I mean, look at those really weird spines!
This unusual creature is Chondrocidaris brevispina, which appears to be much less spiny than the urchins we're probably all more familiar with. Those pinkish pimply bumps towards the creature's right-hand side are the bases of missing spines, which articulate with their bumps via a type of ball joint.
As the name 'brevispina' suggests, the spines are quite short. Those pink swellings on the end of each are apparently sponges, which leads me to wonder what restricts the sponges' downward spread along each spine. Their lower limit seems very well defined. If this was a 'normal' urchin I'd wonder if that was due to the action of the structures known as pedicellariae, which are capable of nipping & crushing (& are sometimes venomous as well) – they'd certainly take care of an overgrowth of sponge!
Alas! Information on C.brevispina seems fairly scanty – a pity as I really would like to know more about those spines. Hopefully someone more learned in this area than I am can help me out 🙂