Body slender and attenuate. Prostomium triangular and acute, with three lobes and usually two eyespots. Gills in two groups, usually meeting at the middorsum; three gills of each group in a transverse row, the fourth one just behind the middle one of the three; bases of gills on each side fused.
6-10 long, slender and tapering chaetae with a fine tip on each side in front of gills.
Fourteen thoracic segments with notopodia with capillary chaetae, the posterior twelve also with neuropodia with uncini. Twelve uncinigerous abdominal segments, none with rudimentary notopodia. Uncini with three rows of teeth, outer rows with 4-7 teeth, the middle row with 1-3 teeth. No long cirri above the neuropodia, but rudimentary cirri may be present. Pygidium with a pair of lateral cirri and a number of long cirriform papillae (A. baltica-detail).
Worm up to 18 mm long.
A thin secretion encrusted with a layer of mud.
In alcohol very pale yellow to whitish.
In Scandinavia in more or less brackish waters. There are very few records of the species, probably because it has been mistaken for juveniles or young of other species; hitherto it has been reported from silt and mixed bottoms at depths between 5 and 77 m.
Nova Scotia. Jan Mayen, Trondheimfjord, Oslofjord, Kattegat, the Öresund, western Baltic to Blekinge, West Britain.