The anatomy of red coral is relatively simple (Grillo et al., 1993 ; Allemand, 1993) : the tissues cover the axial skeleton like the fingers of a glove. The tissue morphology is a classical branching : two layers of tissue (each consisting of a single layer of cells) cover a gelatinous non-cellular layer called the mesoglea. A network of small canals run through the mesoglea, like a circulation system whose function is not yet understood. These little canals communicate with big canals lying parallel to the axial skeleton and with the polyps which form the mouths of the coral ( and not one animal alone, as is often thought). These polyps which have 8 tentacles can retract completely into their little holes and disappear from view. Their opening and closing cycles and their function are not well known (Russo et al., 1993). Their closure does not mean that the coral is dead. The mesoglea also contains small crystals of calcium, called spicules, near the outer edge of the animal, which could play an important part in mechanical protection against abrasion.

The skeleton has a double origin : apical skeletogenesis (which makes up the centre of the skeleton) is the fusion of the spicules between themselves. Once the centre of the axis is formed, the thickness increases by secretion of layers of deeper tissues (or skeletogenic epithelium); Allemand et Grillo, 1992 ; Grillo et al., 1993 ; Allemand et Bénazet, 1996), and not, as was suggested for a long time(Lacaze-Duthiers, 1864 ; Grassé, 1987), from the migration, then fusion of the spicules from the mesoglea. This is the only skeleton with some commercial value : it is formed of calcium carbonate (CaCO3), crystallised as calcite. It also has an organic fraction, representing about 1.5% of the total weight of the skeleton (Allemand et al., 1994). This organic fraction is made up of proteins and glycosaminoglycan complexes, and will play a key role in controlling the calcification process, providing a framework with exceptional mechanical properties (the rupture pressure is 10 times superior to that of concrete!). The red colour of coral is due only to the pigmentation of its skeleton by carotene-like substances (Merlin et Delé, 1983).