![Diadema setosum [CC-BY-SA-3.0 Steve Cook]](https://www.polypompholyx.com/wp-content/uploads/2014/02/Diadema_setosum-224x300.jpg)
It’s amazing how informative an anus can be. Take this sea urchin. The orange pucker in the middle of the spines is its “around-the-bum”, although zoologists would insist on writing that in Greek as “periproct“. The bright orange ring-piece is characteristic of this species, and marks it out as Diadema setosum, rather than any of …