Despite their name, brain corals are completely brainless and are not as intelligent as their name implies. Their spherical shape and grooved surface of a sea brain resembles an underwater brain, but the animals within the stony exterior are simple invertebrates related to jellyfish and sea anemones. Brain corals belong to a group of hard corals, or stony corals. Their structure is made of calcium carbonate, or limestone, which hardens into a rock-like exoskeleton. These skeletal structures become cemented together to form a sphere that gives brain corals their shape. Brain corals grow very slowly as each generation adds to the limestone skeleton. Some brain corals can live up to 900 years. Because of their massive, sturdy structure, they form the foundation of coral reefs and can grow to 6 feet tall.