A class of the phylum cnidaria which includes the fresh-water hydras, the marine hydroids, many of the smaller jellyfish, a few special corals, and the Portuguese man-of-war. The Hydrozoa may be divided into seven orders: the Hydroida, Milleporina, Stylasterina, Trachylina, Chondrophora, Siphonophora, and Spongiomorhipa. See separate article on each order.