The exclusively aquatic phylum Cnidaria is represented by polyps such as sea anemones and corals, and by medusae such as jellyfish. A polypoid or a medusoid cnidarian is a radially or biradially symmetrical, uncephalized animal with a single body opening, the mouth. The mouth is surrounded by tentacles studded with microscopic stinging capsules known as nematocysts that are the agents of offense and defense. The less preferred name of the phylum, Coelenterata, is based on this attribute.
All cnidarians are carnivorous, with cnidae and tentacles active in prey capture.
Green algal symbionts, termed zoochlorellae, occur in both marine and freshwater cnidarians. Anthozoa is the absence of a medusa. The cnidarian larva is the planula, a pear-shaped, entirely ciliated animal. At metamorphosis, the planula settles on and attaches to the substratum, where it metamorphoses into a polyp. The primary polyp produces additional polyps asexually, by budding, stolonic outgrowth, or some other process, to form a clone or a colony.