Page:Natural History, Mollusca.djvu/31

Rh by a shell, resembling in appearance that of a Snail, but tinged with blue (Ianthina), which is furnished with an elaborate apparatus for swimming,—

To the hinder part of the foot is attached a kind of float, consisting of many small bladders of thin membrane, united in a group, and looking somewhat like coarse froth. By means of these, the shell floats securely on the broad sea.

Another bladder-swimmer, and like the former, an ocean-species, is the Litiopa. "This is a small snail, born amid the gulf-weed, where it is destined to pass the whole of its life. The foot, though rather narrow and short, is of the usual character, and, having no extra hold, the snail is apt to be swept off its weed; but the accident is provided against, for the creature, like a spider, spins a thread of the viscous fluid that exudes from the foot, to check its downward fall, and enable it to regain the pristine site. But suppose the shock has severed their connexion, or that the Litiopa finds it necessary to remove, from a deficiency of food, to a richer pasture, the thread is still made available to recovery or removal. In its fall, accidental or purposed, an air-bubble is emitted, probably from the branchial cavity, which rises slowly through the water, and as the snail has enveloped it with his slime, this is drawn out into threads as the bubble ascends; and now, having a buoy and bladder whereon to climb to the surface, it waits suspended until that bubble comes into contact with the weeds that everywhere float around."