hydroid

/huy"droyd/, adj.
1. noting or pertaining to that form of hydrozoan that is asexual and grows into branching colonies by budding.
n.
2. the phase of a hydrozoan coelenterate that consists of polyp forms usually growing as an attached colony.
[1860-65; HYDR(A) + -OID]

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      any member of the invertebrate order Hydroida (class Hydrozoa, phylum Cnidaria). Hydroids have three basic life-cycle stages: (1) a tiny free-swimming ciliated planula larva about 1 mm (0.04 inch) long, which settles and metamorphoses into (2) a sessile (attached), usually colonial polyp stage, which in turn liberates (3) a gamete-producing male or female medusa (“jellyfish”). This cycle is exemplified by the genus Obelia (q.v.), whose members are widely distributed throughout the world. Many hydroids have, through evolution, suppressed the medusa by retaining it on the sessile hydroid colony. Colonies of hydroids are typically 5 to 500 mm (0.2 to 20 inches) or more high and are branched; the branches bear the individuals, or zooids (hydroid polyps). Each zooid consists of a tubular body that has two layers separated by a thin, jellylike mesoglea (layer of connective tissue), a terminal mouth, and surrounding circlet(s) of tentacles. The zooids are joined basally to a common living tube called the stolon that runs the length of the colony. The living tube, which is assumed to permit the exchange of food between individuals, is protected within a tough, chitinous sheath, the perisarc. Colonies of hydroids grow vegetatively by increase in the number of hydranths. Reproductive polyps (gonozooids) occur intermittently on the colony. They release either medusae (typically) or planula larvae (if the medusae are retained or reduced), depending on the species. Members of some species can retract their polyps within a protective extension of the perisarc, the hydrotheca, but others lack such a structure. Most hydroids inhabit marine environments, but some have invaded freshwater habitats. An example is the Hydra (q.v.), a genus that has the distinction of being solitary rather than colonial. In addition, the life cycle of Hydra lacks a medusa stage. There are about 2,000 species of hydroids.

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Universalium. 2010.

Look at other dictionaries:

  • Hydroid — may refer to:Marine BiologyColonial, plant like animals closely related to jellyfish, with stinging cellsany member of the invertebrate order Hydroida (class Hydrozoa, phylum Cnidaria). Hydroids have three basic life cycle stages: (1) a tiny free …   Wikipedia

  • hydroid — [hī′droid΄] adj. [ HYDR(A) + OID] 1. like a hydra or polyp 2. of or related to an order (Hydroida) of hydrozoans, including the hydras and many colonial marine species n. any hydroid hydrozoan …   English World dictionary

  • Hydroid — Hy droid, a. [Hydra + oid.] (Zo[ o]l.) Related to, or resembling, the hydra; of or pertaining to the Hydroidea. n. One of the Hydroideas. [1913 Webster] …   The Collaborative International Dictionary of English

  • hydroid — hy·droid hī .drȯid adj of or relating to a hydrozoan esp resembling a typical hydra hydroid n HYDROZOAN esp a hydrozoan polyp as distinguished from a medusa …   Medical dictionary

  • hydroid — I. adjective Etymology: ultimately from New Latin Hydra Date: circa 1864 of or relating to a hydrozoan; especially resembling a typical hydra II. noun Date: 1865 hydrozoan; especially a hydrozoan polyp as distinguished from a hydrozoan jellyfish …   New Collegiate Dictionary

  • hydroid — n. [Gr. hydor, water; eidos, like] (CNIDARIA) Polyp of coelenterates; any member of the Hydroida …   Dictionary of invertebrate zoology

  • hydroid — 1. noun Any of many colonial coelenterates that exist mainly as a polyp; a hydrozoan 2. adjective Of or pertaining to such creatures …   Wiktionary

  • hydroid — adj. pertaining to or characteristic of an asexual hydrozoan polyp (Zoology) …   English contemporary dictionary

  • hydroid — [ hʌɪdrɔɪd] noun Zoology a coelenterate of an order (Hydroida) which includes the hydras …   English new terms dictionary

  • hydroid — hy·droid …   English syllables

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