Octocorallians have an internal skeleton. Some internal skeletons contain calcareous spicules. Spicules are either scattered of fused. They stiffen and protect the polyps. Other octocorallians have internal skeletons made of protein.
Reef-building corals secrete an external skeletal cup of calcium carbonate. This skeletal cup protects the polyp: when the polyp contracts, it's almost completely inside the skeletal cup. The stomach cavity of reef-building corals also contains radiating calcareous walls. These walls extend up form the polyp's base and reinforce the skeleton.