Hydrozoa class of the phylum Cnidaria, with small and plant-like colonies of polyps, and with small and fragile jellyfishes as generative stages, called hydromedusae. In most hydrozoans, the polyps form colonies with individuals attached to the central hydrorhiza, which creeps over the substrate. Hydrozoans feed on fine suspended material collected with tentacles or, as hydromedusae, generally on zooplankton. Some hydrozoans (Siphonophora) form great floating colonies of specialized polyps, like the Portuguese man-of-war.