Body somewhat dorsoventrally flattened.
Tentacles few, short, broad. Eyespots present. Upper lip broad, rounded triangular. Two pairs of gills (on segments 2 and 3). Gills dichotomous with short stems and short tips. Anterior gills usually twice as long as the posterior ones (P. tetrabranchia-lateral view).
Distinct but not prominent lateral lobes on segments 2-4. Ten distinct ventral shields on thorax. Notopodial chaetae from segment 4. Thirteen chaetigerous thoracic segments. Uncinigerous tori from segment 5, prominent, moderately long on thorax and the anterior four abdominal segments; short, more or less protruding on the rest of abdomen. Uncini in double rows on segments 11-20. Notopodial chaetae with finely denticulate, narrow brims on one side. Uncini with one big and several smaller teeth (P. tetrabranchia-chaetae).
Maximum number of segments at least 57.
Pygidium without appendages.
Worm up to 40 mm long.
A thin, transparent layer of secretion encrusted with sand grains.
In alcohol yellowish pink with a lighter longitudinal band through the notopodia.
P. tetrabranchia is found on mixed bottoms of clay and silt with stones. Vertical distribution 55-138 m.
Norwegian coast from North Møre to Oslofjord.