Coral Organisms

The polyp grows by extension of vertical calices which occasionally septate to form a new, higher, basal plate. Over many generations this extension forms the large calcareous structures of corals and ultimately coral reefs.
Formation of the calcareous exoskeleton involves deposition of the mineral aragonite by the polyps from calcium and carbonate ions they acquire from seawater. The rate of deposition, while varying greatly across species and environmental conditions, can be as much as 10 g / m² of polyp / day (0.3 ounce / sq yd / day). This is light dependent, with night-time production 90% lower than that during the middle of the day.
Nematocysts are stinging cells at the tips of the calices that carry poison which they rapidly release in response to contact with another organism. The tentacles also bear a contractile band of epithelium called the pharynx. Jellyfish and sea anemones also carry nematocysts.
The polyps interconnect by a complex and well developed system of gastrovascular canals allowing significant sharing of nutrients and symbiotes. In soft corals these range in size from 50–500 micrometres (0.0020–0.020 in) in diameter and allow transport of both metabolites and cellular components.