Cnidaria Cnidaria comes from the word cnidos which means stinging nettles. All these creatures sush as jellyfish and coral are amrmed with stinging cells and nematocysts.Cnidarians are united based on the presumption that their nematocysts have been inherited from a single common ancestor.Many thousands of cnidarian species live in the world's oceans, from the tropics to the poles, from the surface to the bottom. Some even burrow. A smaller number of species are found in rivers and fresh water lakes.

There are four major groups of Cnidaria:
Anthozoans are probably the most famous cnidarians: they include the corals that build great reefs in tropical waters, as well as sea anemones, sea fans, and sea pens. They also have a long and diverse fossil record, extending back at least 550 million years. The oldest anthozoans are probably some of the polyp-like and sea pen-like fossils from the Vendian (late Precambrian).