SNAILS. 95 
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 
