Massive worm = eunicid worm, those are the ones that get several feet and have monster jaws. A majority of fire/bristle worms that hitch hike on our rocks hide all day and stay small, usually under a couple inches. Best place to get them is ask your local reef club if they spot any, they will be more than gladly to give u some. I never encountered any online store that sells them, occasionally you can find some nut job selling fire worms on ebay as tank cleaners. Medusa worms are actually a kind of cucumber, death = cuke nuke.