A corallite is the skeletal cup, formed by an individual

stony coral Scleractinia, also called stony corals or hard corals, are marine animals in the phylum Cnidaria that build themselves a hard skeleton. The individual animals are known as polyps and have a cylindrical body crowned by an oral disc in which a ...

polyp, in which the polyp sits and into which it can retract. The cup is composed of

aragonite Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, (the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including prec ...

, a crystalline form of calcium carbonate, and is secreted by the polyp. Corallites vary in size, but in most

colonial Colonial or The Colonial may refer to: * Colonial, of, relating to, or characteristic of a colony or colony (biology) Architecture * American colonial architecture * French Colonial * Spanish Colonial architecture Automobiles * Colonial (1920 au ...

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 The Southeastern Pennsylvania Transportation Authority (SEPTA) is a regional public transportation authority that operates bus, rapid transit, commuter rail, light rail, and electric trolleybus services for nearly 4 million people in five coun ...

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 In zoology, a mesentery is a membrane inside the body cavity of an animal. The term identifies different structures in different phyla: in vertebrates it is a double fold of the peritoneum enclosing the intestines; in other organisms it forms com ...

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 In corals, the coenosarc is the living tissue overlying the stony skeletal material of the coral. It secretes the coenosteum, the layer of skeletal material lying between the corallites (the stony cups in which the polyps sit). The coensarc is comp ...

, the layer of living tissue that covers the

coenosteum In corals, the coenosteum is the stony skeletal material secreted by the coenosarc, the layer of living material lying between the corallites (the stony cups in which the polyps sit). The coenosteum is composed of aragonite Aragonite is a carbo ...

, the part of the skeleton between the corallites. In colonial species, when the corallites each have a surrounding wall, the colony is said to be ''plocoid''. When the walls are tall and tubular, the colony is ''phaceloid'', and when several polyps share a common wall, the colony is ''cerioid''. Sometimes the polyps are in valleys on the surface of solid corals, they are then known as ''meandroid''. Branching corals have two forms of corallites, axial and radial. The axial corallites tend to be shallow and are found near the tips of the branches, while the radial corallites are on the sides of the branches. Corallites can be rounded or polygonal and may be inclined (tilted obliquely to one side). As long as the colony is alive, the polyps and coenosarc deposit further calcium carbonate under the coenosarc, thus deepening the corallites. Each polyp has a fixed adult size and, when it is beginning to get submerged in the corallite, it secretes a new floor (tabula) beneath itself. Over time, a series of floors builds up below the living polyps, resulting in a thickening and lateral expansion of the coral.

References

{{reflist Cnidarian anatomy