Page:The aquarium - an unveiling of the wonders of the deep sea.djvu/59

32 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 themselves 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 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