8 



POPULAR CONCHOLOGY. 



foot for the purpose, contrive to advance with 

 considerable agility, by coiling it up tightly, 

 and suddenly relaxing it, by which means, 

 serving as a spring, it urges the body to some 

 distance. There are many species of Mollusca 

 which are unable to move along the rough 

 surface of the ground, but manage to ad- 

 vance rapidly enough along the smooth marine 

 plants, by a kind of sliding motion. Those 

 tribes which do not creep along the ground, 

 but move through the water, are obliged to 

 have recourse to various methods to enable 

 them to swim; some float along the surface, 

 borne as the winds and waves direct ; others, 

 by squirting out suddenly a stream of water, 

 which they have taken into their shells, send 

 themselves forward in an opposite direction; 

 and others swim, by using their foot as a fin, 

 and moving it from one side to the other. In 

 those Mollusca which affix themselves per- 

 manently to a submarine object, the method of 

 attachment varies considerably ; some, for in- 

 stance, are fixed by the shell, some by a con- 

 tinuation of the foot either in a mass, or divided 

 into separate fibres, some by a tube, and others 



