The covered-eyed group consists only of two natural divisions—the Rhizostoma, or many-mouthed medusæ, and the Monostoma, or one-mouthed medusæ. In both the coloured eye-specks at the margin of the umbrella are larger and more numerous, than in the naked-eyed group, and they are covered with a hood. The proboscis of the one-mouthed order terminates in a square mouth, the four angles of which are prolonged into tentacles with a solid hyaline axis. They have a fringed membrane along their under-surface, containing numerous stinging thread-cells. Sixteen canals, connected with the stomach or cavity of the proboscis, radiate over the flattish, cup-shaped umbrella; eight of these are branched, and terminate in the circular canal which runs round its fringed edge, and they form the nutrient and respiratory system of the animal, while the eight simple and alternate canals terminate in eight openings at the rim of the umbrella, through which the refuse or indigestible part of the food is discharged, thus forming an exception to the other pulmograde medusæ, and indeed to the Hydrozoa in general, which eject it at the mouth. All the canals are lined with cilia, whose vibrations maintain the circulation of the fluids, and perform the duties both of a heart and respiratory apparatus. Dr. A. Krohn has observed that in three species of the genus Pelagia belonging to the covered-eyed medusæ, the young are at once developed as medusæ without the intervention of the hydra form. [Illustration: Fig. 115. Rhizostoma.]