366 AMPHIBIA xm. 6- 



groups and are more fully adapted than most for a terrestrial life, but 

 return nearly always to the water to breed. Bufo itself is found in 

 almost all possible parts except in Australia and Madagascar; related 

 genera, many of them with special features, are found all over the 

 world. Curiously enough only one genus, Nectophrynoides from East 

 Africa, is viviparous, the young being in that case provided with a 

 long vascular tail, by means of which they maintain contact with the 

 wall of the 'uterus', even though embedded in a mass of embryos. 



Hyla and other tree-frogs, very widely distributed, are similar to 

 the bufonids but have pads on the toes by which they climb, and 

 many other adaptations to arboreal life. Gastrotheca (= Nototrema), 

 the marsupial frog, is a genus in which the young develop in a sac on 

 the back of the female, this sac being in one species protected by 

 special calcareous plates. Rana and its allies, the true frogs, are also 

 cosmopolitan. A number of frogs related to Rana have taken to a tree- 

 living habit, developing pads on the toes. Polypedates is a widespread 

 genus and there are several others, each independently derived from 

 ranids. This is therefore a striking illustration of parallel evolution — 

 the hylid tree-frogs having arisen from bufonids and probably several 

 sorts of polypedatids from ranids. 



Burrowing with the legs has also been evolved several times by 

 anurans. In Breviceps (Fig. 210), which digs for ants, there is a large 

 snout, as in other anteaters. 



7. Subclass Apoda (= Gymnophiona = Caecilia) 



These (such as Ichthyophis) are burrowing, limbless creatures living 

 like earthworms, in the tropics. They show several interesting primi- 

 tive features, including the retention of small scales in the skin. They 

 are specialized, however, in having a very short tail and some features 

 suited to their terrestrial life, such as copulatory organs. The animals 

 are blind, the place of the eyes being taken by special sensory ten- 

 tacles. The eggs are large and yolky and cleavage is meroblastic; they 

 are laid on land and the embryos develop around the yolk sac, but 

 often have long, plumed gills. Vivipary is common, including in the 

 aquatic form Typhlonectes. 



8. Adaptive radiation and parallel evolution in modern Amphibia 



Even this superficial study of the 250 genera and about 2,000 species 

 of modern amphibians shows that the features we have already 

 recognized in fish evolution are found also in evolution on land. It is 

 difficult in a short time to gain an impression of the very great variety 



