Did I see a bristle worm? or a potential serpent of death?

[saltwater]

I heard fireworms are pretty destructive but trying to search the net I get all kinds of answers. If I could find him I guess I could count antennae, measure it's thoracid anticompilative region and compare it to it's carbuncle diamater to determine type of polychaete I have encountered. Wait just let me get my flux capacitor and ionic phase inducer to measure... Needless to say I got a little lost way before the protective satae.
This is why I appreciate everyone's help. I wonder if they make a "marine worms for dummies(me)" book.