This is a polychaete in the family Syllidae. If you look at the tail end, you can see that it
is about to breed: these animals can do so with schizogamy: production of stolons which are budded off and become pelagic. The stolons form complete new animals with prostomium, segmnets and pygidium. The stolons differ from the stock animal in a number of respects, such enlargment of the eyes, reduction of the gut, and different musculature. The stolons die after breeding.