MOLLUSCA. - 
Mollusca. at the anterior extremity, and consists of a simple aperture 
‘=~-~ entering into the gullet, or rather stomach. 
It is surround- 
ed by four flattened moveable tentacula, two of which in 
some are in part united with the cloak, while in others they 
are free to the base. In their structure they resemble the 
branchiz. The stomach is full of cells, the bottom of each 
pierced with a biliary duct. A singular organ, termed the 
crystalline process, cylindrical, cartilaginous, and_transpar- 
ent, is found in some species projecting into the cavity of 
the stomach. The liver is large, surrounds the stomach, 
and pours out its contents by numerous openings. The in- 
testine terminates posteriorly by a tubular anus. 
The branchiz consist of two ribbands on each side, ex- 
tending the length of the body, free on the sides and mar- 
gin, and striated transversely. These plates are frequently 
of unequal size. The blood is brought to these by means 
of pulmonic veins, without the intervention of the heart. 
The aérated blood is transmitted to a systemic heart, con- 
sisting of one or two auricles, and a ventricle. 
The reproductive organs of the Bivalvia, hitherto exam- 
ined, consist of an ovarium occupying the sides of the body, 
and penetrating the membranes of the cloak. They appear 
to have the organs of both sexes incorporated, and to pro- 
pagate without intercourse. Lamark is disposed to con- 
sider impregnation produced by the male fluid dispersed 
through the water; a supposition unsupported even by 
analogy in the animal kingdom. Many species are ovovivi- 
parous ; in which case the eggs when ripe pass into the gills, 
where they are hatched. 
The methodical distribution of the Bivalvia appears to be 
attended with peculiar difficulties, in consequence of the 
uniformity which prevails in the structure and disposition 
of their organs. The characters furnished by the shell, 
though useful in the construction of generic as well as spe- 
cific distinctions, have been abandoned by those who prefer 
a knowledge of the structure, rather than the form of an 
animal. The characters derived from the presence of a 
byssus, a foot, or syphons, appear to be nearly of co-ordi- 
nate importance. M. Cuvier gives the preference to those 
founded upon the appearances of the syphon, by the aid of 
which the genera may be distributed into five families, an 
arrangement which, though liable to some objections, may 
be adopted with advantage. These groups, however, may 
be considered as occupying a much higher rank, and each 
as including numerous families. 
lst Subdivision. 
Cloak open. 
There are no syphons, the anterior margin of the cloak 
being as open as the mouth of the shell. When the valves 
open, the water comes immediately in contact with the 
branchiz and mouth. The margin of the mantle has a 
double fringe of filaments. 
lst Tribe. 
Valves closed by one adductor muscle. 
A. Pectenide. Animals free or fixed only by a byssus. 
Furnished with a foot. 
Into this family, contemplated by Lamark, the following 
ill assorted genera may be placed: Pecten, Lima, Pedum, 
Plagiostoma, Vulsella, Placuna, Gryphza, Perna, Gervil- 
lea, Inoceramus, Malleus, and Crenatula. 
B. Ostreade. Shell cemented to foreign bodies. 
destitute of a foot. 
To this family the following genera are related: Ostrea, 
Anomia, Spondylus, and Plicatula. 
2d Tribe. 
Shell closed by two adductor muscles. 
The two genera, Avicula and Meleagrina (of Lamark), 
form one family of this tribe; the genus Pinna another ; 
and the Arcadz a third, including Arca, Pectunculus, Nu- 
cula, Cucullea, Trigonia, and Castalia. 
2d Subdivision, 
Body 
Cloak more or less closed, forming syphons. 
367 
Mollusea. 
The further division of this group depends on the modi- === 
fications of the syphons, or aperture of the cloak. 
1st Tribe. 
The union of the cloak forming only one syphon. This 
is situate posteriorly opposite the anus, and serves for the 
ejection of the excrements. The other large opening allows 
the water to enter to the mouth and gills. 
This tribe may be divided into two families. The first, 
Mytilde, will include the genera Mytilus, Modiolus, and 
Lithodomus, which are furnished with a byssus. The 
second, Unioride, will embrace Unio, Hyria, Anodonta, and 
Tridina. They want a byssus. 
M. Cuvier is disposed to place in this group the genera 
Cardita, Venericardia, and Crassatella. 
2d Tribe. 
Cloak closed posteriorly, and anteriorly forming three 
apertures. The first serves for the passage of the byssus, 
and is the largest. The second admits water to the bran- 
chize and mouth ; and the third is opposite the anus. The 
valves are closed by one adductor muscle. There are only 
two genera belonging to this tribe, Tridacna and Hippopus. 
In the two remaining tribes there are three openings in 
the cloak. Two of these are posterior, and near each other ; 
sometimes, indeed, they are tubular and united. There is 
no byssus, but always a foot. 
3d Tribe. 
Anterior opening large, allowing the water free access to 
the mouth and gills, and the feet freedom of motion. The 
structure of the animals is yet too imperfectly examined to 
enable any one to establish families on permanent charac- 
ters. The attempt which Lamark has made may be con- 
sidered as a complete failure, independent of the wanton 
changes of nomenclature with which it is chargeable, whilst 
the efforts of Cuvier have not been attended with greater 
success. The following are the principal genera belonging 
to this tribe: Chama, Isocardia, Cardium, Donax, Cyclas, 
Corbis, Tellina, Loripes, Lucina, Venus, Capsa, Petricola, 
Corbula, and Mactra. 
4th Tribe. 
Anterior opening small, and not exposing the mouth or 
gills. 
In this tribe the mantle is closed in front ; and even when 
the valves are open, neither mouth nor gills are visible. The 
anterior opening serves for a passage to the foot, and the 
posterior openings, in the form of two long tubes, united by 
a common membrane, serve for the entrance and exit of the 
water, to the mouth and branchiz, and the ejection of the 
feeces, the dorsal syphon serving the latter purpose. The 
cuticle of the shell covers also the exposed portion of the 
cloak, so that, when the animal is removed from the shell, 
it remains as a loose membrane on the margin of the valves, 
as was first observed by Reaumur. All the genera prefer 
concealment, burrowing in sand, mud, or wood, with the 
head downwards, and the syphons rising to the surface. 
The following genera belong to this tribe: Mya, Lutraria, 
Anatina, Glycemeris, Panopea, Pandora, Gastrochena, Bys- 
somia, Hiatella, Solen, Sanguinolaria, Pholas, Teredo, Xylo- 
phaga, Clavagella, and Fistulana. 
Secr. []—Acepala Tunicata. 
Covering soft or coriaceous. 
The formation of this interesting group of animals was 
first publicly announced by Lamark in his Histotre Natu- 
relle des Animaux sans Vertebres, tom. iii. p. 80, (1816.) 
The labours of Desmarest, Lesueur, and Cuvier, aided by 
the descriptions of Ellis and Pallis, paved the way for the 
masterly efforts of Savigny, to whom we owe the most ex- 
tensive, new, and accurate information yet given. concerning 
the animals of this group. His observations are contained 
in his Recherches Anatomiques sur les Ascidies composées, 
