A group of predominantly marine species of invertebrate animals known for their specialized cells called cnidocytes, which they use primarily for capturing prey. This category includes diverse forms such as jellyfish, sea anemones, corals, and hydras. They are characterized by their radial symmetry, presence of a single body cavity, and complex life cycle often involving asexual and sexual phases. They are a fundamental part of the animal kingdom and have significant ecological roles in their habitats.