A relatively big and strong animal.
Prostomium rounded, broader than long, with four reddish eyes in a trapezoid arrangement. Peristomium shorter than the following segments, with 2 pairs of tentacle cirri. The dorsal ones being as long as the lateral tentacles; ventral ones half the length of the dorsal ones. Palps hardly visible dorsally, fused for the full length. Median antenna shorter than lateral antennae, about 2x the prostomium length. Lateral antennae 4x or more as long as prostomium. Nuchal epaulettes triangular, reaching the first chaetiger.
Dorsal tentacular cirri as long as lateral, ventral half that size. Dorsal cirri of first segment as long as the median antenna or longer. Dorsal cirri of second segment half that size or shorter. All other dorsal cirri equal in size and clearly shorter than segment width. Ventral cirri absent.
Blades of compound chaetae with a large tooth and a smaller above it (P. prismatica-detail). Posterior parapodia with an additional needle shaped chaeta. Bayonet-chaetae on posterior half of the body.
Pharynx long, anteriorly with 9 weak papillae. Trepan with 9 large and 9 small teeth, alternating.
Body up to 26 mm long for 105 segments.
White, yellowish or pale greenish with 3 dark brown or black lateral lines dorsally. Tips of antennae and cirri and the epaulettes often reddish.
Between algae, sponges, etc. and on muddy, sandy or gravel bottoms. From the eulittoral down to 700 m.
Arctic, North Pacific, North Atlantic up to English Channel.
Pelagic forms from the Öresund to the Bay of Kiel.