The Liverpool Standard and General Commercial Advertiser.
Liverpool, Merseyside, England, 1838-08-24 00:00:00

MECHANICS' INSTITUTION.

LECTURES ON THE STRUCTURE AND HISTORY OF THE
ARTICULATED 8j MOLLUSCOUS CLASSES OF ANIMALS.

We continue Dr. Grant's lecture of Wednesday week from
our last publication.
The crustaceous animals are furnished with masticating
organs, by which they may seize their food whilst swimming
or walking. Their strong mandibles are provided with contri-
vances similar to those of the large polyopterous insects. The
stomach also is near the head of the animal, and is provided
with strong teeth. There are four solid lumps in their sto-
mach to give additional firmness to the teeth, so that a proper
grinding action may be carried on. The crustaceous animals

are welf provided with a liver. When we examine it we End
it to be full of little tubes, being in fact composed of a myriad
of them. As the food is intended to undergo masticat►on,
the stomach is the best place into which to pour the liquid in-
tended to assist digestion. The liver, accordingly. pours its
secretion into it. The blood is conveyed through the respira-
tory organs into the heart, from which it is propelled forwards
and backwards throughout the whole system. As we ascend
upwards in the scale of animals, we find that fewer functions
are given to each organ, and the reverse is the case as we de-
scend to the inferior animals. No less singular than the
power by which the arachnidm and crustaceous animals have
of reproduction is that of the planaria, for, if they are cut in
two, each part will reproduce a portion of its body correspond-
ing to that part which has been separated, and in this manner
the creature will go on multiplying as often as we cut it
asunder.

The crustaceous animals are the spiders and the scorpions
of the deep, for they are organised to consume the living and
the dead—the putrid and the fresh. They afford us a whole-
some nutritious food, yet most of them we know to subsist
upon putrid animal matter. Putrid carcases of fish we often
find surrounded by a host of crustaceous animals which are
partly devouring each other, and partly the fiocculi from the
decayed carcases of the fish. They are the very scavengers
of the deep. They do not keep to the bottom of the ocean,
for every stratum of it teems with crustaceous animals up to
its highest surface. Even the very spray itself which is dash-
ed up by the paddles of our steam-packets has been found to
abound with them.

On Saturday evening, Dr. Grant delivered his sixth lec-
ture upon the molluscous animals, previous to the commence-
ment of which he took a rapid survey of the remarks which
be had made in the preceding lectures upon the articulated
animals.

