; phylum Cnidaria, class Hydrozoa)
An order of cnidarians that have a well-developed polypoid generation. They are fixed, solitary or colonial, and many have a chitinous exoskeleton, although some (e.g. Hydra) are naked. There are free medusae or aborted structures on the polyps. The medusae have ocelli or ectodermal statocysts, or both.
Subjects: Zoology and Animal Sciences.