hy·dro·zo·an 
(hī′drə-zōən)
(hī′drə-zōən)n.
Any of various cnidarians of the class Hydrozoa, including the hydras, hydroids, and siphonophores, which exhibit alternation of generations between the polyp and medusa stage or exist in only one or the other stage.
[From New Latin Hydrozōa, class name : Hydra, type genus; see HYDRA + -zōa, pl. of -zōon, -zoon.]
hy′dro·zoan adj.