An order of the class Hydrozoa of the phylum Coelenterata, characterized by an extremely complex organization of components of several different types, some having the basic structure of a jellyfish, others of a polyp. The components may be connected by a stemlike region or may be more closely united into a compact organism. The medusalike components include swimming bells, a gas-filled float, bracts which are tough gelatinous bodies presumed to be protective, and gonophores which produce the eggs and sperms. Polyplike components include gastrozooids with mouth and one long hollow tentacle; dactylozooids without mouth and often with one tentacle; and gonozooids which bear the gonophores.