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Hydrozoa
Hydrozoa (hydrozoans, from ancient Greek ὕδρα, ''hydra'', "sea serpent" and ζῷον, ''zoon'', "animal") are 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. A few genera within this class live in fresh water. Hydrozoans are related to jellyfish and corals and belong to the phylum Cnidaria.
Some examples of hydrozoans are the freshwater jelly (''Craspedacusta sowerbyi''), freshwater polyps (''Hydra''), ''Obelia'', Portuguese man o' war (''Physalia physalis''), chondrophores (Porpitidae), "air fern" (''Sertularia argentea''), and pink-hearted hydroids (''Tubularia'').
==Anatomy==
Most hydrozoan species include both a polypoid and a medusoid stage in their lifecycles, although a number of them have only one or the other. For example, ''Hydra'' has no medusoid stage, while ''Liriope'' lacks the hydroid stage.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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