Echinothrix from Lembeh Strait Photo (above) by Efdixon
Another day when pictures say more than words! What is an anal cone???
The anal cone is the weird, prominent bulb-shaped bit (its white in the species below) that is sitting on the top of the test (i.e., the body) in sea urchins of the family Diadematidae, which are well-known for their big long sharp spines....
Video below (by Ulesja) was taken from a diadematid filmed in Banderas Bay, Puerto Valarta, MX, in January 2011 (thanks to the original videographer for clarifying the locality!).
But what function does it serve??
The Anal cone seemingly falls into the "poorly understood" category but I did find some explanations..
Another peculiarity of this family is that there is an elongated anal cone or sac which is commonly inflated like a small balloon in the centre of the upper side. This may gently roll around its attachment and is believed to have a respiratory function".
Hendler et al. (1995) in the book Sea Stars, Sea Urchins, and Allies, Echinoderms of Florida and the Caribbean note in the tropical Atlantic Astropyga magnifica
As in Diadema antillarum, the anal cone enables the animal to eject feces away from the body.So here's a diversity of anal cones from various diadematid urchins! If they are for ejecting feces then these are some of the most aesthetically pleasing feces ejecting structures that I've seen among animals!
I believe this is Echinothrix from Hawaii
Photo above by JLambus
Astropyga from Sulawasi (photo below by pauldub)
Astropyga from Indonesia
Photo above by PacificKlaus
Another by PacificKlaus. Looks to be Echinothrix? from Okinawa, Japan
Astropyga from Lembeh, Indonesia
Another by Christian Loader. Maybe Echinothrix from Lembeh,
Diadema? from tropical Atlantic
Photo above by marcsdad50
Echinothrix from Sulawasi (photo below by fisheyedave aka David Cropp)
Here's one from Diadema (the Philippines). Photo below by maxette