The moluscous tribe of animals is almost exclusively con-
fined to the ocean and its precincts, which are peopled with
one thousand times more beings than the other portions of
the globe. They are generally attached to rocks at the bottom
of the sea, and consequently depend upon the ocean to bring
their food to them. 1 hey are formed to digest the lowest
forms of organic substances which exist in the sea. The de-
velopment of their nervous system is very remarkable. Their
spinal cord is disposed as a ring around the oesophagus, and
on this ring we can commonly perceive distinct ganglia.
They are therefore called cyclo-gangliated beings. There
are classes in which the visual organs are equivocal, and
others which present no traces of them. This is because it
would be painful for them to see approaching danger, which
they could not avoid by any muscular energy. They have a
complicated digestive apparatus. We find here, for the first
time, as we ascend in the scale of creation frem the monads,
an auricle united with a ventricle to assist the action of the
heart. In the lower tribe of the molluscous animals there are
some which are called acephalous. They have a short sto-
mach, but no head. The term molluscous has been given to
this tribe of animals on account of the softness of their bodies,
for when the exterior shell is removed there appears to be no
internal skeleton such as we see in the lobster, scorpion, and
other animals. These acephalous or headless molluscs com-
prise two very different groups of animals, of which the one
have their shells united with a valve, and the other have a
tough. exterior cutaneous covering, in which are two open-
ings, the one for inward current and the other for an outward.
There are a number of different forms of this tunicated
animal. We sometimes find an immense number of them
aggregated together, of a bright red or a black colour. They
cause the currents to pass through their bodies by means of
minute vibratile cilia, and thus are enabled to intercept the
nutritious matters floating along and use them as food. The
bivalved shell fish are called conclifera, on account of there
being two pieces of shell upon their back. This shelly co-
vering is composed of carbonate of lime—is extravascular
and laminated. The carbonate of lime is united with
animal matter, which gives it toughness ; for when it is
deprived of this by means of acids or heat, it presents the
appearance of burnt lime, and is used as burnt lime for the
purpose of manuring the land, and to serve as mortar. The
laminae are always added from within, and form a continued
piece over the exterior of the body. The ligament is a very
important part. Its natural state is one-half open; when the
shells are closed the ligament is on the stretch. When the
animal becomes languid the valve opens, and it always dies
with the valve opened. It is not in the power of the animal
to will that condition which is most fit for respiration, but
that which is most unfit for the condition on which life de-
pends. These conchiferous creatures are important in the
economy of nature. They serve as the components of great
calcaverous rocks, which sometimes appear to be formed of
one description of shell, as if there had been myriads of these
creatures in one• locality. Sometimes the upheaving action
from balow has raised these shells in a vertical direction.
Their situation plainly indicates to the geologist or zoologist
that there must have been a great disturbing force to dislo-
cate them. Nay, in some situations, the volcanic heat has
been so intense as to dissolve the shells and convert them
into a beautiful limestone, presenting no trace of their origi-
nal form. We see, therefore, that there may have been vast
deposits of shells, which have now assumed the highly crys-
talline character of primitive limestone. A celebrated
French philosopher has attempted to prove that all the lime-
stones which compose the crust of our globe are due to
animal life. During the life of the animal the laminae are
firmly united together. They are soft upon their first forma-
tion, and afterwards crystallize. The oyster lies upon its
left side and keeps its right side free. This it can only do in
the young state, or by an extraordinary offort when it is old.
Those oysters, which contain the pearls, inhabit deep waters,
and are found at depths varying from 10 to 30 fathoms. The
divers who risk their lives in the eastern climes, where they
are produced, descend these depth in quest of them from 40
to 50 times a day. This they are enabled to do by discharg-
ing from their lungs every portion of carbon. A single
oyster sometimes contains so many as 200 pearls. These
pearls are but morbid excrescences formed by the mantle in
the interior of the shell. When particles of sand gain
admittance between the surface of the mantle and the shell,
they are painful sourses of irritation to the animal. It cannot
push them out; and how is it to get rid of them? The animal
is stimulated to a more rapid secretion of calcareous matter,
and it coats over the intruders with the same substance which
covers the interior of its shell. There are certain worms
likewise which bore into their shells for the purpose of con-
cealment, as well as for the animal matter which they con-
tain. The animal repairs these apertures, and, in doing so,
forms round nodules, which are struck off, and highly es-
teemed by those engaged in the pearl fishery. The oyster,
being fixed to a rock, has no eyes; but the clam, which can
move itself by means of its shells, is furnished with organs
of vision. Many animals are fixed to the rock by means of
a byssus, or beard. In the muscle this is so extremely tough
and indigestible, as to occasion some inconvenience in eat-
ing that animal. We sometimes see that the inhabitants of
these bivalve shells have two auricles and a ventricle. This
arises from the animal being divided into two portions. Not
only is the auricle divided into two parts, each of which has
the same functions, but it suits the form of some of the inha-
bitants of the bivalve shells to have the ventricle likewise
similarly divided. From the earliest periods of history, we
find this tribe of animals to have been esteemed on account
of the pearls they contain, and our island was once tributary
to ancient Rome to a yearly amount of these precious orna-
ments. At the present day, the shells are used to decorate
the persons of our sable brethren, and manufactured in this
kingdom into an infinite variety of forms for ornamental and
useful purposes.