The underside of a seastar (starfish) showing the mouth and tubefeet.
Starfish (also called sea stars) are any echinoderms belonging to the class Asteroidea. The names sea star and starfish are also used in a broader sense to include the closely related brittle stars, which make up the class Ophiuroidea.
Starfish exhibit a superficially radial symmetry. They typically have five or more arms which radiate from an indistinct disk (pentaradial symmetry). Starfish do exhibit some superficial remnant of this body structure, evident in their larval pluteus forms.
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 starfish's arms which function in locomotion and aid with feeding.