350 MOLLUSCA, 
Mollusca. In the last work of the former naturalist, this method is de- gin of this oral disc, which is strengthened by a band of Mollusca. 
~~ parted from, and the six classes to which we have already muscular fibres, are placed the arms or tentacula. Beyond =——— 
referred, are constituted of equal rank, instead of being 
placed in subordination to the two primary divisions under 
which they can be suitably distributed. 
DIVISION I.—MOLLUSCA CEPHALA. 
~ Head distinct from the body, bearing the lips or jaws. 
The head, or the anterior part of the body on which 
Zoologists have bestowed that denomination, possesses more 
or less freedom of motion, and, on the dorsal aspect, sup- 
ports either tentacula or eyes, frequently both. The ani- 
mals of this division exhibit so many modifications of form 
and structure, in all the series of organs, that the positive 
characters which they possess in common are few in num- 
ber. They easily admit, therefore, of subdivision into in- 
ferior groups which exhibit well-marked characters of dis- 
tinction. Two of these groups occupy a primary rank, and 
admit of the others being included under them as subordi- 
nate sections. In the first of these, the animals are all in- 
habitants of the water, and perform their progressive mo- 
tion through that element by organs fitted for swimming. 
They are destitute of any ventral disc on which to crawl. 
In the second group, including animals which inhabit the 
land, as well as those which live in fresh water and in the 
sea, progressive motion is performed by means of crawling 
along the surface of objects, the body resting on a ventral 
disc termed a foot. 
Sect. 1—Natantra— Organs of progressive motion 
Jitted for swimming. 
The organs of motion are situate near the anterior ex- 
tremity of the body, and consist either of flexible tentacula 
or membranaceous expansions. All the species reside in 
the sea. They are nearly of the same specific gravity with 
the surrounding fluid in which they float about, having their 
motions in a great measure regulated by its changes. It is, 
however, probable, that, by means of some contractile 
movements, they are capable of varying their density, and 
of rising or sinking in the water. They swim slowly, even 
with their utmost efforts. The animals of this section be- 
long to the classes which Cuvier has termed Cephalopoda 
and Pteropoda. 
Crass I—CrryaLoropa. 
Fins in the form of tentacula, surrounding the mouth. 
The Cephalopoda, in reference to their external appear- 
ance, may be regarded as consisting of two parts ; the tunic 
or sac, which contains the viscera, and the head, surround- 
ed by the tentacula. The skin is usually mottled with 
minute coloured spots, the colour varying in intensity in differ- 
ent spots, and evenin different parts of the samespots. These 
are confined to a thin layer on the outer surface of the true 
skin. In a living state these spots change their colour in 
rapid succession as if a coloured fluid was expelled from 
them or replaced in variable quantity. The sac is, insome 
species, in the form of a purse, destitute of any appendages, 
while in others, it exhibits fin-like expansions. It varies 
considerably in its consistence : in some, it is strengthened 
on the back internally, by corneous ribs or testaceous plates, 
and in others, it is protected externally by spiral shells. In 
some species, it is connected with the head by an interven- 
ing space, which may be regarded as a neck, but in others, 
the tunic and head are continuous behind. In all, it ex- 
hibits, after death, great changes of colour. 
On the summit of the head there is a flattened disc, in 
the centre of which is seated the mouth. Round the mar- 
this circle of arms, in some species, there are situated two 
organs, larger in their dimensions than the arms, which may 
be denominated fee¢. Both the arms and feet are covered 
on their central aspect with numerous suckers, by which 
they are enabled to attach themselves to different bodies, 
and to seize their prey ; and in their axis, both a nerve and 
artery may be observed. These arms and feet are capable 
of being moved, at the will of the animal, in every direc- 
tion, and are the organs by which progressive motion is per- 
formed. In the space between the head and tunic in front, 
there is an opening or finel with a projecting aperture. 
This funnel opens into the cavity of the sac, and serves 
both to convey water to the gills, and to carry off the differ- 
ent excreted matters. P 
The brain in the Cephalopoda is contained in an irregu- 
lar hollow ring, in the cartilaginous border of the oral disk. 
This cartilage is thickest on the dorsal aspect, and contains 
the parts which have been denominated cerebrwmn and cere- 
bellum, the remaining part of the canal being occupied with 
the collar, which surrounds the esophagus. The nerves, 
which proceed directly from the brain to the parts which 
they are destined to influence, are few in number. From 
the cerebrum a few small nerves issue, which go to the 
mouth, and the base of the feet, while some proceed to 
form ganglia at the mouth, and others supply the feet. The 
cerebellum, besides furnishing the collar which encircles 
the gullet, contributes to the formation of the large ganglia 
which supply the arms, the optic and auditory nerves, 
those for the funnel, the tunic, and the viscera. From the 
size of the animals, the ganglia of the nerves are very. dis- 
tinetly displayed. The anastomosing branches of the nerves 
of the arms are likewise conspicuous. Each nerve, at the 
base of each arm, sends out two filaments, one to the nerve 
of the arm on each side. In this manner a chain of nerves 
is formed round the base of the arm, probably calculated to 
enable them to act more readily in concert. From the 
abundant distribution of nerves to the different parts, it ap- 
pears probable that the sense of touch exists in a tolerably 
perfect manner. ‘There is no proof of the development of 
organs for the display of the senses of smell and taste. 
The Cephalopoda are furnished with two eyes, one on 
each side of the head. The external membrane on the 
inner side, which may be compared to the sclerotiea, differs 
in many particulars from the covering of the same name in 
the eyes of thevertebral animals. While it surrounds the 
contents of the eye from the entrance of the optic nerve to 
the pupil, it is greatly separated from the choroides. Im- 
mediately within its cavity, there is a bag, with a peculiar 
membranaceous covering, which contains numerous glan- 
dular bedies, similar to the milt of fishes, by which the eye 
is supported, and which probably act as secreting organs 
(although M. Cuvier could not detect any excretory canals), 
and likewise an expansion or ganglion of the optic nerve. 
The concave or anterior surface embraces the choroides. 
This membrane, after enclosing the vitreous humour, forms 
a zone or diaphragm, which may be compared to the ciliary 
processes, with an aperture in the centre for the reception 
of the crystalline lens. The circular margin of this aperture 
is lodgedina circular groove of the lens, andintimately united 
with it, so that the lens is divided into two unequal hemis- 
pheres. Its central surface is coated, as in the higher 
classes of animals, with the coloured mucous pigment which 
has been denominated pigmentum nigrum. In the cepha- 
lopoda, however, it is of a purplish-red colour. 
The optic nerve, after entering the sclerotica, expands 
into a large ganglion, from the peripheral surface of which, 
issue numerous neryous filaments. These pierce the cho- 
roides by as many holes, and go to form, by their reunion, 
the retina. This important membrane extends to the cili- 
