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Sea urchins are the Class Echinoidea of the Phylum Echinodermata. Like the rest of the Echinoderms, literally "spike-skinned", they are entirely marine. They are usually globe-shaped, and protected by calcareous plates and spines.p65 Urchin is an old word for hedgehog, and in many foreign languages these animals are called as "sea hedgehogs".
Like other echinoderms they have five-fold symmetry (called pentamerism) and move by means of hundreds of tiny, transparent, adhesive 'tube feet'. The symmetry is not obvious in the living animal, but is easily visible in the dried test.
Sea urchins mostly feed on algae and small animals. They have a special chewing apparatus called Aristotle's lantern, after the Greek philosopher Aristotle who was fascinated by sea urchins. With this apparatus they can scrape organisms stuck to the surface over which the urchin is moving.
Sea Urchin, a prickly-looking marine animal, spherical in shape and covered with long movable spines. It somewhat resembles a hedgehog and is sometimes called the �hedgehog of the sea.� Sea urchins are found on the ocean bottom, usually near rocky shores. Sea urchins may be brown, black, purple, green, white, or red. Most are about two to four inches (5 to 10 cm) in diameter, including the spines.
In addition to its spines, the sea urchin also has pedicellariae (three-jawed pincers atop slender stalks) and tiny tube feet projecting from its body surface. The movable spines (which in some species are solid and in others hollow and filled with poison) are used for locomotion and protection. The pedicellariae (which in some species contain poison glands) are used for defense and for cleaning the body by removing larval animals and small crustaceans. The tube feet are hollow, muscular projections ending in suckers. They are flexible and can be extended beyond the spines to grip objects on the ocean floor.
March 18th, 2013
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