GUSTOIMAGES / SCIENCE PHOTO LIBRARY GUSTOIMAGES / SCIENCE PHOTO LIBRARY
Seashells. Shells of unidentified sea snails. These shells are calcium-rich structures secreted by gastropod molluscs for protection. The animals can withdraw entirely into their shells when threatened. The chambers at the apex are the oldest, with successively larger whorls being added as the animals grow. Some shells spiral outwards to form a cone (lower centre), while others form a flattened spiral (lower right).
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