AMPHIBIA. 



255 



long ccimpresjud tail witli which it swims actively. It breathes liy 

 means of gills or branchia;, of which there are two sets, one external 

 and the other internal. At first theie 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 develo])rnent 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 



Fig. IS-I. — D(jvelopiiieiit uf tiic common Frcjg. a Tadjiole, viewed from above, showing 

 tlie exterujil braiicliiip (</) ; h Side view of a somewhat older specimen, showing the 

 fisli-lilie tail ; c Older specimen, in wliicii tlie hinrl-legs have made their apiieaiance ; 

 d Sjiecijnen in which all the limbs are pi-csent, but tlie tail has not been wholly 

 absorbed. (After Bell.) 



same division. Thus the transitory condition of the j'oung Frog, 

 in which it breathes by external brancliia", is to a certain extent 

 permanently represented by the permanent condition of a perenni- 

 branchiate Amphibian, such as the Protevf:. The stage at which the 

 external branchiiie have disappeared, but the tail is still present, and 

 the limbs are developed, is permanently leiiresented in the common 

 tailed Amphibians, such .-ts the Newts. 



The order Anovra comprises the three families of the Frogs, 

 Toads, and Surinam Toads. The Frogs {Hmiiihr) are distinguished 

 by having a tongue wdiich is fixed to the front of the mouth, and 

 can be protruded at will, while tlie upper jaw is always armed with 



