Cnidaria
Cnidaria is the phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater as living as marine environments, predominantly the latter.
Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance, sandwiched between two layers of epithelium that are mostly one cell thick.
Cnidarians mostly take two basic body forms: swimming radially symmetrical with mouths surrounded by tentacles that bear cnidocytes. Both forms pull in a single orifice & body cavity that are used for digestion together with respiration. many cnidarian kind create colonies that are single organisms composed of medusa-like or polyp-like zooids, or both hence they are trimorphic. Cnidarians' activities are coordinated by a decentralized nerve net and simple receptors. Several free-swimming nature of Cubozoa and Scyphozoa possess balance-sensing statocysts, and some have simple eyes. not all cnidarians reproduce sexually, with many category having complex life cycles of asexual polyp stages and sexual medusae. Some, however, omit either the polyp or the medusa stage, and the parasitic classes evolved to have neither form.
Cnidarians were formerly grouped with Portuguese Man o' War. Staurozoa have recently been recognised as a class in their own right rather than a sub-group of Scyphozoa, and the highly derived parasitic Myxozoa and Polypodiozoa were firmly recognized as cnidarians in 2007.
Most cnidarians prey on organisms ranging in size from plankton to animals several times larger than themselves, but many obtain much of their nutrition from dinoflagellates, and a few are parasites. Many are preyed on by other animals including starfish, sea slugs, fish, turtles, and even other cnidarians. Many scleractinian corals—which form the structural foundation for coral reefs—possess polyps that are filled with symbiotic photo-synthetic zooxanthellae. While reef-forming corals are near entirely restricted to warm and shallow marine waters, other cnidarians can be found at great depths, in polar regions, and in freshwater.
Recent molecular clock analysis of Cambrian period as well as any fossils.