In all latitudes the medusæ are highly luminous, especially in warm seas. Professor Vogt remarked that flashes of light passed over their disk when they touched one another in swimming, and they appear at intervals like globes of fire among the lesser lights of the Noctilucæ; if from involuntary nervous contraction, as is most likely, the light must be electric.
The medusæ are infested by many parasites. Entozoa are often abundant in their gelatinous substance, and crustaceans of various kinds and colours, such as shrimps, sand-hoppers, and a galæmon of glassy transparency, move about in the substance of their disc and arms, entering unscathed by the poisonous darts which inflict instant death on others of their class. The Libanea crab, of gigantic size compared with its host, is in the habit of taking up its abode between the four columns of the Rhizostoma. But the most singular intruder is the Philomedusa Vogtii, which is a polype with twelve thick short tentacles, its whole body and tentacles being covered with cilia and thread-cells. These polypes live in the disk, arms, and stomach of the medusæ, and, when taken out, their stomachs are found to contain fragments of the tentacles of their host, and even the thread-cells with their stings. The larger polypes devour the smaller ones, and the latter live for weeks within the larger ones without apparent inconvenience to either.[[24]]
Mr. M‘Cready mentions that the larvæ of the medusa Cunina octonaria swim as parasites in the cavity of the bell of the medusa Turritopsis nutricula, which not only furnishes a shelter and dwelling-place to the larvæ during their development, but it also serves as a nurse, by permitting the parasites, which adhere by their tentacles, to take the food out of its mouth by means of their long proboscides. They undergo many transformations, and become nearly perfect medusæ while within their nurse.
Medusæ of different species are met with in every sea from the equator to the poles. They are eminently social, migrating in enormous shoals to great distances. The largest shoal of young sea nettles on record was met with in the Gulf Stream, off the coast of Florida, by a vessel bound for England. The captain likened them to acorns; they were so crowded as completely to cover the sea, giving it the appearance from a distance of a boundless meadow in the yellow leaf. He was five or six days in sailing through them, and in about sixty days afterwards, on returning from England, he fell in with the same school, as the sailors call it, off the Western Islands, and was three or four days in sailing through them again. Mr. Piazzi Smyth, when on a voyage to Teneriffe in 1856, fell in with a vast shoal of medusæ. With a microscope he found part of the stomach of one of these creatures so full of diatoms of various forms—stars, crosses, semicircles, embossed circles and spirals—that he computed the whole stomach could not have contained less than 700,000. The flinty shells of the diatoms ejected in myriads by the medusæ, accumulate in the course of ages into siliceous strata, which, heaved up by subterranean fires, at length become the abode of man. Thus gelatinous transparent beings indirectly aid in forming the solid crust of the earth by means of the microscopic vegetation of the sea.
Ciliograde Hydrozoa.
The ciliograde Acalephæ, which form four orders and many genera, and which swim by means of symmetrical rows of long cilia, are represented on the British coasts by the Cydippe pileus and the Beroë Forskalia ([fig. 116]), little delicately tinted, gelatinous, and transparent animals that shine in the dark.
The Cydippe pileus is a globe three-eighths of an inch in diameter, like the purest crystal, with eight bands of large cilia, stretching at regular distances from pole to pole. A mouth, surrounded by extremely sensitive tentacles, is situated at one pole, the vent at the other. The Cydippes poise and fix themselves to objects by means of two very long tentacles, fringed on one edge by cirri, that is, short curled tentacles. These cirrated tentacles, which in some species stretch out to more than twenty times the length of the animal, can be instantaneously retracted into cavities at the posterior end of the body, while, at the same time, the marginal filaments are as rapidly coiled up in a series of close spirals. The whole of these complex organs are enclosed within the limits of a pin’s head.
Fig. 116. A, Cydippe pileus; B, Beroë Forskalia.
The manner in which these little gems swim is beautiful; sometimes they rise and descend slowly, like a balloon, and when they glide along the surface of the water in sunshine, the cilia on the eight meridional bands exhibit the most brilliant iridescence. The long cirrated tentacles follow all their motions in graceful curves, or hang indolently down, and sometimes they are suddenly stretched to their full length, and as suddenly retracted, and in all their varied convolutions the cirri that fringe them are in constant vibration, and exhibit all the tints of the rainbow. Sometimes these creatures whirl round their axis with great rapidity, but, active as they are, no nervous system has yet been discovered in them.