Body of this scale-worm wide, tapering posteriorly. Dorsal and ventral surfaces smooth.
Prostomium bilobed, with a median antenna on a large base, a pair of lateral antennae inserted below the eye stalks, a pair of eyes on bulbous eye-stalks and a pair of long papillate palps. First segment with a few capillary chaetae between the dorsal and ventral tentacular cirri. Scales overlap longitudinally exposing the mid-dorsal surface. They occur on segments 1, 3, 4 and 6, then alternately to the end of the body. The first pair of scales are small and round, the rest larger and more irregularly rounded, all margins and surfaces smooth. From the 5th pair of scales the outer lateral area is folded over to form a pocket. Spinning glands, which are visible through the body as an iridescent cord, are present from the 8th chaetiger. Notopodia are bulbous lobes decreasing in size posteriorly, with chaetae on the 1st and 2nd segments only. Notopodial chaetae are all capillaries. Neuropodia fleshy and flattened with chaetae in a sinuous row (P. oerstedi-parapodium). Neuropodial chaetae of four kinds: 1) spinose capillaries, from the 2nd chaetiger; 2) stout, tapering to a fine point with a spinose outer dorsal tip; 3) scythe-like unidentates with spinose inner curved edge; from the 8th chaetiger, 4) fine, brush-tipped unidentates (P. oerstedi-detail).
Up to 100 mm for 80 chaetigers.
Thick and tough, constructed of threads from the spinning glands with attached sand and mud particles.
Pearly white, prostomium yellow to red, and the scales transparent.
Sublittoral to 760 m in mud, shell and shell sand.
Northeast Atlantic between Norway and Northwest Africa, Mediterranean, northern North Sea up to Skagerrak and Kattegat.