12 
PROCEEDINGS OF THE PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
gold is exceedingly beautiful. The pearls, of which 
we shall have more to say presently, consist of identically 
the same material as this layer. Covering the mother-of- 
pearl is a thick coating of brown horny material called the 
epidermis; while over that is a delicate skin, termed the 
periostracum. These two last layers not only protect the 
limey part of the shell from being worn away by 
friction, but protect it from the decomposing action 
of the carbonic acid and other gases contained in 
the water. An examination of the inner surface of the 
shell gives us a clue to the mechanism by which the animal 
is enabled to open and close it. Along the hinge-line of 
each valve will be observed a groove or channel. When 
the valves are brought together these form a small 
chamber, which is filled with an exceedingly tough and 
elastic ligament. The tendency of this ligament is to force 
the valves asunder, so that the natural position of the shell 
is to be about half-open, and in this position the creature 
generally keeps it when at ease. On being alarmed, it 
instantly closes its shell, which it does by contracting two 
powerful muscles, one of which extends between the two 
valves at either extremity of the shell. These are termed 
respectively the anterior and posterior adductor muscles, 
and the four scars which mark the points at which they 
are attached to the shell may be distinctly seen on exami¬ 
ning a fresh specimen. So long as the shell is kept closed, 
the ligament is being compressed; so that whenever the 
adductor muscles are relaxed, the shell springs open in 
consequence of the elasticity of the ligament. Judging 
from the analogy of such creatures as crabs and lobsters, 
and from the fact that so many empty shells are thrown 
up by the water, many persons suppose that the mussel and 
other molluscs cast off their shells periodically in order to 
form larger ones. This, however, is impossible, as the 
mussel, unlike the lobster, is inseparably and organically 
attached to its shell. If we take a mussel and look at the 
outside of the shell, we shall see that each valve is marked 
with a series of lines concentric with the outline of the 
shell. Each of these “ lines of growth,” as they are called, 
represents the outline of the shell at a certain stage of its 
growth, and thus we find that the shell increases in size by 
being added to at its outer margin. If we now open the 
mussel, we shall find that the creature is completely 
enveloped in a thin membrane, consisting of two halves or 
“ lobes,” one of which lines the interior of each valve of 
the shell, and corresponds to it in shape. We shall also 
observe that this membrane—or “mantle,” as it is called 
—is considerably thickened round its margin, and that 
there is a scar on the inner surface of each valve correspon¬ 
ding to, and caused by, this thickened margin. It is by 
means of this mantle that the mussel builds up its shell—a 
process which it is continually, though vary slowly, 
carrying on. The manner in which it secretes lime, homy 
material, &c., from the water, and constructs its Bhell out 
of these by means of its mantle, is one of the many secrets 
which Man has tried in vain to wrest from Nature. 
The fact of each mollusc constructing its shell after the 
same pattern is another of Nature’s mysteries which we 
can admire and wonder at, but cannot explain. We now 
turn from the shell to consider the creature which it con¬ 
tains, and which, if less attractive to the general observer, 
is equally interesting to the naturalist. If we take a spe¬ 
cimen from the river, and lay open the shell by passing a 
sharp knife between the valves in order to sever the ad¬ 
ductor muscles, we shall see a tongue-like process lying in 
the centre of the fleshy mass displayed. This is termed 
the “ foot,” and although it scarcely corresponds with 
what people generally conceive a foot to be, yet the term 
will be found to be appropriate enough if we watch a live 
mussel, and observe it gradually pushing its way from 
stone to stone, or burying itself in the mud by the aid of 
this organ. At the base of the foot, and in the vioinity of 
the hinge-line of the shell, lie the digestive system and the 
heart, which are intimately associated with one another. 
The digestive track commences with a mouth which is 
situated near the front adductor muscle, and which is pro¬ 
vided with no teeth, but is guarded by four soft leaf-like 
processes, which perform the function of the lips of higher 
organisms. The intestine passes through the ventricle 
of the heart. The heart consists of a ventricle and 
two auricles, the whole being enclosed in a delicate 
covering or pericardium. The purified blood—for mussels, 
and oysters too, possess blood—is a clear colourless fluid, 
and is received from the gills into the auricles, from 
whence it is passed into the ventricle, which, by its pulsa¬ 
tions, propels it through the blood-vessels. The gills are 
four in number,—two occurring on either side of the foot, 
and lining the lobes of the mantle in the form of delicate 
plate-like membranes, which, from their peculiar form, 
give the name of Lamellibranchiata, or “ plate-gilled,” to 
this class of molluscs. The inner surfaces of these gills, 
when examined under the microscope, are seen to be 
covered with exceedingly delicate filaments, termed “cilia,’ 5 
which are constantly and rapidly vibrating backwards and 
forwards. The object of these is to create a current of 
water through the gill-chamber, which brings with it a 
supply of fresh oxygen to purify the blood. On the 
surface of each gill is a net-work of minute blood-vessels, 
and by this means a large quantity of the blood is exposed 
at one time to the influence of the oxygen. The nervous 
