Please help identify marine animal

I saw this in an aquarium the other day: Unfortunately, it wasn’t listed on the sign next to the tank, and there was nobody around I could ask. Any ideas? Some kind of sea urchin, maybe?

Sea mouse, perhaps?

It’s a polychaete worm, in the same phylum but a different class than earthworms and leeches. The sea mouse (per Sailboat’s post and link) or a close relative would be my guess.

Cool! Never heard of them before:

And they do look strange when upside down. Thanks, guys.