Hydroida

(invertebrate zoology) An order of cnidarians in the class Hydrozoa including usually colonial forms with a well-developed polyp stage.

An order of the coelenterates which includes the fresh-water hydras, the attached and usually colonial hydroids, and many of the smaller jellyfish. It is the largest order of the class Hydrozoa.

The order Hydroida includes two principal suborders, Gymnoblastea and Calyptoblastea. The Gymnoblastea are those hydroids which lack protective cups around the hydranths and gonozoids. Jellyfish produced by these athecate hydroids are Anthomedusae. The Calyptoblastea include the hydroids with protective cups around the hydranths (hydrothecae) and around the gonozoids (gonothecae). Jellyfish of these thecate hydroids are called Leptomedusae. Two minor suborders are Limnomedusae and Chondrophora.

Anthomedusae are typically ovoid jellyfish, often with eye-spots. Leptomedusae are usually flattened or saucer-shaped, have statocysts (sense organs of balance), and lack eyespots. The gonads of Anthomedusae are generally on the wall of the stomach just above the mouth, and those of Leptomedusae below the radial canals.

Young hydranths of gymnoblastic hydroids are small with five or more tentacles, but subsequently grow much larger and add more tentacles. Calyptoblastic hydranths, in contrast, emerge from a bud with a full complement of parts; they do not grow, live for only about a week, undergo regression and absorption by the colony, and are then replaced by new hydranths.

The fresh-water hydras are simple, motile polyps which do not produce colonies (see illustration). Buds separate from the parent and become individual polyps. Simple gonads develop on the body and there is no medusa. Hydras are sometimes included in the Gymnoblastea and sometimes placed in a suborder by themselves, the Hydrida.

Hydra. (a) Longitudinal section. (b) Movements. (After T. I. Storer et al., General Zoology, 6th ed., McGraw-Hill, 1979)

Hydroids are species in which the polyp stage is usually dominant. Most hydroids are found near the shore attached to various supports such as rocks, wharves, boats, mussels, barnacles, worm tubes, crab and snail shells, and seaweeds. Sometimes the medusa stage is well developed and the hydroid stage may be lacking. Medusae are abundant both in coastal waters and in the open sea.

Hydras and hydroids have been used extensively in research on problems of growth, development, and regeneration. They have a high capacity for reorganization. Missing parts are quickly replaced, a bit of stem can produce a new hydranth, and completely disorganized masses of cells can reconstitute a new polyp. See also Hydrozoa; Regeneration (biology).

This article does not cite any references or sources. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (December 2009)

Hydroida is an obsolete cnidarian order which united such animals as hydras, hydromedusae, and many marine attached hydroids. However, the group is paraphyletic and not composed from close relatives. But for the largest part, this group makes up what today is usually considered the subclass Leptolinae (or Hydroidolina) which also includes the colonial jellies of the Siphonophora which were not part of the Hydroida.

The "hydroid" cnidarians typically which grow up into large, elegantly branched forms. All the zooids of a colony are asexually produced from one parent zooid.

Examples of "hydroids" are:

• Craspedacusta sowerbyi, the freshwater "jellyfish"
• Hydra
• Air ferns, novelty "plants" and aquarium ornaments.
• Obelia
• Aequorea victoria of the suborder Leptomedusae, the crystal jelly- a bioluminescent jellyfish.

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)