noun, plural: coral polyps
A coral reef is a colony of sessile adult coral polyps connected to each other. There may be hundreds or thousands of coral polyps in a compact colony that forms the reef. Each coral polyp is an individual invertebrate that has a sac-like body. It has an opening or mouth (called a coelenteron) surrounded by tentacles. The coelenteron serves both as an oral cavity where food enters and as a passageway for the release of digested wastes. The tentacles surround the coelenteron. The tentacles capture food from water and bring it into its mouth. The tentacles are described to be stinging due to the presence of nematocysts (or cnidae). Nematocysts are coiled stinging cells that are ejected in order to entangle and poison a prey (e.g. copepods).
Coral polyps in a reef are sessile. They get calcium and carbonate ions from their marine habitat in order to build a hard skeleton made of limestone (or calcium carbonate). The skeleton provides them protection since the body of a coral polyp is soft and delicate. The body of coral polyps is clear. However, because of their symbiosis with algal species such as zooxanthellae their body tends to show colors due to the pigments of zooxanthellae that live within the cells of the clear tissues of the coral polyp.