Hydrozoans are small, predatory animals with stinging cells belonging to the Phylum Cnidaria with true jellyfish, coral, and sea-anemone. Most hydrozoans have an alternation of generations which includes a sessile, benthic polyp stage, and a plankktonic medusa, jellyfish stage. Siphonophores are colonial hydrozoans that are composed of both medusa and polyp stages. Read more about hydrozoans at Wikipedia.
