VI. SCYLLEA. The distinguishing characters of this genus depend on the two tentacula, the mouth being furnished with jaws, and the back with membranaceous expansions, bearing on their upper surface the branchial The foot is very narrow, forming a groove on the un der surface. The body is ovate, and compressed late rally. There are two expansions or crests on each side, and one at the tail. The branchix arc in the form of plumes, covering the upper surface of the crests and the back. The tentacula are each contained within a large funnel-shaped sheath.
The mouth is placed at the hase of the tentacula, sur rounded with a semicircular lip. The jaws are horny, and cut by crossing each other. The tongue is furnish ed with reflected points. The gullet is longitudinally plaited. The stomach is short and cylindrical, with a ring of hard longitudinal scales. The liver consists of six unequal globules. The bile is poured into the car diac extremity of the gullet.
The Scyllea pelagica has been long known to natu ralists, and appears to be very common in the equatorial seas. It is commonly found adhering to the stems of the fuel's natans.
VII. Git.nrcus. In this genus, whose true characters are still imperfectly explored, the head is furnished with four simple tentacula. On each side the body arc three or four horizontal membranaceous expansions, whose margins arc fringed with the branchial filaments.
This genus was instituted by R. Foster, and the spe cies with which naturalists have been longest acquaint ed, is the Doris rad•ata of Gmelin, which is figured in the Phil. Trans. vol. Ili. tab. iii. It is not uncommon in the Atlantic Ocean.
It is probable that a more minute examination of the animals of this order, than has hitherto taken place, would lead to the formation of several tribes and fami lies. The number of the tentacula, the structure of the
mouth, the connection between the cloak and the foot, the appendages of the anus, and the disposition of the branchim, lay claim to the notice of the systematical zoologist. But the study of these animals can only be carried on at the sea-shore, where they can be preserved alive in their native element.
The animals of this order differ from the preceding in the marginal distribution of the branchim. Between the foot and the cloak, or dorsal covering of the body, the gills are placed, above the one, and beneath the other. These are in the form of little leaves, which constitute a fringed band on each side of the body ; in some sur rounding the whole body, in others interrupted front or behind. Independently of this remarkable position of the organs of respiration, M. Cuvier arranges the ani mals in which it occurs into two distinct orders, and places them far apart in his system. It is not easy to account for this distribution ; as the only character of impot tance by which he has marked their separation, is derived from the circumstance, that, in the animals of his first order, the sexes, though united, require the mutual union of individuals, while, in the other, each individual is supposed to be capable of impregnating itself. In these last, however, the other organs, of feeling, diges .ion, or protection, have no common properties. Averse to any arbitrary arrangements of this kind, we have here brought them together under one order, and shall now proceed to consider the characters of the genera.