Hydractinia Genome Project Portal

Cnidaria is a highly diverse invertebrate phylum that includes corals, jellyfish, sea anemones, and the common laboratory hydrozoan Hydra. Other, less familiar cnidarians include sea fans, siphonophores, zoanthids, and the parasitic myxozoans. The character that unites all cnidarians is the presence of unique stinging or adhesive structures called cnidae. Cnidae are post-golgi capsules made in a specific sensory cell type called a cnidocyte. The most common cnidae are used for stinging and called nematocysts.

Many cnidarians form colonies by asexual reproduction. Many species of cnidarians also exhibit a dimorphic life cycle that includes two different adult morphologies: the polyp and the medusa (i.e., a jellyfish). Cnidarians lack cephalization, a centralized nervous system, and distinct respiratory or excretory systems. As the sister group to bilaterian animals, they occupy an important and informative evolutionary position on the animal tree.