MOLLUSCA. 
Mollusca. ary zone, and, like it, appears to unite itself with the groove 
>>’ of the lens. 
The vitreous humour is contained in a peculiar vesicle, 
having the lens seated in a concavity on its external sur- 
face. The lens divides easily into two parts, the line of 
separation being the groove which receives the ciliary liga- 
ment. The separated surfaces are flat, and the outer por- 
tion is in the form of a planoconvex lens. Each portion 
consists of a number of concentric layers of variable thick- 
ness, composed of radiated fibres, becoming less and less 
distinct towards the centre, near which the laminated and 
radiated appearances cease to be perceptible. An imper- 
fect representation of this structure is given by Sir E. Home, 
probably from preparations by Mr. John Hunter, in the 
Phil. Trans. vol. Ixxxiv. tab. 5. p. 26. 
The conjunctiva supplies the place of a cornea, and covers 
directly the crystalline lens, as there is no aqueous humour. 
This membrane, in some, is continuous with the skin, but 
in others, there are imperfect eye-lids formed by its dupli- 
cature, previous to passing over the lens. The skin, at the 
opening of the pupil, formed by the sclerotica, in the ab- 
sence of an wvea and iris, is strengthened by a membrane 
which appears to be muscular, and probably assists in the 
contraction or enlargement of the aperture. 
The animals of the cephalopodous class, besides contain- 
ing complicated eyes, are likewise furnished withears. These 
are situate in the annular cartilage which supports the arms. 
Tn this cartilage, there are two cavities, in each of which 
there is a bag filled with a gelatinous, transparent fluid, and 
containing a calcareous substance, differing in its consis- 
tence according to the species, from the brittleness of starch 
to the hardness of bone. The auditory nerve penetrates 
the walls of this labyrinth, and ramifies on the membran- 
ous bag which it contains. There is no external opening, 
nor any apparent alteration in the thickness of the invest- 
ing integuments. 
The digestive system of the Cephalopoda exhibits seve- 
ral appearances by which it may be distinguished. The 
arms which surround the mouth, seize the animals which 
are to serve as food, and bring them to the mouth. The 
mouth is situated in the centre of the disc, round which the 
tentacula are arranged. It is surrounded with a slight fold 
of the skin, which may be compared to lips, and which is 
rough on the central aspect. Within the lips are the two 
mandibles, of a deep brown colour, hard, horny consistence, 
and in form resembling the beak of a parrot. Where free, 
they are conico-tubular, but where covered, they are open 
at the central side. The under beak, unlike the same 
organ in birds, is the largest, the most crooked, and em- 
braces the upper, or the one on the dorsal margin of the 
mouth. These jaws are merely able to open and shut, as 
they possess no lateral motion. They are supported by the 
muscular bed of the mouth, which serves as a mould to fill 
the cavity towards the point. The tongue is situate be- 
tween the beaks, and is armed with reflected teeth. These 
teeth, in consequence of the undulatory motion of the sub- 
stance of the tongue, expedite the progress of the food into 
the gullet. 
The salivary glands are four in number, and are placed in 
pairs. The glands of the first pair, seated on each side of the 
muscular bed of the mouth, are divided into numerous lobes, 
the excretory ducts of which pour their fluidinto the beginning 
of the gullet. The second pair, seated lower down and be- 
low the eyes, are not so much divided, and send out separ- 
ate canals, which unite and pour their contents into the 
mouth. 
The gullet is furnished with a lateral expansion, not un- 
like the crop of gallinaceous birds. ‘The stomach is mus- 
cular, like the gizzard of fowls, and the cuticle is thick, and 
separates easily from the other membranes. At the pyloric 
opening of the stomach, there is another aperture equally 
large, which leads into the spéral stomach, or caecum, as it Mollusca. 
It may —\o~ 
has been improperly termed by some anatomists. 
with greater propriety be denominated the duodenum, as it 
performs some of the offices of that part of the gut in the 
higher orders of animals. This stomach is conical, closed 
at the distal extremity, and performs about a turn and a 
half, like a spiral shell. Its inner surface is covered with a 
ridge, which traverses it in a closely spiral direction. The 
bile flows into it near the apex, and towards its base glan- 
dular orifices, pouring out a thick, yellow fluid, may be ob- 
served. The intestine, after leaving the pylorus, n some 
species, makes one or two turns, in others, it proceeds di- 
rectly to the anus. This opening is seated at the base of 
the funnel, on its posterior or dorsal side. 
The “ver is of considerable size, of an orange-yellow co- 
lour, and of a soft and spongy texture. It gives rise to two 
hepatic ducts, which proceed to the extremity of the spiral 
stomach, where, by a common orifice, they empty the 
orange-coloured bile which they contain. 
The organs of circulation consist merely of veins and ar- 
teries. The veinswhich have their origin in the feet, mouth, 
and annular cartilage, coalesce, and form two branches, 
which afterwards unite into a common trunk. This vessel, 
after descending through part of the viscera into the abdo- 
men, divides into two branches, each of which may be con- 
sidered as a vena cava, conveying the blood to the lateral 
hearts. Each vena cava, at its origin, is joined by an equal- 
ly large vessel, which empties its contents in a direction 
nearly at right angles with the former. These veins arise 
in the stomach, intestines, liver, and organs of generation. 
The vena cava receives a second large vessel, nearly in the 
same direction as the first, which has its origin in the tunic 
and the supports of the branchiz. From the size of the 
vena cava, in consequence of the union of these two branches, 
and the appearance of muscular ridges on its inner surface, 
it has been compared by some to an auricle. 
On each side, in the common cavity of the tunic, and 
near the gills, an aperture may be observed, the entrance 
to a bag or cavity. Each cavity is traversed by the vena 
cava of that side, and in its passage exhibits a curious con- 
formation. The surface of the vein is covered with spongy, 
glandular bodies of different shapes. These, upon being 
pressed, pour out an opake, yellow, mucous fluid. Within, 
these glands communicate by very wide ducts with the ca- 
vity of the vein. Indeed, when air is blown into the vein, 
it readily passes through the glands into the bag, and thence 
into the cavity of the tunic; and when air is blown into 
the bag, it likewise penetrates the gland, and passes into 
the veins. The arteries with which these glands are fur- 
nished are comparatively minute. 
It appears probable that these glands separate some prin- 
ciple from the blood, and that this is conveyed away by the 
ejection of the water from these venous bags into the com- 
mon cavity. Were it practicable to analyse the yellow 
mucus which these glands contain, some light might be 
thrown on the subject. Indeed, it appears not improbable, 
that this arrangement is analogous in its functions to the 
urinary system in the most perfect classes. 
Each vena cava enters its corresponding lateral heart or 
ventricle, through an intervening valve. Each lateral heart 
is situate at the base of each gill, is pear-shaped, black, and 
moderately thick, with numerous pits on its inner surface. 
Its narrow end terminates without any valvular structure 
in the pulmonary artery. In the genus octopus, the lateral 
hearts are naked ; but in the genera Loligo and Sepia, there 
is suspended from each, by a slender footstalk, a spongy 
round body, which is concave beneath. The footstalk con- 
sists of fibres, which are attached to the surface of the heart, 
but there is no communication by ducts or vessels. The 
use of this organ is unknown. 
The animals of this class continually reside in the water, 
