Coral

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Coral, small, sedentary marine animal, related to the sea anemone but characterized by a skeleton of horny or calcareous material. The skeleton itself is also called coral. Although most corals form colonies by budding, there are some solitary corals; in both types the individual animals, called polyps, resemble the sea anemone in form.
Corals grow in warm and temperate climates and in the cold water found at greater depths, but they are most abundant in warm, shallow water; over 200 coral species are found in the Great Barrier Reef of Australia. In many shallow-water species the polyps contain unicellular plants, which may provide the high oxygen concentration required by such corals.

Stony Corals 

In the large group known as stony corals, or true corals (Madreporaria), each polyp secretes a cup-shaped skeleton, the theca, around itself. Some solitary corals of that group may reach a diameter of 10 in. (25 cm); in the colonial forms the individual polyps are usually under 1-8 in. (3 mm) long, but the colonies may be enormous. The body of each polyp is saclike, consisting of a wall of jellylike material surrounding a digestive cavity, with a single opening, the mouth, at the unattached end. The mouth is surrounded by tentacles used to capture small prey and is invaginated to form a pharynx leading into the body cavity. Thin sheets of tissue (mesentaries) extend radially from the wall to the pharynx, dividing the cavity. A second set of radial divisions is created by folds (septa) of the outer skeleton and body wall, which extend upward from the floor of the body cavity. Reproduction occurs both sexually and by budding. Sexual reproduction is by means of eggs and sperm, which are produced in the mesentaries and shed into the water. Fertilization results in a free-swimming larva, which attaches to a surface and secretes a skeleton, becoming (in colonial forms) the parent of a new colony. 

As new polyps are produced by budding they remain attached to each other by thin sheets of living tissue as well as by newly secreted skeletal material. The great variety in the form of various colonial corals, which may be treelike and branching, or rounded and compact, depends chiefly on the method of budding of the particular species. In the brain corals, for example, each theca merges with the one next to it on either side, forming long rows of polyps separated by deep channels. In some of the branching corals the polyps occupy small, discrete pits on the surface of the skeleton. As a colonial coral produces more polyps the lower members die and new layers are built up on the old skeleton, forming a large mass. In tropical and subtropical regions these massive corals, along with other plants and animals, may form a coral reef. Most of the reef-forming corals belong to the stony coral group. 

Soft Corals 

The soft corals (Alcyonaria) are a group of soft, often feathery forms, with skeletons composed of calcareous or horny particles imbedded in the body wall. Each polyp of a soft coral has eight tentacles. Among the well-known soft corals are the sea pen, sea pansy, whip coral, and organpipe coral. The precious red coral (Corallium) of the Mediterranean Sea, used for jewelry, also belongs to that group. The spicules of its skeleton are fused together. 

Classification: Stony and soft corals are classified in the phylum Cnidaria, class Anthozoa.

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