Page:Popular Science Monthly Volume 8.djvu/39

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29
HYDROIDS.

zoöid often returns to its hydra-life and reproduces itself by budding in the old fashion, and finally becomes "the progenitor of a new colony, every member of which may in its turn bud off a pile of medusa-disks."

The bodies thus detached have all the characteristics of the fully-developed medusæ. Each consists of an umbrella-shaped disk divided along its margin into lobes, generally eight in number, and of a stomach terminating in a probosciform mouth. As the creature grows, the spaces between the marginal lobes fill up; from its border long tentacles are developed, and a fringe of tendril-like filaments sprout forth from the margin. The young medusa eats voraciously, and grows proportionately large; the Chrysaora, which we have been describing, attaining a diameter of fifteen inches, and the Rhizostoma sometimes reaching to three feet. These medusæ are familiarly known as sea-nettles. When they have reached full development the generative organs appear in four chambers arranged round the stomach, and are contained in curious fluted membranous ribbons which hold the sperm-cells in the male, and ova in the female. The fertilized embryos repeat the same wonderful cycle just described, developing into a hydroid from which medusa-disks are budded off.

The relation which late investigations have established between the stationary hydroids, and the medusæ, forms one of the most interesting cases, yet known, of the curious phenomenon called alternate generation. In the majority of cases we find a non-sexual, plant-like form interposed between the ovum and the directly or indirectly sexual form of medusa, though this is not always the case, as direct development has been observed from ovum to medusa.

The nearest approach, in the adult form, to special organs are the digestive cavity, and the cnidæ. The stomach, however, possesses no true parietal walls, and in one form—the fresh-water hydra—the stomach will do duty for the skin, and the skin for the stomach, if necessary; they seem to be able to live very comfortably, and digest their food without difficulty when turned wrong-side outward.

The cnidæ are barbed filaments inclosed in tiny sacs, which they can shoot out at will, for their own protection, or for the capture of their prey, as the case may be. In the hydra the sac is ejected, and a central dart is projected into the body attacked. There must be a minute poison-sac in communication with the darts, as it is found that any soft-bodied victim, released from the clasp of the tentacles, is invariably dead, no matter how short the time of its imprisonment may have been. The effects of the cnidæ in the medusae are very well known, and have gained for them their popular name of sea-nettles. Many an unlucky swimmer has found himself wrapped in the long thread-like filaments of these transparent, floating bells, and been almost maddened as he found himself inextricably inclosed in what