Description:
Sea stars exhibit a superficially radial symmetry. They typically have five "arms" which radiate from a central disk (pentaradial symmetry). However, the evolutionary ancestors of echinoderms are believed to have had bilateral symmetry. Sea stars do exhibit some superficial remnant of this body structure.
Sea stars do not rely on a jointed, movable skeleton for support and locomotion, but instead possess a hydraulic water vascular system that aids in locomotion. The water vascular system has many projections called tube feet on the ventral face of the sea star's arms which function in locomotion and aid with feeding. Sea stars usually hunt for shelled animals such as oysters and clams. They have two stomachs. One stomach is used for digestion, and the second stomach can extended outward to engulf and digest prey. This feature allows the sea star to hunt prey that is much larger than its mouth would otherwise allow. Sea stars are able to regenerate lost arms. A new sea star may be regenerated from a single arm attached to a portion of the central disk.
Most Sea star have 5 arms, but some have more or fewer. The radial canal which is across each arm of the sea star has tooth-like structures called ampullae, which surround the radial canal. On the end of each arm or ray there is a microscopic eye which allows the sea star to see, although it only allows it to see light and dark, which is useful to see movement. The mouth is located underneath the sea star, on its ventral surface. The spiny upper surface is called the aboral or dorsal surface.
