Kalmar "Hydrozoa" tana nufin nau'in dabbobi masu rarrafe waɗanda ke cikin phylum Cnidaria, wanda ya haɗa da murjani, jellyfish, da anemones na teku. Hydrozoans da farko dabbobi ne na ruwa kuma ana siffanta su da kasancewar ƙwararrun ƙwayoyin cuta da ake kira nematocysts, waɗanda suke amfani da su don tsaro da kama ganima. Yawanci suna da nau'in polyp, wanda aka haɗa su zuwa sama, da nau'in medusa, wanda a cikinsa suke yin iyo. Wasu misalan gama-gari na hydrozoans sun haɗa da mutumin-yakin Portuguese da hydra na ruwa.