Reef-building corals secrete an external skeletal cup made of calcium carbonate. This skeletal cup gives shape to the polyp and protects it when it contracts. The stomach cavity of the polyp also contains radiating calcareous walls. These walls extend up from the cavity and reinforce the skeletal cup. The stomach cavity of the coral also house long, mobile filaments which contain the reproductive cells.
Corals can reproduce either sexually or asexually.
Synchronous spawning occurs in many corals. The corals release sperm and eggs into the water at the same time. At least one-third of the corals on the Great Barrier Reef in Australia reproduces using this method. Asexual reproduction occurs in several different ways. Environmental disturbances may dislodge some polyps or portions of the colony and deposit them on other parts of the reef.