Hogg, on the Water-SnaiL 
97 
dissolved out, with a brisk effervescence ; the basement mem- 
brane only remaining, as in the older shell. 
If the young animal be viewed under a power of 150 
diameters, the whole mass is sufficiently transparent, to show 
that the shell is an important part of the whole structure, and 
not " suspended in mucus but has a hardened and definite 
form long before it issues from the egg, or comes in contact 
with the external air, to produce any hardening effect upon it. 
Mr. Bowerbank has observed, that in the fully-formed shell 
the mode of effecting repairs in the periostracum, affords 
evidence of a high degree of vitality." As to the term extra- 
vital, I know not what it means ; and, I believe, no one who 
has bestowed care and attention in the investigation of the 
works of the Great Creator, will for one moment assume the 
smallest speck to be an extra-vital production, or addition. 
Indeed, it appears to me that it would be as reasonable to 
deny the vitality of bone, or the growth of the lower organized 
cartilage, as to deny it to the shell of the pectinibranchial and 
pulmonated Mollusks. 
Dr. Carpenter says : — 
" It may now, however, be stated as an ascertained fact, that shell 
always possesses a more or less distinct organic structure ; this being, in 
some instances, of the character of that of the epidermis of higher animals, 
but in others having more resemblance to that of the dermis, or true 
skin." 
From repeated examinations, I believe, with Mr. Bower- 
bank : — 
" That the structure of shell is analogous to bone in some respects, and 
is formed much in the same manner as in particular kinds of bony matter, 
by the deposition of carbonate of lime within the cells of the membranes, 
which enter into the composition of the shell, or by the aggregation and 
coalescence of the calcigerous cells when the membrane is very sparingly 
produced ; and that it is made up of three strata. Each stratum being 
formed of innumerable plates, composed of elongated cellular structure ; 
each plate consisting of a single series of cells parallel to each other. 
These plates of cellular structure are deposited alternately in contrary 
directions, so that each series of cells intersects the one beneath it, at 
nearly right angles." 
If to a portion of the periostracum a small quantity of very 
dilute acetic acid be added, to dissolve out the calcareous 
matter, and it be then viewed under a magnifying power of 
250 diameters, it will be seen to be composed of oval cyto- 
blasts, exhibiting distinct nuclei, beneath which will be found 
a fine membrane studded with minute spots, apparently the 
escaped contents of the cells. This membrane has a regular 
series of corrugations or folds arranged throughout its whole 
extent, which gives to the shell in certain positions an 
