xiii. 6 MODERN AMPHIBIA 365 



the still more modified forms, such as Necturus, external gills are 

 present and the lung is so reduced that the animals can live walking 

 along the bottom. Proteus from European caves is a blind urodele 

 with external gills and no pigment. Siren shows almost entirely larval 

 characteristics and has no hind limbs. 



The more typical terrestrial newts are of several sorts. In North 

 America the common genus Amby stoma (usually written Amblystoma) 

 has eleven species, many adapted to special habits, including A. mexi- 

 canum, in which some races become mature without metamorphosis, 

 because of lack of iodine in the water, whereas others, the axolotls of 

 Mexico, are genetically neotenous. 



The common British newt Triturus vulgaris is a typical example of 

 the more definitely terrestrial urodeles, though it is not able to live in 

 very dry situations. However, the limbs support much of the weight 

 of the body, and their soles are applied to the ground and turned 

 forwards. The tail shows various degrees of reduction to a circular 

 organ, but in the breeding-season, when both sexes return to the 

 water, it develops a large fin, especially in the male. The common 

 newts of America form a distinct family, including Plethodon and 

 many specialized forms, such as the blind Typhlomolge, inhabiting the 

 waters of caves. 



6. Frogs and Toads. Subclass Anura 



Among the frogs and toads are very many suited for special modes 

 of life, and it must again be emphasized that this is far from being a 

 static and precariously surviving group. We have already mentioned 

 the frog Ascaphns, which lives in mountain streams in the north-west 

 of the United States and has reduced lungs, showing a combination 

 of specialized and primitive features. Internal fertilization is assured 

 by a penis-like extension of the cloaca. In this genus and the New 

 Zealand Leiopelma (Fig. 210) there are several primitive features, 

 including tail muscles (absent in all other anurans), amphicoelous 

 vertebrae, free ribs, abdominal ribs, and persistent posterior cardinal 

 veins. 



In Alytes y the midwife toad of Europe, the male carries the eggs 

 wrapped round the legs. Pipa is a related and still more specialized 

 aquatic frog from South America; it has no tongue, and, curiously 

 enough, has developed an elaborate arrangement by which the young 

 are carried in pits on the back. Xenopus of Africa is related to Pipa, 

 but without the habit of carrying its young (Fig. 210). 



The bufonid toads are among the most successful of all amphibian 



