298 AMPHIBIA xii. 3- 



3. The frogs 



Perhaps the most successful of all amphibia are those belonging 

 to the genus Rana, abundant in every part of the world except in 

 the south of South America, on oceanic islands, and New Zealand. 

 Ranid frogs are typical of the highly specialized subclass Anura, whose 

 members usually inhabit damp places such as marshes or ditches, 

 living for most of their life in the grass or undergrowth and feeding by 

 catching flies and other insects with their tongue. They are preyed 

 upon by birds, fishes, and especially snakes, and escape from these by 

 their hind legs, used either for jumping or swimming. The young 

 develop as tadpoles in the water, where they are omnivorous. The 

 various species differ in size and small points of colour, though there 

 are also some that depart widely from the usual habits, e.g. R. fossor 

 which burrows. R. temporaria is the species found in Great Britain, 

 R. esculenta is a slightly larger form found on the continent of Europe 

 and occasionally in east England, R. pipiens is the common small 

 North American frog; R. catesbiana the giant bull-frog, whose body 

 is up to 9 in. long, also lives in North America. R. goliath of the 

 Cameroons is over a foot long, but is mainly aquatic. 



4. Skin of Amphibia 



The earliest amphibia possessed the scales of their fish ancestors, 

 but these were soon lost in most lines, though retained in some 

 Apoda; perhaps they were too heavy to be worth while for creatures 

 contending for the first time with gravity, unaided by water. Some 

 frogs carry dermal plates on the back, however, fused to the neural 

 spines (Brachycephahis of Brazil). Amphibia differ from reptiles in 

 that the skin is moist and used for respiration; on the other hand, the 

 skin also shows a character typical of land animals in having heavily 

 cornified outer layers. The epidermis therefore consists of several 

 layers in the adult frog and is renewed at intervals by a process of 

 moulting. The moult is under the control of the pituitary and thyroid 

 glands and does not occur if either of these be removed, the kera- 

 tinized cells merely accumulating in those circumstances as a thick 

 skin. Local thickenings of the epidermis often occur in amphibia, for 

 instance to form the horny teeth by which the larva feeds. Such thick- 

 enings are also a conspicuous feature of the warty skin of the toads 

 (Bufo), which mostly have a drier skin and are more fully terrestrial 

 than are the frogs. The fact that the epidermis of amphibia can pro- 

 duce local thickenings is of interest in considering the origin of 

 feathers and hairs. In larval amphibians the skin is ciliated. 



