Sea stars or starfish are marine invertebrates belonging to the phylum Echinodermata. It's class is Asteroidea. They exhibit a superficially radial symmetry. Starfish typically have five or more "arms" which radiate from an indistinct disk. Sea stars do not have movable skeletons, but instead it does posses a hydraulic water vascular system. The water vascular system has many projections called tube feet, located on the ventral face of the sea star's arms, which function in locomotion and aid with feeding.
There are about 1,800 living species of sea star, and they occur in all of the Earth's oceans. The greatest variety of sea stars are found in the tropical Indo-Pacific. Areas known for their great diversity include the tropical-temperate regions around Australia, the tropical East Pacific, and the cold-temperate water North Pacific.
Photo Credit: Blood star: Taken at the New England Aquarium (Boston, MA), December 2006. Copyright © 2006 Steven G. Johnson and donated to Wikipedia under GFDL and CC-by-SA.