CLASSIFICATION OF AMPHIBIA. 465 



male nor female, but hermaphrodite. Differences in nutri- 

 tive 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, but most of all because it is a 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. esculentd), not indigenous in Britain, common on 



the Continent, greenish in colour, without the black patch : 

 the North American bull-frog {R. catesbiana], sometimes eight 



inches in length, with a very sonorous croak : 

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



Hyperolius : 

 some toothless frogs, such as the American Vendrobates. 



(b) Those allied to the toad, all toothless : — 



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



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

 many : 



the obstetric toad — Alytes obstetricans, the male of which carries 

 ' the eggs on his back and legs : Hylodes in tropical America, 

 with rapid development without metamorphosis : 



the South American Ceratophrys, of which some species have 

 bony plates in the, skin of the back : Pelobates, which like 

 many others lives for the most part underground : the 

 brightly coloured tree-toads, such as Hyla, with adhesive 

 discs at the ends of the digits : Nototrema, with a dorsal 

 egg-pouch in the females: Liopelma hochstetteri, the only 

 Amphibian in New Zealand. 

 2 G 



