64 EVENINGS AT THE MICROSCOPE 



First, a delicate glassy central tooth, tapering to a fine 

 point, and cut into minute saw-teeth along each edge. 

 Then a series, of five on each side, of similar glassy 

 pointed leaves, bending inward; and outside these, on 

 either hand, are a great number of stout dark-colored 

 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 cu- 

 rious than is its structure. During life it is only the front 

 portion not more than one-third of the ribbon that is in 

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

 teeth projecting and hooking backward. The remainder 

 has its edges rolled over toward each other, forming a 

 tube closed at its extremity, which, as I have already ob- 

 served, 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 con- 

 ferva has begun to form a thin green growth on the glass 

 sides of the tank, the Mollusca are incessantly engaged in 

 feeding on it, and rasping it away with this toothed rib- 

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

 lower lip expands; the tongue is protruded, and applied 

 to the surface for an instant, and then withdrawn; its teeth 

 glitter like glass-paper, and in the Pond-snail it is so flex- 

 ible that frequently it will catch against projecting points, 

 and be drawn out of shape slightly as it vibrates over the 

 surface. 1 ' 1 



Perhaps every variety is accompanied by some varia- 



1 Woodward's "Mollusca," 161. 



