[Illustration: Physalia, or Portuguese Man-of-War.]
It would be hopeless, even were it desirable, to attempt within the limits of such an article as this to give the faintest idea of the number and variety of these Hydroids; and I will therefore say nothing of the endless host of Tubularians, Campanularians, Sertularians, etc. They are very abundant along our coast, and will well reward any who care to study their habits and their singular modes of growth. For their beauty, simply, it is worth while to examine them. Some are deep red, others rosy, others purple, others white with a glitter upon them, as if frosted with silver. Their homes are very various. Some like the fresh, deep sea-water, while they avoid the dash and tumult of the waves; and they establish themselves in the depressions on some low ledge of rocks running far out from the shore, and yet left bare for an hour or two, when the tide is out. In such a depression, forming a stony cup filled with purest sea-water, overhung by a roof of rock, which may be fringed by a heavy curtain of brown sea-weed, the rosy-headed, branching Eudendrium, one of the prettiest of the Tubularians, may be found. Others like the tide-pools, higher up on the rocks, that are freshened by the waves only when the tide is full: such are the small, creeping Campanularians. Others, again, like the tiny Dynamena, prefer the rougher action of the sea; and they settle upon the sides of rents and fissures in the cliffs along the shore, where even in calm weather the waves rush in and out with a certain degree of violence, broken into eddies by the abrupt character of the rocks. Others seek the broad fronds of the larger sea-weeds, and are lashed up and down upon their spreading branches, as they rock to and fro with the motion of the sea. Many live in sheltered harbors, attaching themselves to floating logs, or to the keels of vessels; and some are even so indifferent to the freshness of the water that they may be found in numbers along the city-wharves.[4]
Beside the Jelly-Fishes arising from Hydroids, there are many others resembling these in all the essential features of their structure, but differing in their mode of development; for, although more or less Polyp-like when first born from the egg, they never become attached, nor do they ever bud or divide, but reach their mature condition without any such striking metamorphoses as those that characterize the development of the Hydroid Acalephs. All the Medusas, whether they arise from buds on the Hydroid stock, like the Sarsia, or from transverse division of the Hydroid form, like the Aurelia, or grow directly from the egg to maturity, without pausing in the Hydroid phase, like the Campanella, agree in the general division and relation of parts. All have a central cavity, from which arise radiating tubes extending to the margin of the umbrella-like disk, where they unite either in a net-work of meshes or in a single circular tube. But there is a great difference in the oral apparatus; the elaborate ruffled curtains, that hang from the corners of the mouth, occur only in the Species arising from the transverse division of the Polyp-like young. For this reason they are divided into two Orders,--the Hydroids and the Discophorae.