232 VERTEBRATE ANIMALS. 



the limbs are fully developed, and the gills have given place 

 to lungs, the tail disappears, and the animal now takes to the 

 land as a perfect frog. 



The development of the Frog is a good illustration of the 

 general zoological law, that the transitory embryonic stages 

 of the higher members of any division of the animal kingdom 

 are often represented by the permanent condition of the lower 

 members of the same division. Thus the transitory condition 

 of the young Frog, in which it breathes by external branchiae, 

 is to a certain extent permanently represented by the perma- 

 nent condition of a perennibranchiate Amphibian, such as the 

 Proteus. The stage at which the external branchiae have dis- 

 appeared, but the tail is still present, and the limbs are de- 

 veloped, is permanently represented in the common tailed 

 Amphibians, such as the Newts. 



The order Anoura comprises the three families of the 

 Frogs, Toads, and Surinam Toads. The Frogs (Ranidaz) are 

 distinguished by having a tongue which is fixed to the front 

 of the mouth, and can be protruded at will, while the upper 

 jaw is always armed with teeth. The typical Frogs have 

 enormously-developed hind legs, the toes of which are united 

 by membrane, or are " webbed." They swim very power- 

 fully, and can take extensive leaps. The Tree-frogs (Fig. 

 114), on the other hand, are adapted for a wholly different 

 life, inhabiting trees, among which they climb with great ease 

 by the help of suckers developed upon the ends of the toes. 

 They are mostly found in warm countries, especially in Amer- 

 ica, but one species is European. 



In the equally familiar Toads (Bufonidce) the structure of 

 the tongue is the same as in the Frogs, but the jaws are not 

 furnished with teeth. In the Surinam Toads (Pipidce) there 

 is no tongue at all, and usually no teeth. 



ORDER IV. LABYRrNTHODONTiA. This, the last order of 

 the Amphibia, is not represented by any living forms, and re- 

 quires to be little more than mentioned. The Labyrinthodonts 

 were Amphibia which were mostly of large size, and of which 

 some must have obtained absolutely gigantic dimensions, the 

 skull of one species being three feet in length and two in 

 breadth. They were first known to science simply by their 

 footprints, which were found in certain Secondary sandstones 

 (Trias). These footprints consisted of a series of alternately 

 placed pairs of hand-shaped impressions, the hinder print of 

 each pair being much larger than the fore one. So like were 



