366 
Mollusca 
=~ Bivalves. 
MOLLUSCA. 
The generation of Terebratula is that of the ordinary 
;. In two of the larger specimens the ova had in- 
sinuated themselves between the layers of the mantle, and 
partly surrounded the branchial vessels. When so far ad- 
vanced they obscure the organization of the mantle which 
adapts it for respiration: this organization is consequently 
most satisfactorily observed in very young individuals. 
The peculiar internal testaceous apparatus or loop con- 
nected with the hinge and supporting the arms, possesses 
some elasticity, and when acted on by the muscles, becomes 
in its reflected part sufficiently convex to press upon the 
perforated valve and separate it slightly from the opposite 
one ; thus compensating for the absence of the thick arms 
of Lingula, which, in their protrusion, push open the valves, 
and also for that of the elastic fibres, constituting the liga- 
ment of ordinary Bivalves. é 
In the Orbicula lamellosa the same intelligent observer 
traced along the whole circumference of the valves, shining 
cilia projecting for an extent varying from two to four lines: 
they are consequently much longer than in Terebratula and 
in Lingula anatina, and are rather longer than in Lingula 
Audebardii, Brod. On examination under a higher power 
they are observed to be beset with smaller setae, which pro- 
bably give them greater power in determining the respira- 
tory currents. The mantle is similarly vascular to that of 
Terebratula, there being, in the upper lobe, four principal 
trunks (comparatively, however, much shorter than in that 
genus); and two in the lower. These trunks terminate in 
sinuses, situated close, and two strong tendinous membranes, 
which circumscribe the visceral mass, and to which the 
mantle-lobes firmly adhere. Here the veins of both man- 
tle-lobes join, and the common trunk or sinus passes oblique- 
ly through the membrane, and may be plainly seen distri- 
buting ramuli over the liver and ovary. 
The muscles and viscera form a rounded mass, situated 
in the posterior half of the shell. The mouth is seated be- 
tween the base of the arms. The oesophagus passes ob- 
liquely through the tendinous wall of the viscera in a direc- 
tion towards the upper valve: it becomes slightly dilated, 
and is then surrounded by the liver. The intestine is con- 
tinued straight to the opposite end of the visceral cavity, is 
there again contracted, makes a sudden bend upon itself, 
and returns to the middle of the right side of the visceral 
belt, which it perforates obliquely, and terminates between 
the lobes of the mantle a little below the bend of the arms. 
The liver is of a beautiful green colour, and consists of a 
congeries of elongated follicles, closely compacted together, 
which cemmunicate by numerous orifices with the stomach. 
As in Terebratula, there is no salivary gland. 
In Lingula Audebardii, Brod., there is also no salivary 
gland; and Mr. Owen is therefore disposed to believe, that 
the gland described as such in Lingula anatina by Cuvier, 
was only a portion of the liver, from which the colour had 
probably been removed by long maceration in spirit. 
In the want of salivary glands the Brachiopoda would 
seem to agree with the ordinary Bivalves. Destitute, like 
them, of any hard parts about the mouth for comminuting 
alimentary substances, glands for pouring in a fluid to 
blend with the food during that operation, are not wanted. 
The nervous system in Terebratula was not detected by 
Mr.Owen. In Orbicula two small ganglia were found on 
the side of the oesophagus next the perforated valve ; from 
which two filaments, accompanying the oesophagus through 
the membranous wall, immediately diverge and pass ex- 
terior to the anterior shell muscles, proceeding with corres- 
ponding arteries to near the heart, beyond which he could 
not trace them. A single small ganglion is situated on the 
opposite side of the oesophagus, but on a plane posterior to 
the preceding; this is probably the cerebral ganglion for 
giving off nerves to the free spiral extremities of the arms, 
close to the base of which it is situated. 
Onper II.—Bivanvra. 
Mouth destitute of fringed spiral arms. 
The animals of this group form the class Conchifera of 
Lamark, the Bivalvia of the older naturalists. 
The shells exhibit great variety of form and relative size. 
They are joined together at the hinge, which is either plain 
or toothed, and corresponds in position with the back of the 
animal. The connection of the two valves is secured by 
the intervention of an elastic horny ligament, the office of 
which is to keep the valves open. It is either extemal or 
internal. The valves are closed by means of adductor 
muscles, intermixed with tendons, and, passing traversely 
through the animal, adhere to the corresponding places in 
the inside of each shell. By the contractions of these 
muscles the free edges of the valves are brought into con- 
tact, at the same time that the ligament is compressed or 
stretched, according as it is internal or external. The num- 
ber of muscular impressions is employed by Lamark in the 
division of the Bivalvia into two orders, Dimyaires and 
Monomyaires. This distinction, however, he has not at- 
tended to with care, as in his family Myétlacées, which he 
includes in his second order, er those having one adductor 
muscle, there are obviously two adductor muscles, although 
the one is certainly much larger and more complicated than 
the other. Besides these impressions of the adductor mus- 
cles, there are cthers connected with the foot and byssus. 
The cloak lines the inside of the shells. In some cases it 
is entirely open, when the border corresponding with the 
free margin of the shell is thickened, and more or less fring- 
ed with contractile irritable filaments. In other cases the 
cloak in front is more or less united, and even forms tubu- 
lar elongations, which are termed syphons. 
Locomotion is denied to many species of this order. 
Among these some are immoveably cemented to rocks and 
stones, as oysters; a few are attached by a cartilaginous 
ligament, as the Anomiz ; while others are fixed by means 
of a byssus. This last organ consists of numerous filaments 
issuing from a complicated apparatus in the breast, connect- 
ed with a secreting gland and with the shell by the inter- 
vention of tendinous bands. The foot is seated a little to- 
wards the mouth, is usually tengue-shaped, capable of con- 
siderable elongation, with a furrow on its posterior surface. 
This organ, where a byssus is present, is considered as 
employed in spinning and fixing the threads. 
is no byssus, it either acts as a sucker, enabling the animal 
to crawl among the surface of bodies, or as a paw, to dig 
holes in the sand or mud. None of the species can float in 
the water. They either crawl or leap, the last kind of 
motion being effected by suddenly opening and shutting the 
valves. In securing a residence, some of the species bore 
into different substances by means of a rotatory motion of the 
shell. It was at one time supposed that the dwelling was 
formed by a secretion affecting the solution of the surround- 
ing substance. But the very different substances penetrat- 
ed by the same species, as limestone, slate-clay, and wood, 
forbid us to entertain such a supposition. 
The nervous system is here but little developed. The 
superior and inferior ganglia, surrounding the gullet, give 
rise to all the nervous filaments which proceed through the 
body. 
The digestive organs are scarcely less simple. The food 
is soft and swallowed entire, and either brought to the mouth 
by accident, or by eddies produced in the water by the 
opening and shutting of the shells, aided in some cases by 
the syphons. 
It may be proper here to state, in order to understand 
the relative situation of the parts, that, upon laying the ani- 
mal upon its back, and cpening the cloak, the abdomen 
appears to occupy the middle longitudinally, and the bran- 
chia to be arranged on each side. The mouth is situated 
Mollusca. 
a 
When there - 
