Sea urchin shell. Holbox Island, Quintana Roo, Mexico.
Sea urchins belong to the phylum Echinodermata, (from the Greek for spiny skin) which also includes starfish, sea cucumbers, brittle stars, and crinoids. Like other echinoderms they have fivefold symmetry (called pentamerism) and move by means of hundreds of tiny, transparent and adhesive "tube feet". The pentamerous symmetry is not obvious but is easily to distinguish in the dried shell of the urchin.
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