Subclass of solitary and colonial anthozoans which are many tentacled and have an enteron (gastrovascular cavity) divided by numerous paired mesenteries. The basal tissues secrete a corallum (cup) made of aragonite, which has an outer epitheca (skin) and radially arranged septa (see septum). The mesenteries are placed between the septa. The first group appeared in the Ordovician and is divided into four orders: Rugosa; Scleractinia; Tabulata; and Heterocorallia. Scleractinian zoantharians (Mesozoic to Recent) are responsible at the present time for building coral reefs, restricted to tropical areas. They may build wave-resistant structures, sometimes very thick, and are usually associated with other organisms, e.g. calcareous algae. Organic build-ups also took place in the Palaeozoic, and although zoantharians were present they did not always represent the major part of the fauna. Stromatoporoids (Stromatoporoidea), algae, etc., are associated with zoantharians in these build-ups and it is not always certain whether Palaeozoic reefs were comparable with modern or Mesozoic examples. It is unlikely that Palaeozoic zoantharians were reef-frame builders in the sense that modern scleractinians are, and probable that organisms other than corals formed the wave-resistant structures.