It is not strange that the relation between the various phases of this extraordinary series of metamorphoses, so different from each other in their external aspects, should not have been recognized at once, and that this singular Acaleph should have been called Scyphostoma in its simple Hydroid condition, Strobila after the transverse division of the body had taken place, Ephyra in the first stages of its free existence, and Aurelia in its adult state,—being thus described as four distinct animals. These various forms are now rightly considered as the successive stages of a development intimately connected in all its parts,—beginning with the simple Hydroid attached to the ground, and closing in the shape of our common Aurelia, with its white transparent disk, its silky fringe of tentacles around the margin, its ruffled curtains hanging from the mouth, and its four crescent-shaped ovaries grouped to form a cross on the summit. From these ovaries a new brood of little embryos is shed in due time.
There are other Hydroids giving rise to Medusae buds, from which, however, the Medusae do not separate to begin a new life, but wither on the Hydroid stock, after having come to maturity and dropped their eggs. Such is the Hydractinia polyclina. This curious community begins, like the preceding ones, with a single little individual, settling upon some shell or stone, or on the rocks in a tide-pool, where it will sometimes cover a space of several square feet. Rosy in color, very soft and delicate in texture, such a growth of Hydractinia spreads a velvet-like carpet over the rocks on which it occurs. They may be kept in aquariums with perfect success, and for that purpose it is better to gather them on single shells or stones, so that the whole community may be removed unbroken. These colonies of Hydractinia have one very singular character: they exist in distinct communities, some of which give birth only to male, others to female individuals. The functions, also, are divided,—certain members of the community being appointed to special offices, in which the others do not share. Some bear the Medusae buds, which in due time become laden with eggs, but, as I have said, wither and die after the eggs are hatched. Others put forth Hydroid buds only, while others again are wholly sterile. About the outskirts of the community are more simple individuals, whose whole body seems to be hardly more than a double-walled tube, terminating in a knob of lasso-cells. They are like long tentacles placed where they can most easily seize the prey that happens to approach the little colony. The entire community is connected at its base by a horny net-work, uniting all the Hydroid stems in its meshes, and spreading over the whole surface on which the colony has established itself.
There is a very curious and beautiful animal, or rather community of animals, closely allied to the Hydractinia polyclina, which next deserves to be noticed. The Portuguese Man-of-War—so called from its bright-colored crest, which makes it so conspicuous as it sails upon the water, and the long and various streamers that hang from its lower side—is such a community of animals as I have just described, reversed in position, however, with the individuals hanging down, and the base swollen and expanded to make the air-bladder which forms its brilliant crested float. In this curious Acalephian Hydroid, or Physalia, the individuality of function is even more marked than in the Hydractinia. As in the latter, some of the individuals are Medusae-bearing, and others simple Hydrae; but, beside these, there are certain members of the community who act as swimmers, to carry it along through the water,—others that are its purveyors, catching the prey, by which, however, they profit only indirectly, for others are appointed to eat it, and these feeders may be seen sometimes actually gorged with the food they have devoured, and which is then distributed throughout the community by the process of digestion and circulation.
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.
The third Order, the Ctenophorae, are among the most beautiful of the Acalephs. I have spoken of the various hues they assume when in motion, and I will add one word of the peculiarity in their structure which causes this effect. The Ctenophorae differ from the Jelly-Fishes described above in sending off from the main cavity only two main tubes, instead of four like the others; but each of these tubes divides and subdivides in four branches as it approaches the periphery. From the eight branches produced in this way there arise vertical tubes extending in opposite directions up and down the sides of the body. Along these vertical tubes run the rows of little locomotive oars, or combs, as they have been called, from which these animals derive their name of Ctenophorae. The rapid motion of these flappers causes the decomposition of the rays of light along the surface of the body, producing the most striking prismatic effect; and it is no exaggeration to say that no jewel is brighter than these Ctenophorae as they move through the water.
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