Corallite

The vertical blades inside the calyx are known as septa and in some species, these ridges continue outside the corallite wall as costae.

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, the part of the skeleton between 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.