PETER SCOONES / SCIENCE PHOTO LIBRARY PETER SCOONES / SCIENCE PHOTO LIBRARY
Sea urchin (class Echinoidea) on a rock. The sea urchin is a marine invertebrate. As an echinoderm, it is related to the starfish, and like a starfish it has radial symmetry. Its five arms have fused to form a usually spherical shell. Spines protrude upwards from this shell, while tube feet on its underside are used to crawl over rocks. Its mouth is also on its underside and its five jaws scrape up algae (microscopic plants) growing on the rock surface. Sea urchins range in diameter from 1-40 centimetres. Photographed in the Red Sea.
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