Page:Natural History, Mollusca.djvu/107

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roof formed of moss or leaves, or a small quantity of earth, brought there by its motions. When it has succeeded in bringing the aperture of the shell to nearly a horizontal position, it stops. The foot is soon contracted within the shell; the snail then expands, so as completely to cover it, the collar of the mantle, which is at this period very white; and then inspires a quantity of air, after which it closes the respiratory hole. When this is done, a fine transparent membrane is formed with its mucus, and interposed between the mantle and any extraneous substances lying above. The mantle then secretes a quantity of very white fluid over its whole surface, which sets uniformly, like plaster of Paris, instantly forming a continuous covering about half a line thick. When this is hardened, the animal separates its mantle from it by another and stronger mucous secretion; and after a few hours, expelling a portion of the air it had previously inspired, it is enabled to shrink a little farther into the shell. It now forms another lamina of mucus, expires more air, and thus retires farther into the shell. In this way sometimes a fourth, fifth, and even sixth partition are formed, with intermediate cells, filled with air.

Such is M. Gaspard's account; but Mr. Bell remarks that it does not completely explain the manner in which the excavation is formed. "It is not by the pressure of the foot," says the last-named zoologist, "and the turning round of the shell, that this is principally effected. A large quantity of very viscid mucus is secreted on the under surface of the foot, to which a layer of earth or dead leaves adheres: this is turned on