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. All modern scleractinian skeletons are composed of calcium carbonate in the form of crystals of aragonite, however, a prehistoric scleractinian (Coelosimilia) had a non-aragonite skeletal structure which was composed of calcite. The structure of both simple and compound scleractinians is light and porous, rather than solid as is the case in the prehistoric order Rugosa. Scleractinians are also distinguished from rugosans by their pattern of septal insertion. Growth In colonial corals, growth results from the budding of new polyps. There are two types of budding, intratentacular and extratentacular. In intratentacular budding, a new polyp develops on the oral disc, inside the ring of tentacles. This can form individual, separate polyps or a row of partially separated polyps sharing an elongate oral disc with a series of mouths. Tentacles grow around the margin of this elongated oral disc and not around the individual mouths. This is surrounded by a single corallite wall, as is the case in the meandroid corallites of brain corals. Extratentacular budding always results in separate polyps, each with its own corallite wall. In the case of bushy corals such as Acropora, lateral budding from axial polyps form the basis of the trunk and branches. The rate at which a stony coral colony lays down calcium carbonate depends on the species, but some of the branching species can increase in height or length by around a year (about the same rate as human hair grows). Other corals, like the dome and plate species, are more bulky and may only grow per year. The rate of aragonite deposition varies diurnally and seasonally. Examination of cross sections of coral can show bands of deposition indicating annual growth. Like tree rings, these can be used to estimate the age of the coral. Solitary corals do not bud. They gradually increase in size as they deposit more calcium carbonate and produce new whorls of septa. A large Ctenactis echinata for example normally has a single mouth, may be about long and have more than a thousand septa. Distribution Stony corals occur in all the world's oceans. There are two main ecological groups. Hermatypic corals are mostly colonial corals which tend to live in clear, oligotrophic, shallow tropical waters; they