Scleractinia are typical anthozoan coelenterates with the same basic structure and physiology as Actiniaria, from which they differ in the capacity to secrete a complex calcareous skeleton and in being usually colonial. There is no distinction systematically between hermatypic (reef-building)
and ahermatypic species, but the former invariably contain, in the endoderm, immense populations of endozoic dinoflagellates (zooxanthellae). In comparison with conditions in the ahermatypes, these enhance metabolic efficiency by acting as automatic agents of excretion and by increasing the
rate of skeleton formation. They may also assist, qualitatively if not quantitatively, in nutrition, although all Scleractinia (hermatypes equally with ahermatypes) possess highly efficient organs of feeding (nematocysts with tentacles and/or cilia) and of digestion (mesenterial filaments).
In the hermatypes there is a remarkable range in both size and form of the polyps and of the calyces into which they are withdrawn as well as in skeletal form generally. This indicates great powers of adaptability because these have all been through the sieve of natural selection. The precise
significance of these adaptations, although presumably involving food collection, remains obscure in the majority of cases. The present concentration by workers on studies of overall productivity of coral reefs would have greater significance if accompanied by detailed studies of individual
species of a range of hermatypic corals.
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