Bristle worms or Polychaeta are amongst the most abundant of marine organisms. They are distinguished by linear series of external ringlike segments; the grooves between the segments coincide with internal compartments, often separated by transverse septa, containing serially repeated nervous, muscle and excretory systems. Anterior segments bear jaws, eyes and cirri in some species; the terminal segment may bear a cirrus. Parapodia are unique to polychaetes; these thin, fleshy flaps protrude laterally from each body segment. A chitinous cuticle covers the entire body.
It is a diverse group with many species and they usually make up a lot of biomass of the macrobenthos. Unfortunately, this does not mean the group has been well studied. This CD-ROM is the first attempt at a complete and user-friendly identification key for the North Sea. It is not yet possible to provide a definitive checklist of the species in the North Sea. Even in areas as relatively well studied as this, many species have been found only once and it is impossible to tell which species are still to be expected. We have therefore also included species that have been found in the Skagerrak, Kattegat and Öresund.