D. ROBERTS / SCIENCE PHOTO LIBRARY D. ROBERTS / SCIENCE PHOTO LIBRARY
Sea urchin. Coloured X-ray of the shell of a sea urchin (class: Echinoidea). Fused plates form this endo- skeletal test or shell, which encloses the sea urchin body. As in other echinoderms, including starfish, the body is arranged in five sections. This symmetry is here seen in the five double rows of pores; from these pores emerge the movable podia ("feet") usually with suction cups, which enable the sea urchin to move. The external surface of the shell also contains protective spines. Located at the larger hole in the shell (foreground) is the sea urchin mouth, which faces the seabed, while the anus occurs at the smaller opening.
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