Structure and growth
The Scleractinia are one of approximately twenty-five orders of animals belonging to Phylum Cnidaria. This phylum includes corals, soft corals, hydroids, jellyfish and sea anemones, all of which have the same general body plan. They are all symmetrical about a central axis (that is, they are radially symmetrical) and have a sac-like body cavity with only one opening, which serves as both mouth and anus. This opening is surrounded by tentacles which have stinging cells. The body wall, unlike that of any other group of animals except comb-jellies, consists of two cell layers, the epidermis and gastrodermis, separated by a jelly-like layer, the mesoglea. Two forms of this plan occur within the phylum, a polyp form which is usually sedentary and a medusa form which is usually free-swimming. The one is the upside-down equivalent of the other.
Corals are basically anemone-like animals that secrete a skeleton. Some corals are solitary and look just like simple anemones when their tentacles are extended. Others, including most that are seen on coral reefs, are colonial. Although corals are primitive organisms, their skeletons, like those of many other primitive organisms, are often complex. Fortunately it is not necessary to understand much about this complexity in order to identify corals.