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More About Star Fishes ...
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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 (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.
Starfish 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.
Starfish 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, muscles 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 starfish 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.
Sea stars are composed of a central disc with five arms exhibiting pentaradial symmetry. The mouth is located underneath (oral or ventral) the sea star, and the spiney surface covering the species is called the aboral or dorsal surface. On the aboral surface there is a structure called the madreporite which acts as a water filter and supplies their water vascular system with water to move (Gilbertson, 1999).
On the surface of the sea star surrounding the spines are small white objects known as pedicellariae. There consists thousands and thousands of these pedicellariae on the external body. Inside the sea star underneath all the hepatic caeca consist the gonads which are involved in reproduction. The radial canal which is across each arm of the sea star has what are called ampullae which surround the radial canal. The ampullae are teeth like structures.
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Source: Wikipedia Read more about Star Fishes
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