Sea stars or starfish are animals belonging to phylum Echinodermata, class Asteroidea. The names sea star and starfish are 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 (pentaradial symmetry). In fact, their evolutionary ancestors are believed to have had bilateral symmetry, and sea stars do have some remnant of this body structure. Sea stars do not have movable 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.















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Sea star digestion is carried out in two separate stomachs, the cardiac stomach and the pyloric stomach. The cardiac stomach, which is a sacklike stomach located at the center of the body may be everted - pushed out of the organism's body and used to engulf and digest food. Some species take advantage of the great endurance of their water vascular systems to open the shells of molluscs (clams, mussels and the like), and inject their stomachs into the shells. Once the stomach is inserted inside the shell it digests the mollusk in place. Because of this ability to digest food outside of its body, the sea star is able to hunt prey that are much larger than its mouth would otherwise allow (including mollusks, arthropods, and even small fish). Partially-digested food is passed to the inside of the sea star where digestion continues in the pyloric stomach. Due to all of this digestive demand, the sea star's arms are filled with digestive glands called pyloric caeca or hepatic caeca.


001 to 004.jpg Courtesy of The Us Fish & Wildlife Service
005 to 006.jpg Courtesy of The National Undersea Research Program





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