Sea urchin spine. Coloured scanning electron micrograph (SEM) of a broken sea urchin spine. The spine is covered in an epidermis (brown), an outer layer of epithelial cells. The epidermis covers the skeleton of the spine (grey), which 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: x115 when printed at 10 centimetres wide.
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