Cnidaria (IPA: ) is a phylum containing some 11,000 species of apparently simple animals found exclusively in aquatic, mostly marine, environments. On the other hand, their biochemistry and genetic makeup reveal that even the stationary starlet sea anemone is much more complex than it appears at the first glance.
Cnidarians get their name from cnidocytes, which are specialized cells that carry stinging organelles called cnidocysts. As for the etymology, the word Cnidaria comes from the Greek word "cnidos", which means "stinging needle". The corals, which are important reef-builders, belong here, as do the familiar sea anemones, jellyfish, sea pens, sea pansies and sea wasps. The name Coelenterata was formerly applied to the group, but as this name included the Ctenophores, it has been abandoned. Cnidarians are highly evident in the fossil records, having first appeared in the Ediacaran period.
The basic body shape of a cnidarian consists of a sac containing a gastrovascular cavity with a single opening that functions as both mouth and anus . It has radial symmetry, meaning that whichever way it is cut along its central axis, the resulting halves would always be mirror images of each other. Their movement is coordinated by a decentralized nerve net and simple receptors. Several free-swimming Cubozoa and Scyphozoa possess rhopalia, complex sensory structures that can include image-forming eyes with lenses and retinas , and a gravity-sensing statolith comparable in function to the otolith of the vertebrate inner ear. Tentacles surrounding the mouth contain nematocysts, specialized stinging cells, which they use to catch prey and defend themselves from predators. The ability to sting is what gives cnidarians their name.
Traditionally the hydrozoans were considered to be the most primitive, but evidence now suggests the anthozoans were actually the earliest to diverge. Sea anemones, sea fans and corals are in this class. The non-anthozoan classes may be grouped into the subphylum Medusozoa. Under this scheme, Anthozoa is also elevated to a subphylum.
Theoretically, members of Cnidaria have life-cycles that alternate between asexual polyps and sexual, free-swimming forms called medusae. In reality there is a vast variation within the life-cycles of cnidarians.