Some urchins have small bulb of tissue protruding from their upper surface, adjacent to the anus. This bulb is essentially an urchin ‘haemorrhoid’, and is believed to be caused by pressure build-up in the gut of the urchin.
http://www.beautifuloceans.com/creatures/a-m/long-spined-urchin/235-long-spined-urchin-diadema-antillarum.html">http://www.beautifuloceans.com/creatures/a-m/long-spined-urchin/235-long-spined-urchin-diadema-antillarum.html</a>
Who knows???