Astraea is also known as the star coral. They are exclusively marine and have an appearance of stone. They occur in colonies which are fixed and have a spreading nature. They are sedentary and have polyps. The polyps have multiple tentacles. It has a basal cup made up of calcium carbonate referred as corallite. It is known as the skeleton of polyp. It is secreted by the epidermis of polyp and increases in the size with the passing time. The skeleton is hard and it forms a colony known as corallium.
A group of polyps form coral colony. They secrete a skeleton which can be soft or hard. It makes the red coral which is used for gems. Their skeleton is made up of calcium carbonate. It occurs in the tropical seas and forms one of the stable ecosystems. They make the Great Barrier Reef which extends from the north eastern coast of Australia
