DAVE ROBERTS / SCIENCE PHOTO LIBRARY DAVE ROBERTS / SCIENCE PHOTO LIBRARY
Sand dollars. Coloured X-ray of three sand dollar shells (phylum: Echinodermata), viewed from the upper surface. Sand dollars are related to sea urchins. Like all echinoderms the body is divided into five sections. This can be seen in the five double rows of respiratory pores on the upper surface, and the structure of the shell beneath. Five yellow arrow-shaped teeth are seen inside the shell, which have become displaced from the central mouth. Sand dollars are flattened and they live on the seabed, feeding on organic matter contained in sand. The surface of the shell of sand dollars is often covered in spines (absent here), and they move with spines and tube feet.
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