MOLLUSCA. 
Mollusea. thus proving it to have exuded from the body of the animal. 
—\——~ When a portion of the mouth of the shell of a snail is bro- 
325 
responding increase of the dimensions of the contained ani- Mollusea. 
mal, or cavity for its reception. a aid 
ken off, and a piece of skin glued to the inner margin, re- 
flected outwardly, and fixed on the body of the shell, the 
defective part is again supplied, and the matter added to the 
inner surface of the skin, thus leaving the interposed sub- 
stance between the new formed portion and the fractured 
edge. Similar experiments, repeated on a variety of shells, 
both univalve and bivalve, by different naturalists, leave no 
room to doubt that shells increase in size by the juxtaposi- 
tion of shelly matter from the common integuments. 
Each caleareous layer is more or less enveloped in the 
animal matter which we have already stated as being pre- 
sent ; so that the different layers of successive growth may, 
by various processes, be distinctly exhibited. If the shell 
has been exposed for a short time in the fire, the animal 
matter will appear charred, and its black colour, contrasted 
with the white earthy matter, will indicate the arrangement 
of the different strata; in the same manner as the ivory and 
enamel of a tooth can be distinguished, when subjected to 
similar treatment. The same satisfactory results may be ob- 
tained by a different process. If the shell be steeped in 
weak muriatic acid, the earthy matter will be dissolved, and 
the flakes of albumen will remain as the frame-work of the 
edifice. 
The layers of growth may often be distinguished on the 
surface of the shell, in the form of striz or ridges, more or 
less elevated, but parallel to the margin of the aperture. 
Other inequalities may likewise be observed on the surface, 
at right angles to the layers of growth, such as ridges, 
knobs, and spines. These last derive their origin from the 
inequalities of the skin on which they have been moulded. 
In some univalve shells, the layers of growth parallel to 
the opening cannot be discerned; when exposed in the fire, 
there is little darkening of colour; and when dissolved in 
acids, but a feeble trace of animal matter remains. Jn the 
fire, these shells crack in various directions, but exhibit ne 
trace of ascaly structure. By careful management with the 
file, the shell may be separated into a central layer contigu- 
ous to the skin, and a peripheral layer, both similar in struc- 
ture, though frequently differing in colour. The shells 
exhibiting such characters have been termed porcellaneous, 
from their dense structure, and the fine polish which their 
surface presents. The formation of shells of this kind must 
take place ina different manner from those of the first kind 
which we have noticed. 
If we attend to the form of a young shell belonging to the 
genus cyprea of Linnzus, we may perceive that an addition 
of shelly matter to the margin of the aperture, in the man- 
ner in which it is applied in other shells, would not enlarge 
the cavity, but completely close the aperture. The increase 
of the shell, (accompanied with a corresponding increase of 
its inhabitant), must take place, therefore, either by ab- 
sorption of the accumulated shelly matter of the mouth, and 
an elongation in the direction of the greatest curvature of 
the shell; or the old shell must be thrown off, and a new one 
produced, suited to the size of the animal. The former sup- 
position has not been entertained, the latter is now generally 
received by naturalists, though there are a few individuals 
unwilling to adopt such a view of the matter. The inner 
coat of such shells appears to be a transudation from the 
body of the animal, and the outer one to be laid on the sur- 
face by the loose reflected lobes of the cloak. In many other 
shells, portions of matter, more compact than the other parts, 
may be observed spread on the pillar, and applied to the 
margin of the mouth by a similar process. Mr. Platt, in 
support of Reaumur’s opinion, that shells are formed by jux- 
taposition, against the objections of Mr. Poupart, (Phil. 
Trans. vol. liy. p. 43), erroneously considers the different 
sizes of the cypreze as depending on the thickness of the 
shell increasing according to age, without admitting a cor- 
The shells of the first kind which we have noticed, from 
being formed of cones or layers applied to the inner edge of 
the margin, and extending beyond it, have an imbricated 
structure. Those of the second kind, consisting of layers 
regularly superimposed, have consequently a laminated struc- 
ture; but between the two kinds there are numerous inter- 
mediate links, formed by a combination of the two processes. 
In some cases, the hard parts of the skin are not entitled 
to the appellation of shell, but may rather be considered as 
horn. Such are the coverings of the mandibles of the cut- 
tle fish, the branchial lid of the aplysia, and the operculum 
of the welk. The two last appendices, however, though 
horny in some species, are shelly in others. 
«The position of the shell, with respect to the constituent 
layers of the integuments, exhibits very remarkable differ- 
ences. In some it appears instead of a cuticle, or at least 
without an external membrane investing it. In general, 
however, it occurs-between the cuticle and the skin; a po- 
sition which induced Cuvier (Lec. d’ Anat. Comp. xiv. 11.) to 
consider it as analogous to the mucous web of the vertebral 
animals. Its intimate connection with the muscular sys- 
tem of the animal, and the protection which it affords, seem 
adverse to such a conclusion. In many species the testa- 
ceous substance occurs in folds of the corium, or inserted 
in its substance. In this position it never acquires the so- 
lid texture which shells exposed, or covered only by the 
cuticle, exhibit. Those shells which are thus concealed 
are in general white; those which are more exposed are 
frequently coloured. The colouring, however, does not de- 
pend on the direct exposure to the light, as some have im- 
agined, for many shells which are destitute of a cuticle are 
white, while many of those covered with a dense cuticle are 
finely variegated beneath, 
Between the skin and the shell neither vessels nor nerves 
have been traced; and the manner in which the latter is 
formed, forbids us to expect their existence. Yet the shell 
cannot be considered as dead matter, so long as it remains 
in connection with the living animal. In those animals in 
which the shell is external, there are muscles which connect 
the animal with its internal surface, and the bond of union 
being a substance soluble in water, the muscle can be de- 
tached by maceration. The analogy between shell and 
bone is here obvious, although in the one case the connec- 
tion between the muscle and the bone is permanent, in the 
other, between the muscle and shell, temporary, or frequent- 
ly changed during the life of the animal, But the vitality 
of the shell, if I may use the expression, is demonstrated, 
from the changes which it undergoes when detached from 
the animal: the plates of animal matter harden: the epi- 
dermis dries, cracks, and falls off; and in many cases the 
colours fade or disappear. We confess ourselves unable to 
point out the means employed by the animal to prevent 
these changes from taking place, by any process similar to 
circulation. The difficulty, however, is felt in contem- 
plating the functions of hairs, nails, and feathers, in verte- 
brated animals. 
When the shelly covering consists of two or more pieces, 
they are joined together, as the articulated bones in the 
higher classes of animals, by ligaments. These, in some 
cases, are of great thickness and strength, and, in con- 
sequence of their elasticity, assist in the motion of the dif- 
ferent parts. 
In the molluscous animals the skin secretes a viscous, 
adhesive substance, differing according to the medium in 
which the animal resides, but in all cases calculated to re- 
sist its influence. It is probably owing to the lubricating 
agency of this secretion, that both the cuticle and shell are 
preserved from decomposition. The skin likewise secretes 
the colouring matter by which the shells are variegated. 
