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If a Bipinnaria, a Brachiolaria, a Pluteus, is competent to produce the Echinoderm, which is so widely different from it; if a hydroid polype can produce the higher Medusa; if the vermiform Trematode 'nurse' can develop within itself the very unlike Cercaria, it will not appear impossible that the egg, or ciliated embryo, of a sponge, for once, under special conditions, might become a hydroid polype, or the embryo of a Medusa, an Echinoderm."
They swim about for a time, until they have found a resting-place, where they attach themselves, each one founding a Hydroid stock of its own, which will in time produce a new brood of Medusae. This series of facts, presented here in their connection, had been observed separately before their true relation was understood.
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.
If a 'Bipinnaria', a 'Brachialaria', a 'Pluteus', is competent to produce the Echinoderm, which is so widely different from it; if a hydroid polype can produce the higher Medusa; if the vermiform Trematode 'nurse' can develop within itself the very unlike 'Cercaria', it will not appear impossible that the egg, or ciliated embryo, of a sponge, for once, under special conditions, might become a hydroid polype, or the embryo of a Medusa, an Echinoderm."
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.
To very few does this term recall either the large Discophore, with its purple disk and its long streamers floating perhaps twenty or thirty feet behind it as it swims, or the Ctenophore, with its more delicate, transparent structure, and almost invisible fringes in parallel rows upon the body, which decompose the rays of light as the creature moves through the water, so that hues of ruby-red and emerald-green, blue, purple, yellow, all the colors of the rainbow, ripple constantly over its surface when it is in motion, or the Hydroid, with its little shrub-like communities living in tide-pools, establishing themselves on rocks, shells, or sea-weeds, and giving birth not only to animals attached to submarine bodies, like themselves, but also to free Medusae or Jelly-Fishes that in their turn give birth again to eggs which return to the parent-form, and thus, by alternate generations, maintain two distinct patterns of animal life within one cycle of growth.
This process of normal self-division may take place at all periods of life: it may form an early phase of metamorphosis, as in the Hydroid of our common Aurelia, described in the last article; or it may even take place before the young is formed in the egg.
At any time late in February or early in March, bulb-like projections, more globular than the somewhat elongated buds of the true Hydroid heads, may be seen growing either among the tentacles of one of these little animals, or just below the head where it merges in the stem, Very delicate and transparent in substance, it is hardly perceptible at first; and the gradual formation of its internal structure is the less easily discerned, because a horny sheath, forming the outer covering of the Hydroid stock, extends to inclose and shield the new-comer, whom we shall see to be so different from the animal that gives it birth that one would suppose the Hydroid parent must be as much surprised at the sight of its offspring as the Hen that has accidentally hatched a Duck's egg.
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.
It was a hydroid, a development of that strange animal-vegetable that, sometimes almost microscopic, waves in the sea depths like a cluster of flowers paralyzing its prey with the mysterious force that dwells in its blossom heads! "Put it down, Lakla," the distress in O'Keefe's voice was deep.
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