Sea urchin spine. Coloured scanning electron micrograph (SEM) of a section through a sea urchin spine. The skeleton of the spine is made of calcium carbonate, or calcite, and is symmetrical. The spines, which are moveable and cover the sea urchin's shell, are used to protect it from predators. Sea urchins are marine echinoderms, relatives of the star fish. Magnification: x110 when printed at 10 centimetres wide.
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