What is the structure of coral?

Coral is actually the animal called a polyp, which is related to the jellyfish and the anemone.  The polyp can be the size of a pin head all the way to one foot in diameter, depending on the type of coral.  Each polyp has an opening that is surrounded by stinging tentacles and the sac-like body of the polyp is in the calcium carbonate shell it forms around itself.