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Cnidarians are characterized by their radial symmetry and stinging cells called nematocysts, which they use to capture prey. They are exclusively marine organisms and can be found in various habitats, such as coral reefs, open ocean, and seabeds. The phylum Cnidaria includes jellyfish, sea anemones, hydra, and corals. These animals exhibit two body forms: the polyp, which is typically sessile (attached to a substrate), and the medusa, which is free-swimming. Cnidarians play essential roles in marine ecosystems as predators and prey. They form complex relationships with other organisms, such as symbiotic associations with microalgae or providing shelter for other animals within their tissues. The Cnidaria subcategory under Invertebrates highlights the unique features of these animals while emphasizing their importance in marine environments. This subcategory is a natural fit alongside related terms like Animals, Animals By Taxonomy, and Invertebrates since cnidarians are a diverse group within the animal kingdom that does not possess a backbone or any other internal skeletal structures.