Produce a rigid skeleton comprising CaCO3. Septae, Tentacles and Mesenteries are emplaced in sets of 6. Thus the term, hexacorals.
Tentacles are smooth without additional pinnate fringes.
This group typically possesses zooxanthellae symbionts which aid in nutrition and fromation of the skeleton of the coral. When symbionts are present, we term these hermatypic corals. These are relatively simple tentacles in structure without fringes oramented only by small protruding cells representing the cnidoblasts. Note that this is a colonial coral comprising several individual polyps. In the above figure, one can also observe the sutures between individual polyps.