An onidarian, more commonly referred to as a cnidarian, is a member of the phylum Cnidaria, which includes diverse marine animals such as jellyfish, corals, and sea anemones. Cnidarians are characterized by their radial symmetry, a simple body structure, and specialized cells called cnidocytes that contain stinging organelles used for capturing prey and defense. They typically have two main body forms: the polyp and the medusa. Cnidarians play crucial roles in marine ecosystems, particularly in coral reef environments.