Sea stars (also often called starfish) are echinoderms belonging to the class Asteroidea. The names sea star and starfish essentially refer to members of the Class Asteroidea. However, common usage frequently finds sea star and starfish also applied to ophiuroids which are correctly referred to as brittle stars or basket stars. Sea stars do not rely on a jointed, movable skeleton for support and locomotion (although they are protected by their skeleton), 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 be extended outward to engulf and digest prey.