Coral Anatomy

Coral reefs consist of many diverse species of corals (as well as alga, sponges, worms, bivalves (clams), snails and other organisms secreting hard calcium reef) each with distinct colony and polyp forms. The structure of the polyps and the skeleton of the coral is a rather simple combination. A polyp is made up of two cell layers: the epidermis and the gastrodermis. The non-tissue layer between the gastrodermis and the epidermis is called the mesoglia.

The polyp contains mesentery filaments, which contain nematocysts or stinging cells used in food capture, a pharynx, endothecal dissepiments (horizontal layers of skeletal material) and the columella (the central axis of the corallite found below the mouth). The corallite is the limestone skeleton deposited by one polyp. The wall around each polyp is called the theca.

Other structures include the calice (the upper opening of the corallite), the coenosarc (the coral tissue that stretches over the surface of the coral between the polyps), the coenosteum (the skeletal material around the corallites), and the corallum, which is the skeleton of the coral. The coral anatomy also includes calcarcous plate-like structures known as septa. The septa radiate out from the centre of the corallite to the wall. There are two types of septa: insert septa, which lie below the corallite wall, and exsert septa, which protrude above the corallite wall.

Corals are of two types: perforate and imperforate. Perforate corals have porous skeletons with connections between the polyps through the skeleton. Imperforate corals have solid skeletons.

Many corals have different growth forms. They can be placoid as in Tubastrea coccinea (orange cup coral) and Favia fragum (golf ball coral) they can also be meandroid in which corallites form a series within the same walls, as in the species Dendrogyra cylindrus (pillar coral) and Diplora (brian corals). Other growth forms include cocoid spherical shaped and phalecoid, as in Eusmilia fastigiata (smooth flower coral).