In the ciliograda, the ovaries are more obvious. They consist of two or four vesicular organs, each placed between two of the cilia-bearing arches. In cydippe, they are of a red colour, and nearly cy lindrical shape. The ova are spherical.
The parts in the mograda corresponding to the organs just referred to, are eight round bodies, of small size, situated near the margin of the disc, each formed of a vesicle, containing, at its free tremity, many minute hexagonal corpuscules ; there is attached to each vesicle a digitated dix, which seems to be hollow, and to municate with the circular vessel. These organs were seen by Gaede and by Muller in medusa capillata, and M. aurita, and by Eschscholtz in some species of cyanea, nonia, pelagia, and chrysaora ; Dr. M. wards has observed them also, at certain seasons, in rhizostoma ; and in carybdea supialis, he found, midway between each pair of pendent filaments, and immediately above a little notch in the margin, four spots of a deep brown colour, each of which appeared, under the microscope, to be formed partly by a minute spherical body, having a granular aspect, as if it were filled with eggs, and partly by a little sac, with puckered sides, which is imbedded in the gelatinous substance of the body. These he regards as the ovaries.t But, notwithstanding their having found nular bodies like ova in the organs above described, neither Gaede nor MUller considered them as ovaries. Mailer regarded the granules as excrementitious matters ; and Gaede thought that he saw the ovaries in the plaited branes of the gastric cavities ; whence he observed the ova descend into certain minute vesicles imbedded in the margins of the arms. He remarked that, in medusa aurita, when the cells in the arms were filled with eggs, the plaited membranes had none ; and, on the other hand, when there were no eggs in the arms, the plaited membranes were studded with them.
Cuvier was also of opinion that the ova are formed in the plaited membranes above men tioned, and that they are matured in the mar ff of the arms.* No observations, so far as we know, have hitherto been made on the development of the ova ; but Dr. Grant has recently stated that the ova of equorea are furnished with cilia, and have locomotive powers, like the ova of the porifera and polypifera4 The colours of the acalephce often depend on the tints of their ova: these are generally red, but sometimes brown, yellow, or purple.
VIII. Geographical distribution.—We conViii. Geographical distribution.—We con- ceive that a brief notice of this part of their natural history may, in some measure, illus strate the physiology of the acalephm. They are met with in all seas ; but certain families exist more abundantly in some localities than in others. The ciliograda and pulmograda, for instance, are inhabitants chiefly of the colder regions, while the physograda are seldom found beyond the limits of the tropical zone. Some float in bays, and near land, but the greater number in the high seas. Medusec and cyaneye are met with only in the cold and tem perate zones of the northern hemisphere. Cy dippe lives in the North Arctic Ocean, as well as in the Pacific, under the equator. One species of cestum inhabits the Mediterranean,—another the South Sea. It frequently happens that enormous numbers of one species are met with closely grouped together, so as somewhat to impede a ship's progress for two or three suc cessive days ; after which, not a single indi vidual of the same species is seen. In the European seas, it is chiefly in summer and autumn that the acalephm swim on the surface. In winter, they probably sink to the bottom.