These polyps reside within a cup-like calcium carbonate skeleton. They have a central opening surrounded by tentacles which can be extended to feed on phytoplankton in the water column.

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A coral contains a very thin outer layer of living coral polyps. Although

most corals contain hundreds or thousands of polyps, some contain

only one. The polyp is a hollow, cylindrical animal with a mouth that

is surrounded by tentacles armed with stinging cells for capturing

food. Underneath these coral polyps is the dead coral skeleton

composed of calcium carbonate. Each coral polyp excretes a calcareous

exo-skeleton that lives in a symbiotic relationship with zooxanthellae,

host algae located in the tissue of the polyp, that gives the coral its

green, blue, purple or brown color.