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Hydroida (Leptolinida, hydroids; 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.