The polychaeta is from the family of the Syllidae. You see here the proventricle (a barrel-shaped structure made of rings of muscle cells) and the proboscis of the anterior digestive tract. More interesting information about this organs: http://www.seaslugforum.net/message/17308 .
two pictures
Finding worms in a sample is like finding gold. I have found quite a few, but only once have I found a duplicate, [one of the little ones with red spots] There must be hundreds of different kinds.