What are the physical characteristics of sea urchins?
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The body wall is a firm, globose shell, or test, made of fused skeletal plates and marked by regularly arranged tubercles to which the movable spines are attached. Five rows of the skeletal plates are pierced by pores for the tube feet of the water-vascular system; these are typical of echinoderms and are used for locomotion. The mouth is centered on the lower side of the body and in many species is surrounded by a whorl of gills. A complex jaw and tooth apparatus in the mouth, known as Aristotle's lantern, is used to fragment food. Long, sharp spines are used for protection, and in some species are poisonous. The spines are also used as levers, aiding the tube feet in locomotion and, along with the teeth, are used by some species to dig burrows in hard rock.
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