|Phylogeny of Anthozoa, relationships of the orders still undefined
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
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
cnidocyte trait. The Octocorallia appears to be monophyletic, and primitive members of this group may have been
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
Anthozoa of the
Cnidaria. They are divided into three subclasses,
 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
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