254 VERTEBRATE ANIMALS. 
presents itself as a little fish-like creature with a broad head, a 
sac-like belly, and a long compressed tail with which it swims 
actively, It breathes by means of gills or branchize, of which 
there are two sets, one external, and the other internal. At 
first there are no limbs; but, as development proceeds, the 
limbs make their appearance—the hind-legs first, and then the 
fore-legs. The tail, however (fig. 137), is still retained as an 
instrument of progression. Ultimately, when the limbs are fully 
developed, and the gills have given place to lungs, the tail disap- 
pears, 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 branchie, 
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 branchice have 
disappeared, but the tail is still present, and the limbs are 
developed, is permanently represented in the common tailed 
Amphibians, such as the Newts. 
The order Azoura comprises the three families of the Frogs, 
Toads, and Surinam Toads. The Frogs (Ranide) are distin- 
guished 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 powerfully, and can take 
extensive leaps. The Tree-frogs, 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 America, but one species is European. 
In the equally familiar Toads (Aufonzde) the structure of the 
tongue is the same as in the Frogs, but the jaws are not fur- 
nished with teeth. In the Surinam Toads (Pzfzd@) there is no 
tongue at all, and usually no teeth. 
ORDER IV. LABYRINTHODONTIA.—This, the last order of the 
Amphibia, is not represented by any living forms, and requires 
