MOLLUSOA : THEIB TON"GXJES. 55 



The mode in wMcli the tongue is used may be read- 

 ily seen by watching the actions of a reriwinkle in a 

 marine or a Pond-snail in a fresh-water aquarium. 

 When the conferra has begun to form a thin green 

 growth on the glass sides of the tank, the MuUusca are 

 incessantly engaged in feeding on it, and rasping it 

 away with this 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 instant, and then withdrawn ; its teeth glitter like 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 Trochus, 

 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 again the whole is withdrawn ; and this proceeds with 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 nothing 

 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 cutting down swathe after swathe as he marches 

 along. The latter comparison is more striking, for the 

 * Woodward's " MoUusca," 161. 



