General Anatomy

Echinoderms are characterized by radial symmetry, radiating from a central body (pentamerous). It can be expressed in different body forms such as five arms, a five-part jaw, and five radial canals. The body consists of five equal segments, each containing a duplicate set of various internal organs. They have an endoskelteon that is made from hard calcium carbonate (CaCO3). They have no heart, brain, or eyes, but some brittle stars have light sensitive areas on their arms. Their mouth is situated on the underside and their anus on upper body (except feather stars, sea cucumbers and some urchins).Echinoderms have tentacle-like structures called tube feet with suction pads situated on their extremities. They are many small, flexible, fluid filled tubes that project from the body and are used in locomotion, feeding, gas exchange, and nitrogen excretion. These tube feet are hydraulically controlled by a remarkable water-vascular system. The madreporite is a sieve-like opening on the aboral surface of a sea star through which water enters the water vascular system and connects to a circular ring canal. Five radial canal branch from the ring canal and a portion of the sea star’s water vascular system that runs along inside the ambulacral ridge in each arm. The body is covered by many ossicles, which are small calcium carbonate plates that forms the endoskeleton.