fishieness said:bristle worms can be good or bad. most of the time they are benificial scavengers and very few will harm fish, but they will eat clams.
however, i doubt it is a bristleworm. It appears to have an exoskeleton and it looks like legs go all the way around it. A bristleworm (or the most commonly referty to type, a fireworm) does have these characteristics.
it looks like some type of isopod.[
The legs don't go all the way around