178 SUPPLEMENT 
even a part of the corneous, appear to adhere through all their 
internal or inferior surface, so as to leave nothing free but their 
circumference ; while the corneous opercula of all the ento- 
mostomata are fixed to the skin only by a small part of the 
same surface, at their base, and are free in all the rest. This 
may be perfectly well seen in murex, buccinum, purpura, &c. 
In the hemicyclostomata, the adherence to the foot is made by 
means of one or two processes of the anterior or right edge, 
and the operculum seems to be articulated with the internal 
edge of the shell. 
We must take care to distinguish the piece of the shelly 
envelope of which we have just spoken from the epiphragma, 
because, if there is any relation of the usage, which is completely 
to close the aperture of the shell, there is none of structure, 
nor even of position in relation to the animal. ‘The epiph- 
ragma, or temporary operculum, is, in fact, but an aggregation 
of dried calcareous molecules produced by the edges of the 
mantle, or the collar of certain species of helix, when they 
have completely withdrawn their head and foot within the 
mantle. The stratum, more or less thick, which results, ad- 
heres in no wise to the animal, and several may be succes- 
sively formed, in proportion as the unfavorable circumstances, 
such as cold, great drought, or absence of nourishment, which 
forced it to re-enter the shell, are more prolonged. 
After this digression, into which we have been obliged to 
enter, respecting the shell as a dependence on the skin, or 
seat of touch, we shall now proceed to the examination of the 
apparatus of this sense, and successively of the rest. 
The apparatus of the sense of touch in the mollusca con- 
sists in the tentacula, or the tentaculary cirrhi, with which 
the edges of the mantle may be furnished, and of which we 
have already spoken. We may also include certain tentacu- 
lary appendages, sometimes in the form of a fringed membrane, 
as in the Janthini, or even true flatted tentacula, as in certain 
a 
