ON MOLLUSCA. Ise 
The mollusca which live in mud, in sand, or even in argil- 
laceous earth, really operate so as to sink themselves there, in 
proportion as they augment in bulk; and it is evident that this 
is done mechanically, and by means of their foot. As for 
those which sojourn in hard substances, as in calcareous 
stones, madrepores, and shells, it has been thought that their 
successive sinking was owing to some corrosive juice or acid, 
which was capable of dissolving the calcareous stone. But, 
besides that this is any thing but proved, the fact observed 
by Olivi and Spallanzani of pholades in pieces of lava, and 
that of teredines in living wood, do not permit us to adopt 
this opinion. 
The terrestrial mollusca present, as may well be supposed, 
much fewer variations in the circumstances of their habitat. 
In general it is in humid places, and more or less aquatic, that 
most of them are to be found; but there are also some that 
seem more to court dry places, and exposed to the sun, as is 
the case with certain species of helix. 
Some persons have even believed that many species were 
fixed to soils of a peculiar mineralogical character; but that 
does not appear to be probable. 
What is most certain is, that the terrestrial mollusca, in 
countries where the continuance of some unfavourable circum- 
stance, such as cold or drought, forces them to suspend their 
vital activity, are obliged to withdraw themselves from it, and 
for this purpose they sink more or less into the earth, into the 
anfractuosities of bodies, and thus enter into a sort of torpor, 
analogous to that of the marmots and other animals. This is 
the reason why we find in some places, or rather in one and 
the same place, a great quantity of these animals, or of their 
spoils, which have probably accumulated there for a long 
series of years. 
The scientific study of the malacozoaria is even yet so little 
advanced, that we know but little concerning their total num- 
