(n)coelenterates typically having alternation of generations; hydroid phase is usually colonial giving rise to the medusoid phase by budding: hydras and jellyfishes, Syn.class Hydrozoa
‖n. pl. [ NL., fr. E. hydra + Gr. zw^, on an animal. ] (Zool.) The Acalephæ; one of the classes of cœlenterates, including the Hydroidea, Discophora, and Siphonophora. [ 1913 Webster ]