A quote from another part of the board:
The Sipunculids- aka Peanut Worm feed on detritus, microscopic organisms and organics they extract from the sand. The peanut worm burrows into soft rocks and crevices often hiding during the day and coming out at night to feed. They reproduce sexually and given the right conditions can become quite abundant. I have recently discovered many in my tank and had a rare opportunity to photo one as it looked for food.