Hydroids, the lowest order of acalephs or jelly fishes, including, according to Agassiz, two distinct forms, one resembling polyps, the other like the jelly fishes, there being every possible gradation between the two. , It is in this order that the phenomena of alternate generation have been specially studied by Sars and others. (See Jelly Fish.) There are many plant-like forms which give a mossy cover-ering to seaweeds and stones, producing buds, developing in some cases into free medusa), and in others remaining attached to the parent stalk, both discharging ova which swim off by ciliary processes to establish new fixed hydroid communities. In the tubularians the hydroid is pedunculated, and the bell-shaped medusae are either free as in coryne or. persistent as in tubularia. In the sertularians the hydroid is always pedunculated and attached, protected by a horny sheath, forming a cup around the head, with free medusaa as in campanularia, or free generative buds as in sertularia; their medusae are flatter than in tubularians.

The siphonophorce, like the Portuguese man-of-war, are also hydroid communities. - The common green hydra of fresh water (hydra viridis) is easily seen by the naked eye; the body is a cylindrical tube, with thread cells, and a green coloring matter believed to be the same as the chlorophyl of plants; at the base is a disk-like sucker for its attachment to foreign bodies; it is usually suspended, head downward, from some aquatic plant, changing its position at will. The mouth is at the opposite end, surrounded by 5 to 15 very contractile tentacles, armed with lasso cells, hollow, and communicating with the general and stomachal cavity of the body; by these they obtain their food, which consists of minute aquatic animals. There are no internal organs of any kind, and they are therefore very little higher than the protozoa. They resist without destruction a very great degree of mutilation, each fragment into which they may be divided being capable, according to Trembley, of becoming a complete individual.

Reproduction is either non-sexual, by gemmation in summer, or sexual, by ova and sperm cells in autumn; the buds develop a mouth and tentacles at the free end, and are soon detached, each in its turn producing similar buds; both ova and sperm cells are produced in the same individual, coining in contact in the water; the embryo is at first ciliated and free swimming, afterward becoming fixed, losing the cilia, and developing a mouth and tentacles.

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