This is Alitta succinea, a ragworm. One of the many species in the polytraits databases. Information on its life-cycle traits can be accessed at: http://polychaetes.lifewatchgreece.eu/taxonomy/term/5126/traits.
Credit
Hans Hillewaert, via Wikimedia, via EOL http://commons.wikimedia.org/wiki/File:Nereis_succinea_%28epitoke%29.jpg http://eol.org/data_objects/25702161