356 
MOLLUSCA. 
Mollusca. adhesions, not unlike the Ieech. It is also used as a fin in 
<< swimming. 
By the union of the cloak and foot laterally and posteri- 
orly, a sac is formed, which is open in front forthe protrusion 
of the neck and head. The line of junction between the 
cloak and foot is marked, in general, by peculiarities in the 
condition of the margins of both. 
The zeck is usually divided from the cloak by a collar, or 
thickened margin belonging to the cloak, or rather to the 
shield, while in other cases it is continuous. Underneath, 
the neck is frequently attached to the foot. 
The head supports the tentacula and eyes, is free dor- 
sally, but frequently intimately connected with the foot on 
its ventral side. The portion between the tentacula and 
the mouth is termed the snout, (le mufle of the French, and 
its margin le chaperon). The mouth exhibits various mo- 
difications of fleshy lips and corneous jaws. The inside. of 
the cheeks are covered in some species with reflected teeth, 
to aid deglutition. The tongue can scarcely be detected in 
some of the genera; while, in others, it is a simple tubercle, 
or a strap-shaped, spiral organ, armed with transverse rows 
of teeth. This spiral tongue, where it is fixed to the base 
of the mouth, is broadest, and there also the spinous pro- 
cesses are strongest. The spiral part is narrowest and soft- 
est, and folded up behind the pharynx. M. Cuvier conjec- 
tures, and apparently with plausibility, that the spiral portion 
comes forward into the mouth to act as a tongue, in propor- 
tion as the anterior part is worn by use and absorbed. (See 
his Mémoire sur la Vivipare @eau douce, p. 12; and Mém. 
sur la Patelle, p. 17). 
The organs of respiration exhibit the two modifications 
of lungs and gills, to enable us to divide the Gasteropoda 
into two classes, which we have termed Pulmonifera and 
Branchifera. M. Cuvier appears to have been in some mea- 
sure aware of the importance of the distinction, when he 
instituted his order Pulmonés; but he afterwards suffered 
himself to be more influenced by the presence of an oper- 
culum, the shape of the aperture of the shell, and the sup- 
posed separation of the sexes, than by the characters of the 
respiratory organs. 
Some shells are simply tubular or conical; but the greater 
part are variously convoluted, the volutions being termed 
whorls or spires. These whorls are in general visible and 
distinct, the boundary between each being termed the Line 
of separation. The whorls in some species are simply placed 
in a lateral position, while in others the whorls are formed 
upon a pillar, or columella, which runs in the direction of 
the axis of the shell, the inferior whorl in this case embrac- 
ing the superior one. The pillar is in some cases nearly 
solid, in other instances tubular, with its base either open 
or covered. When the base of the tube of the columella is 
uncovered, the opening is termed the pillar cavity, or um- 
bilicus. 
In general, when a spiral shell is placed upon its base or 
mouth, with the apex towards the observer, the mouth will 
be found situated on the right side, and the whorls will be 
observed revolving in a direction from right to leit, or cor- 
responding with the motion of the sun. These shells are 
termed deatral. A few species have this order reversed, as 
the observer will readily perceive. For upon placing the 
shell in the above-mentioned position, the mouth will be 
found situated on the left side, and the whorls will revolve 
from left to right. These shells are termed sinistral, hete- 
rostrophe, or heteroclite. 
Cuass I.—PULMONIFERA. 
The pulmonary cavity is single and lateral. Its orifice is 
capable of being closed at the will of the animal. The 
bloodvessels are spread, chiefly on the walls and roof, like 
delicate net-work. The opening of the cavity is usually on 
the right side, with the anus behind it, and the sexual ori- Mollusca, 
fice is in the front near the head. In some of the genera, 
these openings are situate on the left side. The shells of 
the former are denominated _deatral, of the latter sinistral. 
This change in the position of the external openings is ac- 
companied by a corresponding alteration in the arrangement 
of the internal organs. The heart, for example, is always 
placed on the side opposite the pulmonary cavity. In the 
dextral shells, therefore, it is sinistral. In both kinds, how- 
ever, all the organs preserve the same relation to the back 
and belly, the head and tail. It is impossible, therefore, to 
conceive a dextral animal changed into a sinistral, by any 
circumstance which could take place at the period of hatch- 
ing, as M. Bosc was inclined to believe. This arrangement 
of the organs must have been not merely congenital, but 
coeval with the formation of the embryo. In some species 
all the individuals are sinistral, while in others the occur- 
rence is rarely met with in a solitary example. The former 
are in their natural state, the latter ought to be regarded as 
monsters. Where the character is permanent, it should 
constitute a generical difference. 
The reproductive system of the animals of this class ex- 
hibits the sexual organs, in general, united in the same in- 
dividual. Mutual impregnation, however, isnecessary. All 
the species are oviparous. The eggs are either naked, as 
in the terrestrial genera, or enveloped in a gelatinous mass, 
like the aquatic kinds. The embryo acquires nearly all its 
members while in the egg, and the shell is of a proportional 
size previous to hatching. Sir Everard Home, when treat- 
ing of the distinguished characters between the ova of the 
sepia, and those of the vermes testacea that live in water, 
(Phil. Trans., 1817, p. 297), and when referring to the ova 
of the vermes testacea, says, “If the shell were formed in 
the ovum, the process of aérating the blood must be very 
materially interfered with, for this reason, the covering or 
shell of the egg, first drops off, and the young is hatched 
before the shell of the animal is formed; this I have seen 
take place in the eggs of the garden snail, but in the tes- 
tacea that live in water, the young requires some defence in 
the period between the egg being hatched and the young 
acquiring its shell, which is not necessary in those that live 
on land; for this purpose the ova are enclosed in chambers 
of a particular kind.” The assertion here made, and found- 
ed on a priori considerations, that the shell is not formed 
until after the egg is hatched, is opposed by every observa- 
tion which we have been able to make on the subject; and 
what is more surprising, it is at variance with his own ob- 
servations on the garden snail, the very example produced 
in its confirmation. The eggs of a snail, laid on the 5th of 
August 1773, were hatched on the 20th of that month, 
and their condition at this time distinctly stated. “On the 
20th,” he says, “the young were hatched, and the shell 
completely formed.” It is much more becoming in a phi- 
losopher to observe how nature operates, than to pronounce 
what she must do. 
Orver I.—TrERRESTRIAL. 
The animals of this order reside constantly on the land. 
When by accident they fall into the water, they appear to 
be incapable of using their foot as a sucker or as a fin, and 
die after a few writhings. The species in general prefer 
moist places, and are seldom very active in dry weather. 
After a shower they speedily leave their hiding places, and 
at this time they may be readily collected. The eggs are 
hatched on land. 
1st Subdivision. 
rh Cloak and foot parallel, and containing the viscera between 
em. 
In this group are included those animals denominated 
slugs in this country. They possess four retractile tenta- 
