HydrozoaHy`dro*zo"a\, n. pl. [NL., fr. E. hydra + Gr. ? an animal.] (Zo["o]l.) The Acaleph[ae]; one of the classes of c[oe]lenterates, including the Hydroidea, Discophora, and Siphonophora.
|coelenterates typically having alternation of generations; hydroid phase is usually colonial giving rise to the medusoid phase by budding: hydras and jellyfishes|