About 700 sea urchin species are known throughout the world. These ocean dwellers are in the same family as sea stars, sand dollars and sea cucumbers. To move around coral reefs and rocks, sea urchins use spines in conjunction with tube feet on their bottom side as stilts. Most sea urchins that wash up on shore consist of only their shell, which is often sold in souvenir shops.
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Sea urchin shells, or tests, are round and composed of skeleton-like plates fused together. The plates, made of high magnesium calcite, are arranged in columns underneath the skin. Regular sea urchins have 10 double columns of plates. There are two major openings in the test: the peristome, where the mouth opens, and the periproct, which is a flexible plated membrane containing the anus.
Spines, at times very sharp, protrude from a sea urchin's shell to help protect it from predators such as sea otters. The ball-and-socket joints uniting spine and shell allow the spines to point in many directions.
Adult sea urchin tests range in size from 1 to 4 inches and can be black or a variety of red, purple, green and brown shades.
Sea urchin shells protect the organism's fragile organs and its coveted roe.
The brittle sea urchin shells are dried and painted for use in many crafts after being picked up on the beach or at souvenir shops. Some uses for sea urchin shells include candle holders, decorative bowls and Christmas ornaments.
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