The entry to the gills is by a large aperture between the margin of the cloak and neck, at the middle, or to wards the right side. These are contained in a cavity on the back of the animal, and consist of leaves arranged in one or more rows, which adhere to the walls of the cavity. At the entrance of this cavity is the anus and oviduct.
The male and female organs are considered not only as distinct, but as occurring on different individuals. The evidence in support of this opinion is in many cases complete. The penis is in some external, and incapable of being withdrawn, while in others it is retractile, and situated in a cavity in the right tentaculum.
The body of the animal is attached to the shell by means of two muscles, which adhere to the pillar near the same place, and shift their position, by an arrange ment not well understood, in proportion as the individual increases in size. These muscles terminate in the foot and mouth.
The animals of this order have not been examined sufficiently in detail to admit of their distribution into natural groups, distinguished by characters founded on important differences of organization. The form of the shell has been resorted to, with the view of assisting ar rangement. The characters thus furnished would be useful and valuable, were they the index of any peculiar internal structure. But, unfortunately, animals widely different in structure, inhabit shells of the same form, and vice versa, so that, however useful the mere concho logist may find the form of the shell to be in his arrange ments, it can only be regarded by the zoologist as occu pying a subordinate place. Without therefore entering into any details regarding the structure of the few spe cies which may have been examined anatomically, we shall merely point out the tribes and families which have been contemplated, and whose characters in a great mea sure depend on the shape of the shell.
The first TRIBE contains animals with spiral shells, having an entire aperture. The anterior margin of the cloak is likewise entire. It includes the greater part of those shells included by Linnxus in his genera Turbo trochus and Nerita.
The first FAMILY of this tribe, represented by the ge nus Turbo, has the aperture of the shell nearly round.
All the species are furnished with an operculum. Some arc known to be ovoviviparous, and it is probable that all the species are so. Some have the sexes distinct, in different individuals, as the Palludinx, while in others the female organs only have been detected. Some of the genera are marine, such as Turbo, Delphinula, Ver. micularia, Turritella, Scalaria, Odostomia, and \Iono donta. The animals exhibit remarkable differences in the form of the hood, the length of the peduncles sup porting the eyes, and the filaments on the sides of the body.
There is only one genus, the species of which reside in fresh water, termed Paludina. It contains the Helix vivapara and tentaculata of British conchologists. Cu vier seems disposed to unite with this genus, the com mon marine shell Turbo littoreus or Periwinkle. The form of the foot and hood, however, arc different, but more especially the tongue, which, in the Paludina, is a fixed tubercle, while in the periwinkle it is strap•shaped, free at one extremity, equal in length to the body, and covered with spines.
The second FAmtLY is represented by the Linnean genus Trochus The aperture of the shell is somewhat quadrangular. The foot of the animal is furnished with an operculum, (unless in the genus Pyramidella, in which it is supposed to be wanting,) and the body on each side is ornamented with filaments, usually three in number, resembling tentacula. All the species are ma rine. and are included in the genera Trochus, Solarium, and Pyramidella.
The third FAMILY, termed Comitylium by Cuvier, is formed of shells having the aperture crescent-shaped, as in the genus I lelix, in which they were formerly includ ed. Two of the genera, Ampullaria and Melania, are fluviatile, residing in the countries near the equator. The remaining genera, Phasianella and Janthina, are marine. The Janthina vulgaris is destitute of an oper culum, but in its stead it is generally furnished with a cellular spongy body adhering to the foot, which, in con sequence of changes produced in its density by un known means, enables the animal to rise to the surface of the water and float. When restrained, it throws out a purple fluid, like the Aplysia, lodged in the margin of the cloak, covering the gills.