xii PHYLUM CHORDATA 431 



staple food, though it also eats decaying leaves and the flesh 

 of dead animals. 



Soon the external gills show signs of shrivelling, and at 

 the same time internal gills, like those of fishes, are devel- 

 oped on the branchial arches. A fold of skin, the operculum, 

 appears on each side, in front of the gills, growing from the 

 region of the hyoid arch, and extends backwards until the 

 gill-slits and external gills are covered and there is only a 

 single small external branchial aperture on each side. On 

 the right side the operculum soon unites with the body- wall 

 so as to close the branchial aperture, but on the left side 

 the opening remains for a considerable time as the sole exit 

 of the water. At this time the tadpole is to all intents 

 and purposes a fish. 



The lungs now appear, and the larva is for a time truly 

 amphibious, rising periodically to the surface to breathe air ; 

 the single branchial aperture, however, soon closes, and 

 henceforth respiration is purely aerial. 



In the meantime the limbs are developed. The hind- 

 limbs appear as little rounded buds, one on each side of the 

 root of the tail. The fore-limbs arise beneath the operculum, 

 and are therefore hidden at first ; soon, however, they 

 emerge by forcing their way through the operculum. As 

 the limbs increase in size, the tail undergoes a progressive 

 shrinking. The mouth widens, the intestine undergoes a 

 relative diminution in length, and vegetable is exchanged 

 for animal diet. The little tailed frog can now leave the 

 water and hop about upon land ; its tail is soon completely 

 absorbed, and the metamorphosis is complete. 



The frogs and toads are all closely allied as regards all 

 the main features of their structure the chief differences 

 between the many genera and species being in such super- 

 ficial characteristics as coloration and proportions. In 



