CLASS GASTEROPODA: ORDER PTEEOPODA. 249 



ter-gilled) have their respiratory organs in a nucleus, and 

 their foot modified into a sort of fin for swimming. 



The Carinaridse (keeled) have the more delicate organs 

 protected by a small shell fringed around the mouth with 

 gills, while the larger part of the animal is exposed. The 

 fin-like foot has a small sucker for attaching to sea-weed, 

 compelling them to swim with the back downward. 



ORDER PTEROPODA. 



General Characteristics. — The Pteropods (wing-footed) 

 move by means of two wing-like fins. The animals are 

 small, yet occur in such numbers as often to color the ocean 

 for miles. They are probably carnivorous, and, in turn, are 

 food for the whale. 



Mg. las. 



Cliidae. — The Clio is only an inch long, but its prehensive 

 apparatus is elaborate. Six labial tentacles bear about three 

 hundred and sixty thousand suckers for seizing and holding 

 its minute prey, which is speedily reduced by the laterally- 

 working jaws full of long, comb-like teeth, and by the tongue 

 rough with sharp, spiny, recurved hooklets. 



