Corals...   are marine organisms, classified as animals back in 1726 with gratitude to the efforts of a French physician called Andre Peysonnel.
They have a different anatomical structure; however, they do have a mouth, a gastro vascular cavity (stomach) and a skeleton.
They grow in colonies in tropical areas between 32 degrees north and south of the equator in salt water of 3 to 4 percent salinity, temperatures between 20 to 30 degrees and can be found in any place from right below the sea surface to 50 meters of depth.
Although they are classified into many different categories, they are widely recognized as two different types; soft corals and hard corals. They have a protective layer of mucus covering all around which acts as a shield from the bacteria (much like a human skin) , which is easily removed by the slightest touch of divers' or snorkellers hands or fin strokes leaving them unprotected and easily infected; that is one of the reasons of corals' decline on earth… us, humans.