106 
PROCEEDINGS OP THE PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
original question of what a shell is. We may define it 
generally as a hard external covering peculiar in its 
structure to the sub-kingdom Mollusca, and serving at once 
as a support and a protection to the mollusk, or, as Tryon 
has expressed it, “A dwelling-place and a citadel.” There 
are two points that I would ask you particularly to bear in 
mind : first, that the shell is part of the animal—that is, 
that they are organically connected, and cannot be separated 
without causing the death of the animal; and, second, that 
at no stage of the animal’s development is the shell cast off 
and a new one formed, as is the case with the covering of 
the crab or lobster. The connection may be said to be 
partly a vital and partly a mechanical one. Thus there 
are muscles which extend from the animal and enter into 
the substance of the shell at certain points, and a frame¬ 
work of animal matter runs through the whole structure 
of the shell; but, on the other hand, only the parts of the 
shell most recently formed can be said to be possessed of 
vitality, and no part possesses in itself the power of repair. 
This is proved by the fact that if it is bored or broken in 
such a way as not to injure the animal, the break is not 
repaired; but if the animal is wounded at the same time, 
then a fresh secretion of shelly material takes place to 
repair the damage. The shell is present in embryonic 
form from the earliest stage in the mollusk’s history; but 
in the so-called naked forms, it is lost as soon as the 
creature emerges from the egg. It is probable that all 
mollusks, with the exception of the argonaut, possess a 
rudimentary shell when hatched. 
So much, then, for what a shell is. Let us look next at 
how it is made. The greater part of the body of a mollusk 
is enveloped in a loose fold of the skin, called the mantle, 
which is tolerably thin and transparent, except at its outer 
margin, where it is thickened into a kind of collar. This 
mantle is the part of the animal which builds up the shell. 
The precise manner in which it does this work is not fully 
understood, and what is known of the process is perhaps 
rather difficult to follow, but I shall make it as plain as I 
can. The margin of the mantle corresponds to the outer 
or growing edge of the shell, and its general surface lines 
its interior. The main structure of the shell is built up by 
the former, which adds layer after layer of material to the 
growing edge, while the general surface of the mantle adds 
its smooth inner lining, and thus serves to increase it in 
thickness. This thickened margin of the mantle consists 
of minute cells of a certain type, known as epithelia, 
which have the power of separating from the blood, as it 
passes through them in the course of its circulation, certain 
mineral as well as animal substances, of arranging these 
in definite form with mathematical exactness, and 
of adding the material thus elaborated to the growing 
margin of the shell. The structure is thus literally built 
up layer by layer, prism by prism. I said that animal 
as well as mineral matter entered into its composition. 
This is proved if we place a shell in an acid which will 
dissolve the latter but not the former. After it has lain in 
the acid for some time, all that remains is a delicate 
spongy framework of animal tissue, exactly corresponding 
to the shape of the shell. This may be considered as the 
foundation or basis of the structure on which the solid 
material is encrusted, as it were. I think we may there¬ 
fore look on the shell as a kind of duplicate of the mantle, 
gradually thrown off by itself in greatly attenuated 
form, but correspondingly strengthened by earthy particles 
deposited in intimate association throughout its tissues. 
When we find, further, that the mantle encloses the breath¬ 
ing chamber and heart, you will understand why it is 
that the shell is looked upon as specially a protection for 
these organs. The general outer surface of the mantle is 
found to consist of cylindrical cells, which have the power, 
as already stated, of forming layers of mother-of-pearl or 
nacre. Some mollusks, however, have the power of com¬ 
pletely enveloping the shell in the mantle, and of thus 
adding a smooth layer to the outer as well as to the inner 
surface. This is the case with the cowries and olives, whose 
shells many people fancy to be polished by artificial means, 
though in truth no human agency could produce such an 
exquisite gloss. 
Shells are almost invariably enveloped in a thin skin 
or covering of animal matter, known as the epider¬ 
mis. This is secreted by the margin of the mantle, 
and is formed at the same time as the main substance of 
the shell—not, as might be supposed, added afterwards, 
layer by layer. Thus it is always freshest on the parts of 
the shell last formed, and is frequently found to be entii ely 
worn off the older parts. It may be looked upon as an 
outer layer of the shell, in which the framework of animal 
tissue is not hardened by limey particles. In most marine 
species it is exceedingly thin, like a mere coating of varnish, 
but in those inhabiting brackish or fresh water it generally 
becomes thick and horny. Its use is to protect the mineral 
substances of the shell from the corroding action of the 
water, or rather of the carbonic acid contained in it. It is 
found to exist even on the internal shells of slugs; and in 
the cowries, just referred to, it occurs underneath the outer 
glossy coating. 
Passing from the question of how the shell is formed, we 
have next to ask what it is made of. Except in two com¬ 
paratively small groups, the hard parts of shells are com- 
l posed of carbonate of lime, together with a small percentage 
