Hydrozoa

• WordNet 3.6
  • n Hydrozoa coelenterates typically having alternation of generations; hydroid phase is usually colonial giving rise to the medusoid phase by budding: hydras and jellyfishes
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• • n. pl Hydrozoa (Zoöl) The Acalephæ; one of the classes of cœlenterates, including the Hydroidea, Discophora, and Siphonophora.
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• • Hydrozoa A class of Cœlenterata, one of two prime divisions of that subkingdom, Actinozoa being the other. The Hydrozoa are aquatic and chiefly marine organisms, single or oftener compound, and nearly always soft and gelatinous (in some cases with a chitinous perisarc). Each individual polypite consists essentially of a simple sac composed of an outer (ectodermal) and an inner (endodermal) membrane, with a simple gastrovascular cavity or stomach-sac, not differentiated into an esophageal tube nor separated from the general body-cavity, developed as an outward process of the bodywall, and usually furnished with tentacular processes. The reproductive organs are external to the body. Reproduction is accomplished either by sexual elements (ova and spermatozoa) or by gemmation or fission; the generative zoöids are developed as medusoid organisms, which may become detached and free-swimming, or remain permanently attached to the parent stock. The class is of world-wide distribution, and includes the numerous creatures known as hydroids, acalephs, medusans, jelly-fish, sea-blubbers, etc. Their forms are endlessly varied, and range in complexity from the simple fresh-water hydra to the complicated structure of the oceanic hydroids, as the Portuguese man-of-war. The classification of the Hydrozoa varies with different writers, and it is difficult to define most of the larger groups into which they have been divided. They are separated into from three to six groups, as the Hydrophora, Discophora, and Siphonophora of Huxley's arrangement, or the Hydroida, Siphonophora, Lucernarida, Discophora, Graptolitidæ, and Hydrocorallinæ of Nicholson's arrangement. The class is sometimes called Polypomedusæ. It was named as a class of Polypi by Owen in 1843.
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• • n.pl Hydrozoa hī-dro-zō′a (sing. Hydrozō′on) one of the main divisions of the sub-kingdom Cœlenterata, the other two being Ctenophora and Anthozoa or Actinozoa; they are chiefly marine organisms, soft and gelatinous, free or fixed, existing everywhere, endlessly varied in form and complexity of structure, including such great groups as hydroids, acalephs, medusans, jelly-fish, sea-blubbers, &c
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