Although the Porpita, a genus of the Velellidæ, has no sail, it is akin to the Velellæ in size and structure. The body of the Porpita consists of two circular cartilaginous disks, united at their edges and surrounded by a blue membranous limb. On the surface of the upper disk there are beautifully radiating striæ, each of which ends at the circumference of the disk in a little protuberance, which gives it the appearance of a toothed wheel. A large sterile polypite occupies the centre of the under-surface of the body, surrounded by a zone full of smaller ones; and the space between the zone and the blue limb is occupied by a narrow area of a reticulated appearance, to which numerous circles of tentacles are fixed, that spread out and radiate all around the margin of the animal. The interior circular rows are simple, short, and fleshy, not extending much beyond the edge of the limb: the succeeding circles are gradually longer, while the exterior row, which extends far beyond the limb, are branched and beset with slender filaments, ending in minute globes, sometimes filled with air, so that a Porpita is like a floating daisy, though differently coloured. The Porpita glandifera, a pretty little inhabitant of the Mediterranean, which only appears in calm weather, is not more than eight lines in diameter; somewhat convex, white, marked by radiating striæ, and encompassed by a dark blue limb. The central polypite and those next to it are whitish, the rest become of a darker blue towards the limb; the tentacles are pellucid and bluish, and the three last rows have little dark blue globes attached to them by slender filaments. The Porpita has a horizontal air-vessel divided vertically into air-chambers like the Velella, but they are much more numerous. In a middle-sized Porpita, four or five lines in diameter, there are twenty-three or twenty-four air-chambers surrounding a central one, and eighty or ninety pneumatic filaments, so that the animal is extremely buoyant. Brown matter, supposed to be a liver, lies directly below the undermost wall of the air-vessel, through which, as well as through the base of the animal, all the pneumatic filaments penetrate; the greater number go straight down into the water, but a portion of them terminate in the walls of the polypites. A complete system of canals, ciliated internally, traverses all parts of the animal; and it may be presumed that the cilia maintain its juices in a state of circulation similar to that in the Velella; and the functions of the polypites, great and small, that are in connection with the liver, are also similar to those of the Velella. The Porpita is armed with thread-cells like all the class. The central polypite is sterile and nutritive; the small ones are both nutritive and reproductive: buds spring from their stems, which become independent male and female medusiform zooids, swim away from their parent and produce abundance of eggs, whence a new generation of Porpitæ arise.