Hexacoralla, a group of corals (q.v.) of the class Madreporaria (q.v.), including all those "true corals" in which the septa are either six or are arranged in multiples of six. The group is divided into three orders, the Perforata, Fungida, and Aporosa. In the former the skeleton is loose and porous in texture as all the solid parts are perforated by numerous pores. Madrepora, of which the well-known stag's horn coral is a familiar example, is the principal genus of the Perforata. The Fungida are characterised by the possession of "synapticulae" - small rods passing from the septa into the body chambers on each side. The mushroom coral (Fungia) is the type of this group. The Aporosa is the largest order; the calcareous structures are here solid. This includes the astrean corals, such as the Brain coral, and most of the simpler single corals such as Flabellnm. Another classification has been prepared, based on the origin of the body wall, but so far it has not been sufficiently worked out to be accepted here. The Hexacoralla is the largest existing group of corals.
It is probable that most of the Palaeozoic "Rugose" corals ought to be included within it.