One of the defining characteristics of echinoderms, including starfish, is their radial symmetry. Radial symmetry applies to animals that have no right or left side, only a top and bottom. Starfish, like other echinoderms, have a five-sided radial symmetry, which is represented by its five arms. The arms extend from a central axis in the center of the starfish. Two of the arms, known as the bivium, connect a starfish's madreporite. The three remaining arms are called trivium and the arm opposite the madreporite is the anterior arm.
The water vascular system is a hydraulic system in the starfish's internal anatomy. This system is used to operate the starfish's locomotive, digestive and respiration capabilities. Water enters the vascular system via the madreporite on the starfish's aboral surface. A tube called the stone tube runs from the madreporite to the central ring canal at the center of the starfish's body. From the central ring canal, the water vascular system structures include a network of tubes -- called radial canals -- connecting the central ring canal to its tubed feet on the body's oral surface.
The aboral surface is responsible for a starfish's navigation. A starfish's arms have pigmented ocellus at each of their tips. The pigmented ocelluses are more commonly known as eyespots and serve as sensory organs for a starfish. Eyespots are not visible on the starfish's aboral surface. The aboral surface features an opening to the starfish's water vascular system, also called a madreporite, and its anus, a hole the starfish uses to excrete wastes. The madreporite is located off-center from the center of a starfish's aboral surface.
The underside of a starfish is called its oral surface. The oral surface features a starfish's mouth, which it uses to eat its prey. The mouth is at the center of the starfish's oral surface. Oral surfaces also have ambulacral grooves in the center of all five arms. Two rows of moveable spines surround the ambulacral grooves; the spines cover up the starfishes' soft tissues when the tissues are not in use. Ambulacral grooves also possess tube feet known as podia. Starfish use their podia to grab prey and for moving around.