4.08 : SUPPLEMENT 
almost completely, so as to be unable to move there, an im- 
possibility which is often augmented by a ridge of the stone, 
which fills the furrow, formed by the crooks of the two valves. 
When we add that these shells are often very smooth, and 
that one species is so thin as to be merely membranaceous, 
we are led to reject every idea of the possibility of motion, 
whether of rotation or vibration, by means of which these 
animals could file the stone to introduce themselves. We 
must, therefore, have recourse to the theory of a corrosive, or 
solvent fluid, which would act upon the stone, soften it, and 
convert it into a sort of liquid, which the movement of the 
foot of the animal would subsequently expel from the cavity. 
But what is the organ of the animal producing this sort of 
humour, and what is its nature? It is probable, according to 
the opinion of M. de Bellevue, that it is the foot, or abdominal 
appendage, which furnishes the greatest quantity. In the 
pholades, this part constantly passes the coquillaceous enve- 
lope. As to the nature of this liquid, the same observer is 
induced to think that it must be an acid strong enough to 
decompose the calcareous salt, of which the shell is formed, 
but not sufficiently so to attack the animal matter, which also 
enters into its composition. He has, in fact, observed that 
when the rupellarie, which pierce the stone in all directions, 
come to meet, they make an irregular wound, one in the shell 
of the other, but without destroying the membranaceous part. 
He has also observed that the pholades in their cavity are 
bathed in a sort of black slime, tolerably abundant, which 
even penetrates some distance into the substance of the stone, 
when the latter is somewhat soft. He has made the same 
observation on the other lithophagous mollusca, and even on 
certain worms which also lodge in stones. This black slime 
appears to be the result of the corrosive humour of the animal 
mingled with the earthy matter of the stone. M. Fleurian 
having, besides, observed that the pholades and modioli pos- 
