Heart urchin or sea potato - Echinocardium cordatum.
Heart urchins in a rock pool on the beach at Heysham in Lancashire in July 2016.
The thin hollow shells of dead heart urchins are called 'tests'.
Author: Joan Martin
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Photograph size: 7.5 Mpixels (21.5 MB uncompressed) - 2238x3357 pixels (7.5x11.2 in / 18.9x28.4 cm at 300 ppi)