Cnidarians are a diverse group of aquatic animals that include jellyfish, coral, and sea anemones. They are characterized by their radial symmetry and the presence of specialized cells called cnidocytes, which contain stinging structures used for capturing prey and defense. Cnidarians typically have two main body forms: the polyp, which is stationary, and the medusa, which is free-swimming. These organisms primarily inhabit marine environments, although some can be found in freshwater. Cnidarians have a simple body structure with a central cavity for digestion and a single opening that serves as both mouth and anus. They play essential roles in ecosystems, particularly in coral reefs, which provide habitat for many marine species.