48 EVENINGS AT THE MICROSCOPE. 



hooks, arching forward and inward, each notched with saw- 

 teeth, and diminishing in thickness as they recede from 

 the centre. 



The manner of using this elaborate organ is no less 

 curious than is its structure. During life it is only the 

 front portion not more than one-third of the band that 

 is in use ; this is spread out on the floor of the mouth, 

 with the teeth projecting and hooking backwards. The 

 remainder has its edges rolled over towards each other, 

 forming a tube closed at its extremity, which, as I have 



TONGUE OF TROCHUS. 



already observed, is coiled away (in the long-tongued 

 kinds) among the viscera. 



The mode in which the tongue is used may be readily 

 seen by watching the actions of a Periwinkle in a marine, 

 or a Pond-snail in a fresh-water aquarium. When the 

 conferva has begun to form a thin green growth on the 

 glass sides of the tank, the Mollusca are incessantly en- 

 gaged in feeding on it, and rasping it away with this toothed 

 band. " The upper lip with its mandible is raised ; the 

 lower lip expands ; the tongue is protruded, and applied to 



