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Phylum Cnidaria is usually divided into four classes: Hydrozoa, hydroids and fire corals; Anthozoa, corals and anemones; Cubozoa, box jellies; Scyphozoa, jellyfish. Anthozoa, in turn, is split into three sub-classes: Alcyonaria (Octocorallia), containing the soft corals and gorgonians; and Zoantharia (Hexacorallia), containing the stony corals and anemones; and Ceriantipatharia, including the black corals and cerianthids, or tube anemones.

The Stony Corals

The stony or hard corals are the reef-builders. They are in the order Scleractinia, and are sometimes called scleractinian or "true" corals. The skeletons these animals secrete range in shape from the massive, smooth boulders of Pontes and stout-branched Pocillipora that take a pounding at the reef edge to the finely foliated needle coral Seriatopora histrix.

These corals are colonies, comprised of thousands of individual coral animals, or polyps. Each polyp, upon close examination, will be seen to have much the same shape as a sea anemone, with tentacles ringing a central mouth. What makes the stony coral polyp distinctive, and so ecologically important, is that it deposits calcium carbonate around its lower part, forming a skeletal cup. The skeleton is essentially formed of repeated casts of the tiny polyp.