326 
N 
MOLLUSCA. 
Tollusca. The glands from which it proceeds vary much in different 
= — individuals, and even in the same individual in different pe- 
Muscular 
riods of growth. 
The characters furnished by the skin and its appendices 
are extensively employed in the systematical arrangement 
of molluscous animals. Nearly all those characters which 
distinguish the species, and many of those on which genera 
are established, are derived from the form of the shell, the 
tentacula, or the colour. This last character, however, is 
one on which little dependence should be placed. 
There is nothing peculiar inthe Muscutar Systemof this 
class of animals. Where the muscles are inserted in the 
skin, as is usually the case, that organ is in some cases 
strengthened by condensed cellular substance, and even ac- 
quires a leathery density. 
Molluscous animals preserve themselves in a state of rest, 
chiefly by suction and cementation. The organ which acts 
as a sucker, is in some cases simple, soft, and muscular, as 
the foot of the snail, while in others it is compound, and 
strengthened internally by hard parts, as in the arms of the 
cuttle fish. The force with which some animals adhere is 
very considerable, and is strikingly displayed, for example, 
when we attempt to detach a limpet from the rock. 
The rest, which is maintained by cementation, in some 
cases depends on a glairy secretion, which glues the body 
of the animal to the substance to which it is disposed to be 
attached. By such an expedient, the shells of snails adhere 
to rocks, stones, and plants. It is probable that the bivalve 
shells of the genus Cyclas, which readily adhere to the side 
of a glass, secure their temporary attachment by means of 
their glutinous cuticle. In other animals threads are pro- 
duced, termed a byssus, from particular glands, and while 
one extremity is glued to the rock, the other remains in 
connection with the animal. But there is an attachment 
more durable than any of these, which takes place in some 
shells, they being cemented to rocks or stones by calcare- 
ous matter, and retained in the same position during the 
whole term of their existence. 
The locomotive powers of the mollusca are confined to 
creeping and swimming. ‘The former action is performed 
by alternate contraction and relaxation of the foot, or mus- 
cular expansion, which serves as a sucker, and is analogous 
to the motion of serpents. The motion of swimming is 
executed either by the serpentine undulations of the foot 
and the body, or by the action of tentacula, or expanded 
portions of the integuments. Many species are aided in 
swimming, by being able to vary the specific gravity of their 
body at pleasure, and either rise or sink in the water as 
circumstances may require. In some, as the Janthina, there 
is a cellular organ peculiarly destined for this purpose, which 
may be regarded as in some measure analogous to the air- 
bladder of fishes. In all these exertions the progress of mol- 
luscous animals is proverbially slow. Some bivalve shells 
have the power of leaping or shifting their position by a 
sudden jerk, produced by shutting the valves rapidly. This 
is strikingly displayed in the common Scallop, and is less 
perfectly exhibited in the river muscles. In a few instan- 
ces, especially among the slugs, a thread is formed of the 
viscous secretion of the skin, by which the animal is enabl- 
ed to suspend itself in the air from the branches of trees 
like a spider. 
Although the progressive motions of molluscous animals 
are comparatively slow, the other muscular actions are exe- 
cuted with ordinary rapidity. The irritability of some parts, 
as the tentacula and branchiz, is so great, that the protect- 
ing movements are executed almost instantaneously, and 
the organs are contracted or withdrawn into the body. But 
these rapid exertions are only called forth in the moments 
of danger. 
The characters furnished by the muscular system, are of 
great value in the discrimination of species, and in the con- 
struction of genera and higher divisions. They are inti- Mollusea. 
mately connected with the habits of the animal, and merit —=\—== 
the attentive examination of the philosophical naturalist. 
In the molluscous animals the Nervous System is less Nervous 
complicated in its structure than in the higher classes, and system. 
the brain is not restricted in its position to the head. The 
whole nervous system appears in the form of ganglia and 
filaments. The principal ganglion, or the one to which the 
term brain is usually applied, is seated above the gullet or 
entrance to the stomach. It sends out nerves to the parts 
about the mouth, the tentacula, and the eyes. It may be 
considered as analogous to the cerebrum of the vertebral 
animals. From this ganglion proceed two filaments, one 
on each side, which in their descent inclose the gullet, and 
unite underneath to form a second ganglion. From this 
last, which has been compared to the cerebellum, numerous 
filaments are likewise distributed to the parts around the 
mouth, and to the other regions of the body. These fila- 
ments in some cases again unite, and form subordinate gan- 
glia. In many cases the brain and ganglia are of areddish 
colour, and granulated structure, while the nerves which 
issue from them are white and uniform, as in the genus Ap- 
lysia. The covering of the first ganglion, which is analo- 
gous to the dura mater, does not adhere to it closely, but 
leaves a space filled with loose cellular matter. The tu~ 
nics of the nerves are equally detached; and as they can 
be inflated or injected readily, some have been led to sup- 
pose that the nerves were hollow, and others, that the tunics 
were the vessels of the lymphatic system. 
The organs of perception common to the higher classes 
of animals, do not all exist in an obvious - manner amongst 
the mollusea. The touch, that universal sense, is here dis- 
played in many cases with great delicacy; and the tenta- 
cula, and the other cuticular elongations which we have al- 
ready referred to, contribute to augment its resources. The 
sense of sight is by no means universally enjoyed by the in- 
habitants of this class. In a few species, the eye is con- 
structed on the plan of the same organ in the vertebral ani- 
mals. In general, however, it appears only as a black point, 
the peculiar functions of which can only be inferred from 
analogy. In many species there is no trace of an eye, con~ 
sequently they cannot possess that varied information which 
the others derive from that organ. Where eyes exist in 
this class, they are uniformly two in number. In one tribe 
only, namely the cuttle-fish, the rudiments of the organs of 
hearing have been detected. The organs adapted to smedl- 
ing cannot be exhibited, but the existence of the sense is 
demonstrated by the facility with which molluscous animals 
discover suitable feod, when placed within their reach. 
The sense of éas¢e appears to exist, but it is difficult to point 
ont the particular parts of the mouth fitted for its residence. 
As, however, particular articles of food are selected in pre: 
ference to others, it may be concluded from analogy that 
taste regulates the choice. 
In the classification of the mollusca, the characters fur- 
nished by the nervous system, from the difficulty of their 
detection and exhibition, have never come into use. But 
those furnished by the organs of preception are highly priz- 
ed. Of these, the eye is the most obvious and constant. 
It varies in position in different species; but, among indi- 
viduals of the same species its characters are constant. 
In the cutaneous, muscular, and nervous systems, traces 
of a general plan may be observed, according to which they 
have been constructed in the different tribes.. In the or- 
gans which remain to be considered, there is less uniformity 
of structure, each family almost, being constructed aecord- 
ing to a model of its own. 
The time when molluscous animals feed has not been Digestive 
Those which live in the water are system. 
carefully attended to. 
beyond the reach of accurate observation. Those that re- 
side on land usually shun.the light, and creep forth in the 
