You have a member of the Nereidae.
Presently only two species, Neanthes succinea, is considered cryptogenic in Denmark; another, N. virens, considered cryptogenic in the Netherlands.
You may use data, pictures,information etc. available ad the site, as long as you refere to the sourse.
The database as a whole is to be cited as follows:
NOBANIS. Available from http://www.NOBANIS.org. Data of access x/x/201x