in temperature and decrease in pH at a competitive rate to these pathogens responsible for coral bleaching, resulting in species loss. Anatomy For most of their life corals are sessile animals of colonies of genetically identical polyps. Each polyp varies from millimeters to centimeters in diameter, and colonies can be formed from many millions of individual polyps. Stony coral, also known as hard coral, polyps produce a skeleton composed of calcium carbonate to strengthen and protect the organism. This is deposited by the polyps and by the coenosarc, the living tissue that connects them. The polyps sit in cup-shaped depressions in the skeleton known as corallites. Colonies of stony coral are markedly variable in appearance; a single species may adopt an encrusting, plate-like, bushy, columnar or massive solid structure, the various forms often being linked to different types of habitat, with variations in light level and water movement being significant. The body of the polyp may be roughly compared in a structure to a sac, the wall of which is composed of two layers of cells. The outer layer is known technically as the ectoderm, the inner layer as the endoderm. Between ectoderm and endoderm is a supporting layer of gelatinous substance termed mesoglea, secreted by the cell layers of the body wall. The mesoglea can contain skeletal elements derived from cells migrated from the ectoderm. The sac-like body built up in this way is attached to a hard surface, which in hard corals are cup-shaped depressions in the skeleton known as corallites. At the center of the upper end of the sac lies the only opening called the mouth, surrounded by a circle of tentacles which resemble glove fingers. The tentacles are organs which serve both for tactile sense and for the capture of food. Polyps extend their tentacles, particularly at night, often containing coiled stinging cells (cnidocytes) which pierce, poison and firmly hold living prey paralyzing or killing them. Polyp prey includes plankton such as copepods and fish larvae. Longitudinal muscular fibers formed from the cells of the ectoderm allow tentacles to contract to convey the food to the mouth. Similarly, circularly disposed muscular fibres formed from the endoderm permit tentacles to be protracted or thrust out once they are contracted. In both stony and soft corals, the polyps can be retracted by contracting muscle fibres, with stony corals relying on their hard skeleton and cnidocytes for defense.