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The Yekta of the Crimson Sea, are as extraordinary developments of hydroid forms as the giant Medusae, of which, of course, they are not too remote cousins. The closest resemblances to them in outer water forms are among the Gymnoblastic Hydroids, notably Clavetella prolifera, a most interesting ambulatory form of six tentacles.
The proboscis, so characteristic of all Jelly-Fishes, hangs from the central opening; and the tentacles, coiled within the internal cavity up to this time, now make their appearance, and we have a complete little Medusa growing upon the Hydroid head.
So each generation of these interesting creatures is entirely unlike that which immediately precedes or follows it. In other words, a hydroid is exactly like its grandmother and granddaughter, but as different from its parents and children in appearance as a plant is from an animal.
It swims freely about, a transparent, umbrella-like disk, with a proboscis hanging from the lower side, which, to complete the comparison, we may call the handle of the umbrella. The margin of the disk is even more deeply lobed than in the Hydroid condition, and in the middle of each lobe is a second depression, quite deep and narrow, at the base of which is an eye.
For a further description of this largest and handsomest of our Hydroid Polypes, I must refer you to Johnston, or, failing him, to Landsborough; and go on, to beg you not to despise those pink, or grey, or white lumps of jelly, which will expand in salt water into exquisite sea-anemones, of quite different forms from any which we have found along the rocks.
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.
"Great heavens," exclaimed Professor Abel Able, "it is a gigantic hydroid polyps! That is precisely the way in which those little creatures swallow their prey; outside becomes inside, what was the surface of the body is turned into the lining of the digestive cavity, and every time they take a meal the process of introversion is repeated. This monster is nothing but a huge self-sustaining maw!"
But in the case I have been describing, while the Hydroid gives birth to the Medusa, it still preserves its own distinct existence; and the different forms developed on one stock seem to be two parallel lives, and not the various phases of one and the same life.
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.
At a lower level in animal life, that of the hydroid polyps, communism has become so complete that the community has grown into an actual individual, the members not being free, but acting as organs of an aggregate mass, in which each performs some special duty for the good of the community. The social animals differ from the communal in that the individuality of the members is fully preserved.
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