Body of this scale-worm long, tapering posteriorly. Dorsal and ventral surfaces smooth.
Prostomium rounded, with a median antenna on a basal joint which extends as a ridge posteriorly then laterally along the posterior margin. Basal joint of the antenna with an ear-shaped process on either side. Palps very long and smooth, with a basal membrane. Lateral antennae fused to the first segment. First segment uniramous, with numerous capillary chaetae; a pair of small ctenidia on the mid-dorsal surface, a pair of finger-shaped lamellae on the inner surface and a pair of long dorsal and short ventral cirri.
Scales cover the body. They occur on segments 1, 3, 4 and 6, then alternately to the 26th, then on all segments. Scales are round to kidney-shaped with long papillae on the outer margins and with smooth surfaces. First and 2nd segments with 1 or 2, all other segments with 3 ctenidia, attached to the dorsal surface of the notopodia. Cirriform gills occur above the ctenidia on all chaetigers from the 3rd. A pair of dorsal cirri present on the 2nd chaetiger. A small ctenidium is present between the cirri and the body wall.
Notopodia in the anterior region with a terminal and some dorsoanterior finger-like projections absent posteriorly. Notopodia with numerous chaetae, all long spinose capillaries.
Neuropodia extend as far as the notopodia with ventral and posterior finger-like projections and numerous chaetae. Neuropodial chaetae compound with finely grooved blades, unidentate tips and smooth or spinose shafts (N. tetragona-detail).
Up to 200 mm for 300 chaetigers.
Sublittoral from 40 to over 2000 m on mud and oyster banks.
Arctic, North Atlantic up to Mediterranean, Adriatic, North Sea, Skagerrak and Kattegat.