Starfish or sea stars are animals belonging to phylum Echinodermata, class Asteroidea. The name starfish is also used for the closely related brittle stars, which make up the class Ophiuroidea. They exhibit a superficially radial symmetry, typically with five or more “arms” protruding from a central body. In fact, their evolutionary ancestors are believed to have had bilateral symmetry, and sea stars do have some remnant of this body structure.
Starfish do not have skeletons, but instead possess a hydraulic water vascular system. 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 feeding.
Starfish digestion is carried out in a sacklike stomach located at the center of the body. The stomach may be everted – pushed out of the organism’s body and used to engulf food. Some species take advantage of the great endurance of their water vascular systems to open the shells of molluscs, and inject their stomachs into the shells.
Asteroidea contains the genus Asterias.
As these creatures are invertebrates and not actually fish, most marine biologists are pushing to completely replace the term starfish with sea star.