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und in the Silurian rocks. They belong to an extinct group (Graptolithina) supposed to be hydroids.

A division of Hydroidea including the hydra. See Hydra.

; the hydromedusæ. Most of them are known to be the free-swimming progeny (gonophores) of hydroids.

Any small fresh-water hydroid of the genus Hydra, usually found attached to sticks, stones, etc., by a basal sucker.

Any species or marine hydroids, of the genus Hydractinia and allied genera. These hydroids form, by their rootstalks, a firm, chitinous coating on shells and stones, and esp. on spiral shells occupied by hermit crabs. See Illust. of Athecata.

One of the nutritive zooids of a hydroid colony. Also applied to the proboscis or manubrium of a hydroid medusa. See Illust. of Hydroidea.

The group of hydroids to which the fresh-water hydras belong.

The hollow stem of a hydroid, either simple or branched. See Illust. of Gymnoblastea and Hydroidea.

A division of Hydroidea, including those genera that secrete a stony coral, as Millepora and Stylaster. Two forms of zooids in life project from small pores in the coral and resemble those of other hydroids. See Millepora.

Any medusa or jellyfish which is produced by budding from a hydroid. They are called also Craspedota, and naked-eyed medusæ.

The Hydroidea.

The rootstock or decumbent stem by which a hydroid is attached to other objects. See Illust. under Hydroidea.

All the zooids of a hydroid colony collectively, including the nutritive and reproductive zooids, and often other kinds.

One of the calicles which, in some Hydroidea (Thecaphora), protect the hydrants. See Illust. of Hydroidea, and Campanularian.

The Acalephæ; one of the classes of coelenterates, including the Hydroidea, Discophora, and Siphonophora.

of having similar sexual zooids or gonophores and dissimilar hydrants; -- said of certain hydroids.

animals of other groups. They are most highly developed in the tentacles of jellyfishes, hydroids, and Actiniæ. Each of these cells is filled with, fluid, and contains a long, slender, often barbed, hollow thread coiled up within it. When the cell contracts the thread is quickly ejected, being at the same time turned…

A kind of gonophore produced by hydroids of the genus Gonothyræa. It has tentacles, and otherwise resembles a free medusa, but remains attached by a pedicel.

tructure of a medusa, but without a locomotive disk; -- said of the sessile gonophores of hydroids. -- n.

e nearly smooth, and perforated with very minute unequal pores, or cells. The animals are hydroids, not Anthozoa. See Hydrocorallia.