Those look like young ragworm (Nereidae), possibly Neresis sp., but I don't think your worm could be identified definitively from those photos since some of the characters useful for identification are not visible. There's a good online key to polychaetes at http://www.nhm.ac.uk/research-curation/research/projects/taxinfo/index2.html
I am not a worm expert but while exploring salt marshes web sites I found a nice website which include a lot of data (text and image) about the subzones of saltmarshes and their associated organisms. Try it:
I have a lot of polychaete taxonomic experience, to identify your worms you will really need to look closely at the head structures and the legs. We have a phyllodocidae worm that looks just like this too. phyllodocidae have a very distinct paddle on their legs, which it kind of look like your has. Their head structure is different to Nereididae too. If you post some more photos of the head I can assist you further. Also if you are searching the literature Nereidea is no longer a valid family name (since 1971) it is now Nereididae spelling only.
I think this word is a Polychaeta (Annelida) living in sulphurous water slighly putrify substratum .But this is resembling to Nereis diversicolor but not Nereis because of it's different habitat . sincerely
It is a Nereididae; the most probable species, dealing with salt marshes, is Hediste diversicolor, however to be sure you'll have to check the disposition of teeth on the pharynx.
This nereid worm is most likely Nereis diversicolor, but you need to look at the parapodial structure. See illustrations in Barnes (1994) The brackish-water fauna of northwestern Europe, page 91, published by Cambridge, for nereids and other polychaetes. Many salt-marsh and estuarine sediments are low in oxygen.