The phylum Cnidaria includes the corals and related animals characterized by having stinging cells called nematocysts. They use those stinging cells to immobilize prey. Altogether, there are about 10,000 species.

The group of corals (Class Anthozoa) is divided into the hexacorals, octocorals, and tube-dwelling anemones. The hexacorals include the stony corals, black corals, and the sea anemones that often host anemonefish. The octocorals include the soft corals and the gorgonians. The coral polyps of the two groups have multiples of six and eight tentacles, respectively. The former tend to form a calcium carbonate base, while the latter have soft bodies with calcium carbonate fibers for support. The tube-dwelling anemones construct tubes of fibrous material. All of these animals attach themselves to the seabed.

This phlyum also includes the jellyfish, which typically float freely in the water. The different types are true jellyfish, box jellyfish and stalked jellyfish (Classes Syphozoa, Cubozoa, and Staurozoa).

Lastly, the phylum includes the hydrozoans (Class Hydrozoa). Those are animals which form colonies such as hydroids, fire corals and medusas.