MOLLUSCA : THEIR TONGUES. 55 



Tlie mode in wLicli tlie tongue is used may be read- 

 ily seen by watching tlie actions of a Periwinkle in a 

 marine or a Pond-snail in a fresli-water aqnarium. 

 "When the conferva has begun to form a thin green 

 growth on the glass sides of the tank, the Mullusca arc 

 incessantly engaged in feeding on it, and rasping it 

 away with tliis toothed ribbon. " 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 instant, and then withdrawn ; its teeth glitter like 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 flexible that 

 frequently it will catch against projecting points, and 

 be drawn out of shape slightly as it vibrates over the 

 surface." * 



Perhaps every variety is accompanied by some vari- 

 ation in food or manner of feeding. With the TrocJms^ 

 the proboscis, a tube with thick, fleshy walls, is rapidly 

 turned inside out to a certain extent, until a surface is 

 brought into contact with the glass, having a silky lus- 

 tre : this is the tongue ; it is moved with a short sweep, 

 and then tubular proboscis infolds its walls again ; the 

 tongue disapearing, and every filament of conferva be- 

 ing carried up into the interior from the little area 

 which had been swept. The next instant, the foot 

 meanwhile having made a small advance, the proboscis 

 unfolds again, the tongue makes another sweep, and 

 again the whole is withdrawn ; and this proceeds with 

 great regularity. I can compare the action to notlmi;- 

 so well as to the manner in which the tongue of an ox 

 licks up the grass of the field, or to the action of a 

 mower cuttiii"; down swathe after swathe as he marches 

 along. The latter comparison is more striking, for the 



» Woodward's " Mollusca." IGl. 



