Hydrozoa is a taxonomic class of very small, predatory animals which can be solitary or colonial and which mostly live in saltwater. A few genera within this class live in freshwater. These organisms are related to jellyfish and corals and belong to the phylum Cnidaria. Some examples of hydrozoans are: Hydra, Obelia, Portuguese Man o' War (Physalia), Chondrophores, air ferns (Sertularia argenta), and Tubularia. Hydra, a freshwater genus The most widely-known and researched freshwater hydrozoa… (More on Hydrozoa)