KAJ R. SVENSSON / SCIENCE PHOTO LIBRARY KAJ R. SVENSSON / SCIENCE PHOTO LIBRARY
Sea urchin shell. View of the shell of a Red Sea Urchin (Strongylocentrotus franciscanus). Sea urchins are bottom-feeding creatures of the phylum Echinodermata. These animals are characterised by their unusual five-fold radial symmetry. This can be seen in the five double columns of pores radiating from the centre of this shell. The shell, or test, is formed of five fused plates which enclose the sea urchin body. From the pores emerge the movable podia (feet), usually with suction cups, which enable the sea urchin to move. The hole in the shell (centre) is the sea urchin's anus, which faces away from the seabed. The mouth (not seen) opens onto the underside.
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