2628 FROGS AND TOADS 



first process is the sprouting forth of branching external gills from the sides of the 

 neck, which in the larvae of the frogs and toads are subsequently replaced by 

 internal gills, but in the long-tailed forms persist for a longer period. After the 

 disappearance of the external gills, the water is expelled from the gill chamber by 

 one or two tubes, generally discharging by a single orifice, which may be situated 

 either on the lower surface of the body, or on the left side. As soon as the 

 external gills have made their appearance, development is concentrated on the tail, 

 and the absorption of the remainder of the yolk. The vertical fin-like expansion of 

 the tail rapidly increase, and the body becomes relatively smaller and more slender, 

 while the limbs begin to make their appearance as buds, although the date of 

 development of the front and hind pair varies in different groups. In the newts, 

 the front pair of limbs are the first to appear, in the frogs the reverse is the case. 

 In the latter the hind-limbs appear some considerable time before the front pair, 

 the fish-like tail persisting till the sprouting of these, when the change from a 

 herbivorous fish-like animal to one carnivorous and reptiliform begins. The jaws 

 are at first invested with horny teeth, and subsequently with horny sheaths, which 

 eventually disappear; while the tail gradually diminishes in size, and finally is lost. 

 It may be observed that no vertebrae are developed in the frog's tail; and that the 

 long spine in which the backbone of the adult terminates is an outgrowth from the 

 hindmost vertebra. Not less remarkable is the shortening of the intestinal canal, 

 as the creature changes its herbivorous for carnivorous habits. To trace in detail 

 the development of the soft parts would greatly exceed our limits of space. We 

 may mention, however, that in one group of the Tailed Amphibians the external 

 gills of some individuals may be retained permanently, while in others of the same 

 species they are cast at an early period. Then again, the number of these gills is 

 by no means constant, for in the Cingalese caecilian and the salamander there are 

 three pairs of these organs, in the tadpoles of some frogs there are two, and in 

 others, as well as in one genus of csecilians, there are only a single pair. 



Geologically the Amphibians are a very ancient group, their oldest 

 Distribution / . 



representatives occurring in the Carboniferous and Permian rocks of 



Europe and North America. All these ancient representatives of the class belong, 

 however, to the group of L,abyrinthodonts, which survived till the period of the 

 Trias, and are structurally very different from the modern forms, approximating in 

 certain respects to fishes. Indeed, since no Amphibians have hitherto been discov- 

 ered between the Trias and the Wealden, or lower Cretaceous, rocks of Belgium, we 

 are quite unable to assert that the modern representatives of the class are the direct 

 descendants of the Labyrinthodonts. Commencing in the Belgian Wealden, the 

 newts and salamanders occur throughout the greater part of the Tertiary rocks; but 

 the frogs and toads are first known in North America from Eocene beds; while in 

 Europe they are not met with before the Oligocene. 



At the present time Amphibians are distributed over all parts of the world 

 except the polar regions; although they are more dependent upon the presence of 

 water and warmth than any of the preceding classes of Vertebrates. They are, 

 accordingly, most abundant in the tropical and subtropical regions; and as none of 

 them are marine in their habits, even a narrow arm of the sea is generally sufficient 



