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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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