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 branchias 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 mouthy and the muscular tail acquires a large 

 relative size. The eyesj the nasal and auditory organs 

 become distinct, but no limbs are at first visible.: 



A fold of the-, integument in the hyoidean 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 

 succeeded functionally by short processes developed from 

 the opposing faees of the branchial . clefts the internal 

 branchia. The rudiments of the limbs appear, rapidly elon- 

 gate 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- 



II 2 



