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Corals are marine species of the Anthozoa that exist in the form of tiny anemone-like polyps, often living in populations of many identical individuals. These individuals secrete calcium carbonate to form hard bones, building coral reefs in tropical waters.
A coral "head" is actually made up of thousands of identical polyps, each with only a few millimeters. After thousands of generations, these polyps leave a skeletal structure that is characteristic of their species. Each coral head grows through the asexual reproduction of the polyps. Coral also reproduces sexually with gametes, which are liberated simultaneously for a period of one to several consecutive nights during the full moon.
While corals may use poisonous nematocysts at the tentacles to capture the ephemera, the animal receives most of the nutrients from a single-celled algae called zooxanthella. ). Therefore, most corals depend on sunlight and grow in shallow and shallow waters, usually at a depth of less than 60 m (200 ft). Corals can contribute greatly to the physical structure of coral reefs that develop in tropical or subtropical waters, such as the Great Barrier Reef off Queensland. Other corals do not need algae and can live in deeper waters, such as those in cold-water Lophelia that live up to 3,000 meters in the Atlantic.  Another example is the Darwin Mounds in southwestern Cape Wrath, Scotland. Corals are also found off the coast of Washington State and the Aleutian Islands in Alaska.
Although a coral head looks like a living organism, it is actually the head of many genetically identical individuals, which are polyps. Polyps are multicellular organisms with food sources of many smaller organisms, from plankton to small fish.
Polyps are usually a few millimeters in diameter, made up of an outer epithelium, and an inner layer of jelly-like tissue called a foreign substance. Polipes have a symmetrical axial shape with tentacles growing around a middle mouth - the only door to the taste chamber (or stomach), both food and residue go through this mouth.
The long closed at the bottom of the polyps, where the epithelium forms an outer skeleton called a background disk. The skeleton is formed by an increasingly thickened calcium ring (see below). These structures grow vertically and form a tube from the bottom of the polyps, allowing it to bend into the outer skeleton when needed for shelter.
Polyps grow by developing vertical cavities (calices), sometimes divided into partition walls to create a new higher floor plate. Over many generations, this type of development has created large coral reef structures containing calcium, and long lasting coral reefs.
The formation of the outer skeleton contains calcium is the result of the polyp deposition of mineral aragonite from the calcium ions obtained from the sea water. Depending on species and environmental conditions, the deposition rate can reach 10 g / m² polyp per day (0.3 ounces per square yard / day). This depends on the level of light, night output is 90% lower than at noon. 
Nematocyst: A nematocyst that responds to a nearby prey that is touching prickly prickles, open flaps, and prisms injected with prey to paralyze prey, then the tentacles pull their prey into their mouths.
The tentacles of polyps trap bait using the lymphoid cells called nematocysts. These are specialized cells that catch and paralyze prey animals such as plankton, when exposed, it reacts very quickly by injecting poison into the prey. These toxins are usually weak, but in fire corals, they are strong enough to damage humans. Jellyfish and sea anemone also have nematocysts. The toxin that the nematocyst injected into the prey will paralyze or kill the prey, then the tentacles pull their prey into the stomach of the polyps with a stretched epithelium called the pharynx.
Close up of the Montastrea cavernosa polip. The tentacles can be seen clearly.
The polyps are connected through a complex system of digestive channels that allow them to share significant nutrients and symbiotic organisms. For soft corals, these channels are about 50-500 μm in diameter and allow the transport of both metabolites and cellular components. 
In addition to using planktonic as food, many species of coral, as well as other Cnidaria groups, such as sea anemias (eg Aiptasia genus), form a symbiotic relationship with the single-celled algal group. chi Symbiodinium. Normally, a polyp will live the same algae. Through photosynthesis, algae provide energy to corals and help corals during calcification . The algae benefit from a safe environment, and use carbon dioxide and nitrogenous substances that the polyp is emitted.