All the animals of this class inhabit the sea. Some of those, as the Clio and Limacina, frequent the arctic regions, and afford the whale a great part of its suste nance. None of the species of the class have hitherto been detected in the British seas.
M. Cu vier, in his Regne Animal, II. p. 378, subdivides the genera of this class into two orders. The animals included in the first are furnished with a head distinct from the body, and are arranged under the five following genera ; Clio, Cleodora, Cymbulia, Limacina, and Pneu moclermon. Those of the second division, which are destitute of a head, constitute only one genus, named Hyalea.
1. C/io.—In this genus the body is ovate and elon gated, the tunic membranaceous, and the head divided into two lobes, whose summits are furnished with ten tacula. The mouth is transverse, with two lateral lon gitudinal lips. On each side the neck arise two blunt, conical, fin-like expansions, with a fine reticulated sur face, considered as serving the double purpose of fins and branchim. The anus and orifice of generation terminate under the base of the right branchia. The viscera do not fill entirely the cavity of the inner bag. The gut makes only one fold. The existence of eyes has not been ascertained.
The genus Clio was originally instituted by Brown, in his Natural History of Jamaica. It was afterwards embraced and modified by Linnmus and Pallas, in such a manner as ultimately to exclude the species for whose reception Brown originally formed it. It contains two species, the most remarkable of which is the Clio borea lis. Mr. Scoresby, in his valuable work on the Arctic Regions, lately published, states (vol. i. p. 544.) that it occurs in vast numbers in some situations near Spitz bergen, but is not found generally throughout the arc tic seas. In swimming, it hrings the tips of the fins almost into contact, first on one side, and then on the other.
2. Cleodora.—This genus was instituted by Peron, for the reception of Brown's species of Clio. The body is covered with a triangular pyramidal tunic, carrying two membranaceous wings, between which is the mouth, furnished with a semicircular lip.
The species whose characters have been most fully developed, is the Cleodora pyramidata. Brown's Jamaica, p. 386. tab. 43. fig. 1.
3. Cymbulia.—The tunic of the species of this genus is trough-shaped, and cartilaginous. The fin-like ex pansion is single, divided into three lobes, one of which is small, with two tubercles, and a minute fleshy beard. This genus was instituted by Peron, in Annales du Mu seum, vol. xv. . tab. 3. fig. 10, 11.
4. Limacina.—This genus is nearly allied to Clio, in the form of the head and wings, but the body of the animal is contained in a tender shell of one turn and a half, flat on one side, with a large umbilicus in the other.
When the animal swims, the head and fin-like expansions arc protruded from the shell.
This genus was instituted by Cuvier, for the recep tion of the Clio helicina, an animal first described by Captain Phipps, and afterwards by Fabricius, under the name of Argonauta artica. According to Mr. Scoresby, it is found in immense quantities near the coast of Spitz bergen, but does not occur out of sight of land.
5. Pneumodermon.—The body, in the species of this genus, is oval. The neck is narrow, with a fin-like ex pansion on each side. The mouth is nearly terminal, furnished on each side with a fleshy lip, and below with a fleshy chin. On the summit are two bundles of tenta cula, each consisting of a filament, with a tubercle at the end, pierced by a small hole, and considered as exercis ing the office of a sucker. Cuvier, in his Memoire sur et le Pneumoderme, considered the leaf-like ridges which occur on the caudal extremity of the body as the branchix, and even describes the pulmonary vein which conveys the blood from these to the heart. But, in his Regne Animal, he states it as the opinion of his assistant, AI. Blainville, that the fin-like expansions of the neck contain the branchia: on their surface, as in the case of Clio. The rectum and oviduct terminate under the right wing. Cuvier has figured and described the only known species, which he terms Pneumodernzon Pe ronii, the trivial name being in honour of the discoverer, M. Peron.
6. Hyalea.—The animals of this genus, as we have already stated, are destitute of a head. The body is lodged between two plates or valves, united at the base, where they inclose the caudal extremity. The ventral valve is nearly flat, with an uneven margin, narrow an teriorly, but expanding behind, and terminating in three projecting points. From the middle point four ribs diverge forward, and a muscle arises, which, fixed in the superior viscera, enables the animal to withdraw into the shell. The dorsal valve is shorter than the pre ceding, the margin flat and circular, and the middle con vex outwardly. In the space between the lateral margin of the two valves, on each side, the branchim are situ ated, in a duplicature of the tunic, the sides of which are furnished with filaments. The fleshy neck supports the two membranaceous expansions ; between which and the base the mouth is situated, surrounded by two lips, and strengthened within by two fleshy cheeks. The opening of the anus and oviduct are at the base of the right fin.
The Hyalea tridentata, the best known species of the genus, was first noticed by Forskal, in his Descriptiones p. 124, as an Anomia, and inhabiting the Me diterranean.