SWIMMING ANIMALS. 53 



their long tail and limbs. Although these animals are 

 thoroughly at home in the water, where they spend a 

 large portion of their time, their organisation has not 

 been so modified for the exigencies of an aquatic life 

 as to depart to any great extent from the normal type. 

 The same remark will apply with still more force to 

 our Common Snake, which is an expert swimmer. In 

 the Sea- Snakes, however, which pass the whole of their 

 life in the tropical seas, the tail assumes a vertically 

 compressed and paddle-like form, and is thus nearly as 

 efficient a propeller as the tail of a fish. These snakes 

 always swim on the surface of the sea, but it is very 

 doubtful if they can have given rise to the stories of 

 the Sea-Serpent. 



Among the Chelonians the Marine Turtles have been 

 especially adapted for an aquatic life by the modification 

 of their limbs into oar-like paddles ; although it is 

 quite clear that this structure is an acquired one. The 

 Soft Turtles (Trionyces), of the rivers of the warmer 

 regions of the globe, are almost equally good swimmers, 

 although their feet, with the exception of being webbed, 

 retain the ordinary type of structure. The Pond- 

 Tortoise, now restricted to Southern Europe, although 

 occurring in the superficial deposits of this country, is 

 almost equally aquatic. Indeed all Tortoises (except 

 perhaps the gigantic ones of the Galapagos and 

 Mascarene Islands) are excellent swimmers, and thus 

 afford a good instance of how some members of a group 

 have gradually adapted themselves to an almost com- 

 pletely aquatic life. 



