AMPHIBIA. 



255 



long compressed tail with which it swims actively. It breathes by 

 means uf gills or branchiae, of which there are two sets, one external 

 and the other internal. At first there are no limbs ; but, as devel- 

 opment proceeds, the limbs make their appearance — the hind-legs 

 first, and then the fore-legs. The tail, however (fig. 184), 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 is absorbed, 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 embiyonic stages of the higher 

 members of any division of the animal kingdom are often repre- 

 sented by the permanent condition of the lower members of the 



;. 184. — Development of tlie common Frog, a Tadpole, viewed from above, showing 

 tlie external brancliia' (f/) ; b Side view of a somewhat older speeimeu, sliowing the 

 fish-like tail ; c Older specimen, in which the hind-legs have made their appearance ; 

 d Specimen in which all the limbs are present, but the tail has not been wholly 

 absorbed. (After Bell.) 



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 permanent condition of a perenni- 

 branchiate Amphibian, such as the Proteus. The stage at which the 

 external branchiae 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 Anoura comprises the three families of the Frogs, 

 Toads, and Surinam Toads. The Frogs (Raiiidce) 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 



