Body long and strong.
Tentacles numerous, very long.
Three pairs of gills, dichotomous branched with long tips and thick round stems. Anterior gills not much longer than the posterior ones.
Distinct lateral lobes on segments 2 and 3. Fourteen ventral shields on the thorax.
Usually 24 chaetigerous thoracic segments, exceptionally 25. Notopodial chaetae winged, subdistally denticulate. Uncini on neuropodia from the second chaetiger onwards. Uncini in double rows on segments 11-29.
Pygidium without appendages.
Up to 250 mm for 100 segments.
A thin layer of secretion encrusted with mud, often with bends and branches.
In life more or less greyish yellow or brown, gills red. In alcohol yellowish.
On mud or mud mixed with sand, rarely on pure sand, often among Zostera , Fucus or Laminaria , and on mussel or oyster banks; from the eulittoral to about 100 m; euhaline to mesohaline.
Eastern North Atlantic, Mediterranean, Canadian Atlantic, Sea of Okhotsk, Japan Sea. East Greenland, Iceland, the Faeroes, the Norwegian coast to Finnmark, Swedish west coast, Kattegat, the Öresund and the Belts, western Baltic, southern North Sea, eastern Great Britain.