Hexacorallia is a subclass of
Anthozoa comprising approximately 4,300 species of aquatic organisms formed of
polyps, generally with 6-fold symmetry. It includes all of the stony
corals, most of which are
colonial and reef-forming, as well as all
sea anemones, tube anemones, and
zoanthids, ranged within six extant
orders. The hexacorallia are distinguished from the other subclass of Anthozoa,
Octocorallia, in having six or fewer axes of symmetry in their body structure and only single rows of tentacles. These organisms are formed of individual soft polyps which in some species live in colonies and can secrete a
calcite skeleton. As with all
Cnidarians, these organisms have a complex life cycle including a motile
planktonic phase and a later characteristic
sessile phase. Hexacorallia also include the significant extinct orders of the
rugose corals and
tabulate corals