Hydrozoa

Hydrozoa is a taxonomic class of individually very small, predatory animals, some solitary and some colonial, most living in salt water. The colonies of the colonial species can be large, and in some cases the specialized individual animals cannot survive outside the colony.

Classification

Hydrozoa are related to jellyfish and corals and belong to the phylum Cnidaria.

Characteristics

Hydrozoans are distinguished by having a life cycle that involves both asexual and sexual reproduction. Most hydrozoan species include both polyp and medusa forms in their life cycle, although a number of them have only one or the other.

Life Cycle

In those hydrozoans with both a polyp and a medusa stage, the polyp stage often forms colonies, with each individual polyp in the colony specialized for a different task. Thus, some polyps, known as gastrozooids, are specialized for feeding. Others, known as gonozooids, are responsible for reproduction.

Reproduction

In most species, the reproductive polyps release free-swimming medusae. Each medusa is either male or female and produces eggs or sperm which combine to create free-swimming, ciliated larvae. In some species, these stages may be suppressed or absent. Some hydrozoans live in freshwater; these include the "hydra" that is often studied in school biology courses.

Distribution

Hydrozoans are found in every ocean and sea, at every depth, and in many freshwater environments.

See Also

• Cnidaria
• Jellyfish
• Coral

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