Starfish (also known as sea stars) are marine invertebrates belonging to the phylum Echinodermata, class Asteroidea. The names sea star and starfish are also (incorrectly) used for the closely related brittle stars, which make up the class Ophiuroidea. They exhibit a superficially radial symmetry. Starfish typically have five or more "arms" which radiate from an indistinct disk (pentaradial symmetry). In fact, the evolutionary ancestors of echinoderms are believed to have had bilateral symmetry, and starfish do exhibit some superficial remnant of this body structure, which is evident in their larval pluteus forms.
Starfish do not rely on a jointed, movable skeleton for support and locomotion (though 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, located on the ventral face of the starfish's arms, which function in locomotion and aid with feeding.