These beautiful marine invertebrates are among the most primitive of multicellular animals and possess a simple nerve network and digestive system. They belong to a group of animals, the coelenterates, which includes sea firs, jellyfish, sea fans, soft corals, anemones, and true corals.
Unlike jellyfish, corals and their nearest relatives have developed the polyp stage to the exclusion of a free-swimming stage, spending most of their life permanently fixed to the seafloor. The sea anemones consist of solitary polyps and , although devoid of a calcareous or horny skeleton, may reach up to two feet in diameter. Sea pens and sea fans have a horny skeleton, which gives them a passing resemblance to true coral, however, the skeleton of coral is calcareous rather than horny.
The calcareous coral skeleton consists of calcium carbonate crystals which are extracted from the sea and laid down by the epithelial cells of the base and lower body wall of the coral animal, which is known as a polyp. The coral polyp is thus protected by the surrounding exoskeleton, into which it withdraws when danger approaches.
Solitary corals consist of a single polyp encased within its calcareous skeleton and are usually relatively small in size. The individual coral polyps of colonial corals are joined together by their epidermal cells, which cover the surface of the coral skeleton, and also by the interconnection of their digestive and circulation systems which are continuous between polyps.
Living corals exhibit a wide range of colors from creamy white, through reds, greens, browns to black, but once the polyps die these brilliant hues are soon lost.
Although carnivoous, many corals have associated single-celled plants present in their tissues and, since these plants require light to convert carbon dioxide and water into food, the corals concerned are restricted to relatively shallow water. These corals and plants are both capable of living independently of each other, buth their association is so mutually beneficial that they are seldom found apart.
Last Updated:14 November
1996
URL:http:/www.indiana.edu/~r317doc/trail/reef.html
Comments:Underwater
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