Body long and slender with a gradually tapering posterior half. Prostomium rounded rectangular. Eyes medium-sized. Proximal part of proboscis with twelve longitudinal rows of 9-10 papillae, six rows on each side. Distal part with six rows of more or less distinct tubercles. Terminal ring of about 16-17 papillae. Segment 1 not visible dorsally. Tentacular cirri of segment 1 reaching about segment 6. Dorsal and ventral tentacular cirri of segment 2 reaching about segment 10 and segment 6 respectively. Dorsal tentacular cirri of segment 3 reaching as far as those of segment 2. Chaetae present from segment 3. Dorsal cirri of median segments rectangular. Parapodial lobes rounded. Ventral cirri pointed, slightly longer than parapodial lobes. Pygidial cirri about six times as long as broad (P. mucosa-detail).
Up to 150 mm for 275 segments.
Prostomium conspicuously darkly pigmented anterior to eyes. Segment 1 (which can only be seen ventrally) darkly pigmented. Segment 2 and 3 dorsally and ventrally unpigmented or very slightly pigmented. Dorsally from segment 3 large dark spots occur, centred on pseudosegmented areas. Posteriorly the spots may coalesce, forming a more or less continuous longitudinal band. Additional pigmentation at bases of parapodia, on cirrophores of dorsal cirri and in centre of dorsal cirri. Ventral pigmentation much less distinct, with one spot (occasionally two) in the centre of each segment and one at the base of each parapodium. Yellow pigment present dorsally on posterior part of prostomium and anterior part of segment 2. On more posterior segments two small yellow spots per segment present dorsally. Eyes dark reddish-brown. Dark pigmentation usually well retained in preserved specimens, yellow pigmentation disappears.
Common on intertidal sand and mud bottoms, but also occurring on bottoms with stones and shell gravel. Known to about 20 m.
Due to possible confusion with other species, especially Phyllodoce maculata , the full distribution range cannot be determined at present. Arctic, North Pacific, northern Atlantic up to West Africa and Gulf of Mexico. Mediterranean, Adriatic, Black Sea, English Channel, North Sea, Skagerrak and Kattegat, Öresund up to Bay of Kiel.
P. mucosa is easily confused with Phyllodoce maculata , and several characters should always be checked for correct determinations.