The subclass Hexacorallia includes 6 orders: Actiniaria (true sea anemones), Antipatharia (black corals), Ceriantharia (tube anemones), Corallimorpharia, Scleractinia (true or "stony" corals), and Zoanthidea. Hexacorallia includes roughly 4,300 known extant species. As the name suggests, most hexacorallians have hexamerous symmetry (i.e., parts in multiples of 6). The most familiar group of corals, the reef builders, includes around 650 extant species (i.e., around half the known stony coral species). These are found mostly in the clear, shallow waters of the tropics. They are distinguished from the other subclass of Anthozoa, Octocorallia, in having six or fewer lines of symmetry in their body structure and only single rows of tentacles. These organisms are formed of individual soft polyps which in some live in colonies and can secrete a calcite skeleton. As with all Cnidarians these organisms have a complex life cycle including a motile phase when they are considered plankton and later characteristic sessile phase. It also includes the significant extinct orders of the rugose corals and tabulate corals.
Octocorallia (also known as Alcyonaria) is a subclass of Anthozoa comprising ~3,000 species of water-based organisms formed of colonial polyps with 8-fold symmetry. It includes the blue coral, soft corals, sea pens, and gorgonians (sea fans and sea whips) within three orders: Alcyonacea, Helioporacea, and Pennatulacea. These organisms have an internal skeleton secreted by mesoglea and polyps with eight tentacles and eight mesentaries. Although commonly called "soft corals," the Octocorallia are not close relatives of the Scleractinia, or "true corals" living today. Unlike true corals, which have hexaradial symmetry, octocorals have eightfold radial. Notice also that the tentacles of all octocorals, are pennate -- small branches come off of the main tentacle to give a more or less feather-like appearance. All octocorals are colonial