sedentary worm

i had one of these on a clam shell once. didn't do much, and it seemed harmless. not sure what it is though.

Thanks for the reply nanoamydee... so far they don't seem to be a problem. I would like to be able to id them someday, however. It bugs me to not know what I am keeping alive

update: appears the proper id is in Family Vermetidae ... 'shell worm'