Body narrow, tapering posteriorly, dorsal surface convex.
Prostomium with a pair of short antennae without cirrophores, a pair of biarticulate palps and two pairs of small eyes in trapezoid arrangement.
Dorsal tentacular cirri a little longer than ventral ones, second dorsal tentacular cirrus reaching chaetiger 4.
Pharynx with a pair of denticulate jaws; paragnaths very pale and few in number, easily dislodged, typically present only in group Vl, where 1-8 may be found in each group; some individuals, however, may have small numbers of paragnaths in groups I, II, IV.
Parapodia of the first two chaetigers uniramous with three lobes.
Remaining parapodia biramous with 4 lobes. In anterior chaetigers, the dorsal notopodial lobe, notopodial acicular lobe and ventral neuropodial lobe all of similar size and shape: narrow tapering cones; neuropodial acicular lobe shorter, with a short postchaetal lobe. Dorsal cirrus slightly shorter than notopodial lobes, ventral cirrus shorter than ventral neuropodial lobe. Posterior parapodia with the dorsal notopodial lobe becoming shorter than the notopodial acicular lobe, and with dorsal cirrus becoming longer than notopodial lobes. Notopodial chaetae homogomph spinigers anteriorly, with homogomph falcigers appearing in posterior chaetigers. Neuropodial chaetae homogomph spinigers and heterogomph falcigers above and heterogomph spinigers and falcigers below the acicula (N. longissima-detail).
Up to 500 mm for 250 segments.
Body divided into two regions, an anterior region of 35-38 chaetigers in males and 46-49 in females, followed by chaetigers with modified parapodia. No modification of the dorsal or ventral cirri of anterior chaetigers has been reported. Metamorphosed parapodia from the posterior part of the body develop accessory flaps at the base of the dorsal cirri, below the notopodial acicular lobe, on the posterior surface of the neuropodial acicular lobe and at the base of the ventral cirri. The accessory flaps are better developed in males than in females. In males a pygidial rosette is found, but no crenulations develop on the dorsal cirri whereas in females dorsal cirri develop crenulations. In both sexes, the ordinary chaetae arc replaced by paddle chaetae in the modified parapodia.
Both intertidally and subtidally, inhabiting a variety of sediment types from clean sand to mud. It is capable of living in areas of reduced salinity.
The whole North Sea, Northeast Atlantic, Mediterranean and Black Sea.