Anthozoans are exclusively marine, polypoid cnidarians. They include the familiar sea anemones, and other anemone-like groups with skeletons (such as the "stony" scleractinian corals) and without skeletons (such as tube anemones), as well as sea pens, sea fans, blue coral, and black coral. Anthozoans occur from the intertidal zone to the depths of the trenches (to 6000 m). In excess of 6000 species currently exist, comprising about two-thirds of extant cnidarian species; some anthozoans, such as the scleractinian corals, have a rich fossil history.
An anthozoan polyp, like polyps in other cnidarian classes, consists of a tubular body terminating in a mouth surrounded by a ring of tentacles. The tentacles of an anthozoan polyp, unlike those of some other classes, are hollow, so the coelenteric space extends into them. The mesoglea between the ectoderm and the endoderm is relatively thick and cellular in many anthozoans compared to that of polyps in the other classes. The skeletal elements of some taxa are internal, lying in the thick mesogleal layer, whereas other anthozoans have skeletons lying entirely outside the living tissues.
Most anthozoan orders contain exclusively colonial species. An anthozoan colony consists of polyps connected by living tissue, the coenenchyme. The members of a colony are derived by vegetative (asexual) propagation from an original founder polyp that was the product of sexual reproduction, possibly with rare exceptions. At least some individuals of clonal species, like those of colonial species, propagate vegetatively by means such as fission, budding, or laceration, but, unlike in colonial species, the progeny do not remain attached to one another. Anthozoans of other species are incapable of asexual reproduction.