The Arctic Seas

pp 397-430

Hydrozoa of the Eurasian Arctic Seas

  • S. D. Stepanjants

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Hydrozoans comprise one of the four classes in the phylum Cnidaria. Most members of the class have bottom sedentary (seldom pelagic) polypoid and pelagical medusoid stages of the life cycle. The polypoid generation may be an individual organism or a colony with the chitinous (rarely calcareous) skeleton. The polypoid produces the medusoid stage. The medusoid stage is not always represented by a free-swimming medusa, but rather by the medusoid, which remains attached to the colony and produces the sexual stage. Sometimes the typical medusoid or the typical polypoid stage is absent.