Bristle worm-segmented worm of the Polychaeta class.

These polychaetes are a class of annelid worms. The segmented name means each body segment has a pair of fleshy protrusions called parapodia that bear many bristles called chaetae, which consists of the material,chitin.
Errant bristle worms are the type you do not want, as they will potentially harm all marine life, including corals. This means for your reef aquariums saftety, do not trust any worms that resemble a centipede and the way that they move,at all!
Some reading on them.
http://www.coralrx.com/index.php?option=com_content&view=article&id=12&Itemid=22
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