MOLLUSCA : THEIR TONGUES. 49 



the surface for an instant, and then withdrawn ; its teeth 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

 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 lustre : this is 

 the tongue ; it is moved with a short sweep, and then the 

 tubular proboscis infolds its walls again ; the tongue dis- 

 appearing, and every filament of conferva being 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 another sweep, and again the whole is withdrawn ; and 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, on 

 account of the marks of progress which each operator leaves 

 behind him. Though the confervoid plants are swept off 

 by the tongue of the Mollusk, it is not done so cleanly but 

 that a mark is left where they grew; and the peculiar 

 form and structure of the tongue, which I have above 

 noticed, leave a series of successive curves all along the 

 course which the Mollusk has followed, very closely like 

 those which mark the individual swathes cut by the mower 

 in his course through the field. 



The Periwinkle's table-manners differ slightly from those 



of his relations. When he eats, he separates two little 



fleshy lips, and the glistening, glass-like tongue is seen, 



or rather the rounded extremity of a bend of it, rapidly 



* Woodward's " Mollnsca," 161. 



