A
corallite is the skeletal cup, formed by an individual
stony coral polyp, in which the polyp sits and into which it can retract. The cup is composed of
aragonite, a crystalline form of
calcium carbonate, and is secreted by the polyp. Corallites vary in size, but in most
colonial corals they are less than in diameter. The inner surface of the corallite is known as the calyx. The vertical blades inside the calyx are known as
septa and in some species, these ridges continue outside the corallite wall as
costae. Where there is no corallite wall, the blades are known as septocostae. The septa, costae and septocostae may have ornamentation in the form of teeth and may be thick, thin or variable in size. Sometimes there are paliform lobes, in the form of rods or blades, rising from the inner margins of the septa. These may form a neat circle called the paliform crown. The septa do not usually unite in the centre of the corallite, instead they form a columella, a tangled mass of intertwined septa, or a dome-shaped or pillar-like projection. In the living coral, the lower part of the polyp is in intimate contact with the corallite, and has radial
mesenteries between the septa which increase the surface area of the body cavity and aid digestion. The septa, palliform lobes and costae can often be seen through the
coenosarc, the layer of living tissue that covers the
coenosteum or skeleton of the coral.