VI. DORID1UM. In the animals of this genus, there is no dorsal plate or shell, although there is a cavity with a spiral turn in the cloak. The branchim and ac companying organs are nearer the posterior extremity than the species of the preceding genera.
The digestive system is more simple than in the pre ceding genus. There is here no appearance of the spinous tongue, the gullet is short, and the stomach is em bran ac eou s.
The characters of this genus were first developed by Curler, but the name which we have adopted is that of Mekel. Only one species, D. carnosum, is known as a native of the Mediterranean.
The preceding review of the genera of this order in dicates the existence of at least three natural families. The pleurobranchia appears to stand apart, and to have few characters in common with the others, which are all intimately related. But under the head Aplysix, the genera Aplysia, Dolabella, and Notarchus, may be in cluded, while the Acerx will comprehend Bulla, Bullcca, and Doridium.
We have been induced to institute this order, for the reception of the genus Valvata of Muller. The species which it includes are inhabitants of fresh water, and in their general appearance resemble the aquatic pulmoni ferous gasteropoda. They are distinguished, however, from all the other gasteropoda, by the position of the branchix, and the number of tentacula.
The branchix appear in the form of a feather, with a central stem, and a row of compound branches on each side, decreasing in size from the base to the extremity, which is free.
This plume is placed on the neck, near the middle, and a short way behind the anterior tentacula. Near this plume, but on the right side, is a single simple fila ment, like a tentaculum. The anterior tentacula occu py the usual positions, and are setaceous. The eyes are placed at their base behind.
The animal is furnished with a spiral shell, into which it is capable of withdrawing the body. The foot is pro tected by a spirally-striated operculum, which shuts up the mouth of the shell upon the retreat of the inhabitant.
The species are all of a small size, and their internal structure has never been subjected to a rigorous exami nation. Two species are natives of Britain.
1. Valvata cristata. This is the Helix cristata of Mon tagu. The shell resembles a planorbis in its depressed form, the whorls revolving nearly on the same plane, so that the last-formed one embraces the others. The aperture is circular and simple. It is found in ditches on aquatic plants in England.
2. Valvata piscinalis. This is the Turbo fontinalis of British conchologists, but first described by Muller as Nerita piscinalis. The whorls of the shell form in this species a short spire. It is very common, both in Eng land and Scotland.
Tuts order comprehends nearly all the animals with spiral univalve shells which inhabit the sea, together with a few which reside in fresh water.
The foot is usually fortified above, on its posterior ex tremity, with a corneous plate, which acts as a lid to the shell, when the animal is withdrawn into the cavity. The anterior extremity is in some of the species double. The anterior margin of the cloak forms a thick band, or arch, rising from the foot, behind which is the portion of the body that is. always contained in the shell, and which is covered with a very thin skin. Between the margin of the cloak and foot is situated the head, supported on a short neck. The tentacula arc two in number, bearing eyes at their base, or on short lateral processes, which have some claims to be considered as tcntacula. The hood is frequently emarginated, and sometimes fringed. The mouth is more or less in the form of a proboscis, in some cases armed within with spinous lips, or furnished with a long narrow spiral tongue, armed with spines, as in the common periwinkle. The nature of this kind of tongue, whose spiral extremity is free and lodged in the abdomen, is not weH understood.