Creatures of the night -- part 2

On night dives, a number of species appear that are completely hidden during the day.  Tube-dwelling anemones are nocturnal.  At night, they extend their tentacles out of their tubes in the sand.  If disturbed by a bright diver's light, they retract back into their tubes.

Lobsters forage at night.  This spiny lobster was out on a sandy area near the reef.

The next photo is a mystery to me.  I showed it to a local naturalist friend, and he didn't know what it was.  Four arms were sticking out of the sand, something like an anemone, but the spine-covered arms were fewer and structurally quite different from anemone tentacles.  My closest guess is a brittle star, with one arm and its central disc buried in the sand.  Anyone knowing the identity of this animal, please post a comment, and I will add the ID to this post.  The arms were about 3.5 inches long.  All photos were taken at a depth of about 20 feet.