Barrier reefs grow parallel to the coast, are often large and continuous, are separated from land by a wider lagoon, and are only likely to occur where there is a wide continental or insular shelf. The Great Barrier Reef in Australia is the largest barrier reef in the world. Atolls are circular shaped reef crests surrounding a central lagoon. In the Indo-Pacific, they develop during slow subsidence of volcanic islands and grow on the perimeter of the island, while in the Caribbean their origin is not understood as well. A coral may consist of a single polyp or a colony of thousands of polyps that are linked by a common gastrovascular system through which they share food, water, and wastes with surrounding polyps.
Coral polyps have tubular, saclike bodies with a ring of tentacles surrounding their mouths.
The non-tissue layer between the gastrodermis and the epidermis is called the mesoglea. Nematocysts are also found in the tentacle tips. The coenosarc is soft tissue stretching over the surface of the coral between the polyps.