CLASSIFICATION OF AMPHIBIA. 555 



is not, however, so great as it seems, for even at a very early 

 stage, animal food is eagerly devoured. 



With the change of diet, the abdomen shrinks, stomach 

 and liver enlarge, the intestine becomes relatively narrower 

 and shorter. The tail shortens more and more, and as it 

 does so the disinclination for a purely aquatic life seems to 

 increase. Eventually it is completely absorbed, the hind 

 limbs lengthen, and the conversion into a frog is completed. 



It seems that for a considerable time the tadpole is 

 neither male nor female, but hermaphrodite. Differences 

 in nutrition and other conditions cause one kind of sexual 

 organ to predominate over the other, and the tadpole 

 becomes unisexual. In nature there are usually about as 

 many males as there are females, but Yung has shown that 

 by increasing the quality of the food given to young tadpoles 

 from fish-flesh to beef, and from beef to frog-flesh, he could 

 raise the percentage of females to about ninety. 



In many respects the development of the tadpole is very 

 interesting, especially because it is a modified recapitulation of 

 that transition from aquatic to aerial respiration, which must 

 have marked one of the most momentous epochs in the 

 evolution of Vertebrates. 



CLASSIFICATION OF AMPHIBIA. 

 Order ANURA or ECAUDATA. 



The attainment of the adult form is associated with the loss of tail 

 and gills. The body is broad. The long and very muscular hind-legs 

 are powerful in leaping. 



(a) The frog and its allies : 



The British frog ( Rana temporaria), brown in colour, with a black 



patch on the side of the head : 



the Edible frog (R. esculenta}^ not indigenous in Britain, common 

 on the Continent, greenish in colour, without the black 

 patch : 

 the North American bull-frog (R. catesbiand), sometimes eight 



inches in length, with a very sonorous croak : 

 some Asiatic and African "tree-frogs," such as Rhacophorus and 



Hyperolius : 



some toothless frogs, such as the American Dendrobates. 

 (/>) Those allied to the toad, all toothless : 



the toads in the strict sense (Bufo), with poisonous skin : 

 the crimson -bellied Bombinator igneus, the Feuerkrote of Ger- 

 many : 



