Some of these worms are potentially quite toxic, and as such, if you discover a polychaete in your tank that looks like a "wooly-bear" caterpillar don't touch it with your bare hands!
I say this because, although there are really only a very few species with which you must be careful, if you are unfamiliar with the species and there is any potential for injury, why risk it. Brittle worms other wise known as fireworms "sting fish" in relation to a reef aquarium.
If a fish happens to come in contact with a brittle/fireworms long, hollow and brittle setae they usually break off and embed themselves into your fish. Once imbedded the setae cause inflammation, irritation, itching and numbness (usually associated with the toxin they contain,
Here is a good web site on the matter.
http://www.aquarm.net/0697/0697_2.shtml
Terry