Hydromedusae is a term used to refer to a group of aquatic organisms that belong to the phylum Cnidaria. Synonyms for the word hydromedusae include hydroids, hydrozoans, and jellyfish. Hydroids are colonial cnidarians with both polyp and medusa stages in their life cycle. Hydrozoans are a diverse group of cnidarians that include jellyfish, hydroids, and freshwater polyps. Jellyfish are free-swimming marine animals with a gelatinous umbrella-shaped body and trailing tentacles armed with stinging cells. All of these synonyms refer to aquatic organisms that possess stinging cells and are capable of delivering a painful sting to humans and other animals.