The skeleton itself is also called coral. In many shallow-water species the polyps contain unicellular plants, which may provide the high oxygen concentration required by such corals. Some solitary corals of that group may reach a diameter of 10 in. The body of each polyp is saclike, consisting of a wall of jellylike material surrounding a digestive cavity, with a single opening, the mouth, at the unattached end. The mouth is surrounded by tentacles used to capture small prey and is invaginated to form a pharynx leading into the body cavity.
Reproduction occurs both sexually and by budding.
Sexual reproduction is by means of eggs and sperm, which are produced in the mesentaries and shed into the water. As new polyps are produced by budding they remain attached to each other by thin sheets of living tissue as well as by newly secreted skeletal material. The great variety in the form of various colonial corals, which may be treelike and branching, or rounded and compact, depends chiefly on the method of budding of the particular species. In the brain corals, for example, each theca merges with the one next to it on either side, forming long rows of polyps separated by deep channels. In some of the branching corals the polyps occupy small, discrete pits on the surface of the skeleton.