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sea urchin

A globular echinoderm (class Echinoidea), the body of which is formed by fused skeletal plates bearing moveable spines. The anus is usually on the upper surface and the mouth on the lower surface. Sea urchins have a complex jaw apparatus called Aristotle's lantern. They feed by scavenging, grazing, or ingesting sediment. Approximately 5,000 fossil species and 950 living species are known, including sand dollars and heart urchins.

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