Organisms commonly referred to as corals are Cnidarians of the class Anthozoa. This class includes sea anemones, hard corals and soft corals like sea fans and sea pens. Corals are actually simple animals that exhibit predatory feeding, sexual reproduction and also posses a simple nervous system.
Anthozoans exist as solitary or colonial, and mostly sessile polyps.
The polyps are the living part of the coral structure. They are embedded in a hard exoskeleton of calcium carbonate secreted by the external covering of the polyp. To feed the polyp extends its tentacles into the water column. The polyp tentacles contain stinging cells called nematocysts, which catch prey, mainly zooplankton. Corals reproduce sexually, often in mass spawning events.