GASTEROPODA.
The animals which are included in this class are more numerous in regard to species, and exhibit a greater diversity of organization, than those belonging to the other great divisions of molluscous animals. They have long occupied the attention of the zoologist, and various methods have been proposed for their artificial and natural arrangement. Their internal structure has like wise been investigated with a considerable degree of suc cess, and the purposes which the different organs serve in the animal economy, have been in a great measure ascertained.
The Gasteropoda present few characters which are common to the whole class. Their body may be re garded as consisting of a foot, the cloak or tunic, and the head. The foot is a firm muscular expansion, co vering a part or the whole of the ventral surface of the animal. The viscera and head rests on its central sur face, while externally it exhibits a soft even disk, capable of acting as a sucker, with which the animal adheres to objects when at rest, and by which it crawls from one place to another. It is likewise used as a fin in swim ming, being aided at the same time by the serpentine motion of the whole body. The tunic arises on each side of the foot, and forms an arch over it for the recep tion of the viscera. Behind, the sac thus formed is closed, whose front is occupied by the neck and head. The tunic on the back is, in some cases, fortified by one or more testaceous plates ; in others it is protected by a conical or spiral shell.
The head is connected at the sides and on the back with the cloak, and beneath with the foot. In general it is furnished with tcntacula as organs of touch.
The organs of circulation are more simple than in the cephalopoda., There is no pulmonic auricle nor ventricle, the veins which correspond with the vena cava in the higher classes proceeding directly to the lungs or branchim. There is, however, a systemic auricle, which receives the aerated blood, and a systemic ventricle, which transmits it to the different parts of the body. The heart is usually situated on the left side ; but in those genera furnished with a spiral shell, it is always on the side next the pillar, or the one opposite to that to which the spire of the shell is directed.
The organs of respiration exhibit the two modifica tions of lungs and gills. The lungs, destined to respire free air, are usually seated in a cavity on one side, whose orifice is capable of being closed at the will of the animal. The walls of this cavity are lined with a delicate net-work of vessels, in which the blood is ex posed to the decarbonizing influence of the atmosphere. In those animals whose blood is aerated in water, the gills exhibit every variety of form. In some they oc cur in a cavity, or exposed in the form of pectinated ridges, in others assuming the appearance of shrubs. In all cases these organs are well protected, delicate in structure, and exceedingly sensible to any external im pression.
These two conditions in the organs of respiration point out the propriety of forming the Gasteropoda into two subdivisions, which may be termed, according as they respire by means of lungs or gills, Pulmonifera and Branchifera. Curler appears to have been in some
measure aware of the importance of the distinction, when lie instituted his order Pu/mones. But he afterwards suffered himself to be more influenced by the presence of an operculum, the shape of the aperture of the shell, and the supposed separation of the sexes, than by the characters of the respiratory organs. Hence he inserted the pulmoniferous genus Cyclostoma among those which are furnished with gills.
The distinguishing character of this group of Gaste ropodous mollusca, is to respire free air. This is per formed in a pulmonary cavity situated at the side, the entrance to which the animal is capable of closing at pleasure. The blood-vessels are spread upon the walls of this cavity, chiefly on the roof, in the form of a deli cate net-work. The opening of the cavity is usually on the right side, with the anus behind it, and the sexual orifice is in the front near the head. In some of the genera these openings are situated on the left side. The shells of the former are denominated dextral—of the latter sinistral. This change in the position of the ex ternal openings is accompanied by corresponding alter ation in the arrangement of the internal organs. The heart, for example, is always placed on the side oppo site the pulmonary cavity. In the dextral shells, there fore, it is sinistral, while in the sinistral shells it is dex tral. In both kinds, however, 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 hatching, as M. Bose 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 occurrence 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 animals of this subdivision have, in general, the sexual organs united in the same individual, requiring, however, mutual union. They are all oviparous.
This is intended to include all those pulmoniferous gasteropoda which are terrestrial. These reside con stantly on the land where they have been hatched, and where they collect their food and propagate their kind. They admit of division into two tribes, from the circum stance, that in some the shell, which is either flat, or open and subspiral, is concealed in a thickened plate of the cloak, termed the shield, while in others the shell is spiral or conical, external, and contains the body of the animal.
TRIBE I.—The genera included in this tribe are three in number. They all possess four tentacula, capable of being withdrawn into themselves like the fingers of a glove. The eyes are black points, seated at the tips of the superior tentacula.