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.