206 THE INFLUENCE OF INANIMATE SURROUNDINGS. 



ground. In the first place, in all swimmers, the whole body 

 serves as a means of motion ; snakes and eels or similar crea- 

 tures with very long bodies swim exclusively or in preference 

 by a wriggling motion, and even much shorter animals, as 

 bleak, &c., can swim without fins, but then their power of 

 directing their movements is greatly impaired. Such organs as 



FIG. 59. Various animals that swim by means of fins. Above a fish (Dors') ; below a 

 Cachalot Whale ; to the right a Fteropod, Hyulea ; left, a Pteropadous larva 

 (Creteiit), 



serve exclusively or chiefly to enable swimming animals to 

 move in a determined direction are known by the general term 

 of fins. Notwithstanding the widest difference in their structure, 

 and though they may have but small morphological correspondence 

 in different creatures as will be understood, without any more 

 detailed comparison, from the subjoined illustration (fig. 59) 

 they have, without exception, certain peculiar characters, which 



