6 ELEMENTARY BIOLOGY. [CHAP. 



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 hyoidean region, called 

 the opercular membrane, now grows back over the external 

 gills and unites with the integument covering the trunk, 

 leaving only a small aperture or spiracle on the left side, 

 through which the ends of the external gills may, for some 

 time, be seen to protrude. The external gills atrophy, and are 

 succeeded functionally by short processes developed from 

 the opposing faces of the branchial clefts — the internal 

 branchice. 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 

 these 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 the roof of the mouth ; the intestine becomes less and 

 less coiled as, not growing at the same rate as the body, 

 it becomes relatively shorter; and the animal gradually 

 relinquishes its mixed diet for one of animal matter — the 

 perfect Frog being mostly insectivorous. 



The two species Rana temporaria and Rana esculenta, 

 are distinguishable by the following external characters. In 

 Rana temporaria, the interspace between the eyes is flat or 

 slightly convex, and its breadth is usually greater than or 



