|Phylogeny of Anthozoa, relationships of the orders still undefined
Aristotle's pupil Theophrastus described the red coral, korallion in his book on stones, implying it was a mineral; but he described it as a deep-sea plant in his Enquiries on Plants, where he also mentions large stony plants that reveal bright flowers when under water in the Gulf of Heroes. Pliny the Elder stated boldly that several sea creatures including sea nettles and sponges "are neither animals nor plants, but are possessed of a third nature (tertius natura)". Petrus Gyllius copied Pliny, introducing the term zoophyta for this third group in his 1535 book On the French and Latin Names of the Fishes of the Marseilles Region; it is popularly but wrongly supposed that Aristotle created the term. Gyllius further noted, following Aristotle, how hard it was to define what was a plant and what was an animal.
The Persian polymath Al-Biruni (d. 1048) classified sponges and corals as animals, arguing that they respond to touch. Nevertheless, people believed corals to be plants until the eighteenth century, when William Herschel used a microscope to establish that coral had the characteristic thin cell membranes of an animal.
The phylogeny of Anthozoans is not clearly understood and a number of different models have been proposed. Within the Hexacorallia, the sea anemones, coral anemones and stony corals may constitute a monophyletic grouping united by their six-fold symmetry and cnidocyte trait. The Octocorallia appears to be monophyletic, and primitive members of this group may have been stolonate. The cladogram presented here comes from a 2014 study by Stampar et al. which was based on the divergence of mitochondrial DNA within the group and on nuclear markers.
Corals are classified in the class Anthozoa of the phylum Cnidaria. They are divided into three subclasses, Hexacorallia, Octocorallia, and Ceriantharia. The Hexacorallia include the stony corals, the sea anemones and the zoanthids. These groups have polyps that generally have 6-fold symmetry. The Octocorallia include blue coral, soft corals, sea pens, and gorgonians (sea fans and sea whips). These groups have polyps with 8-fold symmetry, each polyp having eight tentacles and eight mesenteries. Ceriantharia are the tube-dwelling anemones.
Fire corals are not true corals, being in the order Anthomedusa (sometimes known as Anthoathecata) of the class Hydrozoa.