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Hydroida is an obsolete cnidarian order which united such animals as hydras, hydromedusae, and many marine attached hydroids. However, the group is paraphyletic and not composed from close relatives. But for the largest part, this group makes up what today is usually considered the subclass Leptolinae (or Hydroidolina) which also includes the colonial jellies of the Siphonophora which were not part of the Hydroida.
Examples of "hydroids" are:
- Craspedacusta sowerbyi, the freshwater "jellyfish"
- Air ferns, novelty "plants" and aquarium ornaments.
- Aequorea victoria of the suborder Leptomedusae, the crystal jelly- a bioluminescent jellyfish.