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Mutations also occur in which part or whole of the upper side is white and unpigmented. Several such specimens are mentioned in the memoir by myself and Dr. MacMunn in the Phil. Trans. already cited, one being a Sole which was entirely white on the lower side, and also on the upper, which was pigmented only over the head region from the free edge of the operculum forwards.
As this Helix has a thick calcareous operculum I removed it, and when it had formed a new membranous one, I again immersed it for fourteen days in sea-water, and again it recovered and crawled away. Baron Aucapitaine has since tried similar experiments. He placed 100 land-shells, belonging to ten species, in a box pierced with holes, and immersed it for a fortnight in the sea.
Slowly and cautiously the horny operculum is pushed out, turned back, and hidden beneath a thick fleshy mantle, which spreads over half the shell. Two long tentacles appear upon its front, like the horns of an ox, and it begins to glide along upon its one huge foot. Had you seen it thus at first, you could not have believed it possible for so bulky a body to be retracted into so small a shell.
When feeding, enjoying itself or moving around, its long neck and body are stretched out before it, armed with its hard operculum, which is like an iron shield, or the end of a battering ram. The operculum fits the entrance to its shell like a trap door. As soon as any danger is near it pulls in its head, and slams itself shut with a loud noise.
Kotzebue's First Voyage, vol. iii. p. 222. The large claws or pincers of some of these crabs are most beautifully adapted, when drawn back, to form an operculum to the shell, nearly as perfect as the proper one originally belonging to the molluscous animal. I was assured, and as far as my observations went I found it so, that certain species of the hermit-crab always use certain species of shells.
Operculum none ? Animal unknown, but certainly floating, and probably pteropodous. This I infer from its habits, and from the analogy of the shell with Spirialis. It is translucent and of a brownish-white hue. Its aspect is that of a Turbo in miniature.
In this complicated tunnel the creature resides, presenting a lovely circular disc of glowing pink as its front door. A few inches beneath the water this operculum or lid is not unlike a pearl, but as you gaze upon it, it slips on one side, and five animated red rays appear, waving like automatic flag signals. Though well housed, it is almost as timorous as the coral polyps.
His last feat was worthy of a cannibal, for it was the savage act of devouring a fellow-Natica. You might suppose that in this case the trap-like operculum would afford an easy entrance to one familiar with its use; but, true to his secret system, the burglar broke in as before. How did he do this? Did he abrade the stone-work with flinty sand until a hole was worn?
Opacity is primarily manifested at the aperture of the infant mollusc where a seeming resemblance to an operculum forms, possibly for the protection of vital organs during nascency. This plaque of frail armour is, however, soon dismantled, and of course much more happens in the never-ceasing process than is revealed to the uninitiated.
In both the exterior surface of the buckler and of the operculum the tubercles are a good deal enveloped in the stone, which is of a consistency too hard to be removed without injuring what it overlies; but you will find them in the smaller cast which accompanies the others, and which, as shown by the thickness of the plate in the original, indicates their size and form in a large individual, very characteristically shown.