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.
In this curious Acalephian Hydroid, or Physalia, the individuality of function is even more marked than in the Hydractinia.
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.
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.
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.