Body long, broad anteriorly, tapering towards pygidium, dorsally convex, ventrally flattened. Tentacular lobe and upper lip large and folded, characteristically forming three rolled lobes. Tentacles very numerous, long (P. medusa-detail). Segments 1-3 ventrally forming a broad, triangular cushion-like structure. Behind this six large and a number of smaller pairs of more or less distinct ventral shields. Notopodial chaetae feathery, starting at segment 3. 10-13 thoracic chaetigers. Uncinigerous tori from the first abdominal segment. Pygidium with finely scalloped edge.
Up to 70 mm for 76 segments.
No real tube, only tunnels in the sediment.
In life yellow, orange or reddish, tentacles lighter. In alcohol pale yellow to yellowish grey.
On mud, clay, fine and coarse sand, and all types of mixed bottoms, also in the sediment between the rhizoids of benthic algae; eulittoral to depths exceeding 1500 m; euhaline to polyhaline.
North American Atlantic, West Greenland, eastern North Atlantic to the Mediterranean, Siberian and Alaskan Arctic, Bering Sea, Sea of Okhotsk.
East Greenland, Svalbard, Iceland, the Faeroes, Norwegian coast, North Sea, Skagerrak, Swedish west coast, Kattegat, the Öresund and the Belts.