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 viembrane^ now grows back over the external 

 gills and unites with the integument covering the trunk, 

 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 faces of the branchial clefts — the mterjial 

 braiichia. 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- 

 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 

 changes its diet from vegetable to animal matters — the 

 perfect Frog being insectivorous. 



The two species, Raiia tejuporaria and Rana esculen/a, 

 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 



