32 THE MODE OP EATING. 



appears unable to bear constant submersion ; his habit 

 is to live a good deal exposed to the air, and even to 

 the hot sun, and this seems essential to his health. I 

 have found that if this little species be collected, pretty 

 as the individuals are, they crawl around the sides for' 

 a day or two, as if seeking a more genial dwelling, and 

 then one by one fall to the bottom and die. There 

 is, however, another genus of univalve mollusca which 

 may be made equally available with the Periwink, if 

 indeed it be not superior for the purpose. I allude 

 to those evenly conical shells, which belong to the 

 genus Trochus, sometimes called from their form. 

 Tops. Two species, T. cinerarius and T. umbilicatus, 

 are scarcely less abundant on our weedy shores than 

 the Periwinkles ; the former of a dull purplish grey, 

 marked with close-set zigzag lines; the latter rather 

 flatter, usually worn at the summit, of a dull olive or 

 green, with narrow reddish bands radiating from the 

 centre. Both are pearly in the interior, but the latter 

 species is brilliantly iridescent. 



These Tops and the common Periwink are very 

 useful inhabitants of a marine tank; they make them- 

 selves at home, and feed readily. It is interesting to 

 watch the business-like way in which they proceed ; 

 I have just been looking carefully at a Top doing his 

 work, watching the modus operandi with a pocket lens. 

 At very regular intervals, the proboscis, a tube with 

 thick fleshy walls, is rapidly turned inside out to a 

 certain extent, until a surface is brought into con- 

 tact 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 



