Organisms of the Class Hydrozoa belong to the phylum Cnidaria. Hydrozoans are mostly marine, though a few are freshwater. Hydrozoans may be solitary or colonial, and are extremely diverse, both in life-cycle forms and morphology. Most hydrozoan species have both a polyp and medusa stage, but others make use of only one or the other.
Like other Cnidarians, Hydrozoans keep form through the use of a hydrostatic exoskeleton. Colonial hydrozoans typically have both a medusa stage and a polyp stage in their lifecycle. They have a base, a stalk, and one or more polyps. Hydrozoan colonies are composed of a number of specialized polyps - including feeding, reproductive, and protective zooids.