Cnidaria is a phylum containing over 10,000 species of animals found exclusively in aquatic and mostly marine environments. They are mainly jellyfish or sea anemones. Their bodies consist of a non-living jelly-like substance that is mostly one-cell thick. have two basic body forms: swimming and stationary, both of which are symmetrical with mouths surrounded by tentacles. Both forms have a single opening and body cavity that are used for digestion and respiration. Many cnidarian species produce colonies that are single organisms with many duplicates or sub-branches.