206 



THE INFLUENCE OF INANIMATE SUKROUNDINGS. 



ground. In tlie 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, <fec., 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 fisli (Dors?) ; below a 

 Cachalot Whale ; to the right a Pteropod, Hyalca ; left, a Pteropodoas larva 

 (Crexisi). 



serve exclusively or chiefly to enable swimming animals to 

 mLOve in a determined direction are known by the general term 

 of fins. Notwithstanding the widest diflference in their structure, 

 and though they may have but small morphological correspondence 

 in difierent creatures — as will be understood, without any more 

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

 they have, without exception, certain peculiar characters, which 



