Well, that's a pretty cool trap, but I don't believe that the worm you caught is a Eunicid, and I'm pretty sure that you will not be able to catch one in a trap. They are very long and they usually leave a good bit of their body embedded in the live rock. So while the head may go in the trap, most of the body will not, and they can retract into the LR in the blink of an eye. Weast had to take out the LR home of his Eunicid to get it out. I tried to do the same with mine but was unsuccessful...haven't seen him in awhile, but I'm sure I've still got him...probably several feet long by now.
FWIW,
Mariner