52 ANIMAL LIFE PAST AND PRESENT. 



the eel, swim by lateral curvatures of the body, in the 

 so-called serpentine manner. 



The only other Vertebrate animals which breathe by 

 means of gills, and can therefore be regarded as primi- 

 tively aquatic, are the young, or larvas, of the Amphi- 

 bians (Frogs, &c.). The young Tadpole, as we all know, 

 is an ugly, large-headed creature, swimming by means 

 of lateral movements of its tail. This tail has a 

 vertical fin-like expansion, differing, however, from the 

 fins of fishes by the absence of the bony or cartilaginous 

 rays found in the latter. We have already alluded to 

 the remarkable metamorphosis undergone by the Tad- 

 pole, in the course of which the tail is lost, the gills are 

 replaced by lungs, and the limbs developed. The 

 adult Frog is an instance of an animal adapted to live 

 partly on land and partly in the water, swimming 

 powerfully in the latter element by the strokes of its 

 long hind legs, of which the toes are fully webbed. 

 The Tailed Amphibians, such as the Newts and Sala- 

 manders, are less specially modified than the Frogs, 

 and may be completely aquatic. All the Newts and 

 Salamanders, including the purely aquatic Giant Sala- 

 mander of Japan, lose, however, their gills in the adult 

 state ; but these are permanently retained in the 

 curious blind Proteus of the caverns of Carniola. 



Among the true reptiles of the present day (all of 

 which breathe by means of lungs during the whole of 

 their existence), there are three groups among which 

 aquatic forms occur. The first of these includes the 

 Crocodiles and Alligators, which swim by means of 



