The septa are secreted by the mesenteries, and are therefore added in the same order as the mesenteries are. As a result, septa of different ages are adjacent to one another, and the symmetry of the scleractinian skeleton is radial or biradial. This pattern of septal insertion is termed "cyclic" by paleontologists. By contrast, in some fossil corals, adjacent septa lie in order of increasing age, a pattern termed serial and produces a bilateral symmetry. Scleractinians secrete a stony exoskeleton in which the septa are inserted between the mesenteries in multiples of six.