Body of this scale-worm long, tapering posteriorly. Dorsal surface smooth with a raised central platform and 5 pairs of ctenidia on the first 7 chaetigers, ventral surface smooth.
Prostomium oval with a median antenna with large basal joint with an ear-shaped process on either side, two pairs of eyes (anterior pair often difficult to observe in dorsal view). A pair of large nuchal organs occur on the posterior margin. First segment with numerous capillary chaetae, a pair of lateral antennae in a terminal position, 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 tentacular cirri. Scales overlap, covering the body. They occur on segments 1, 3, 4 and 6, then alternately to the 26th and then on all segments. The first pair are rounded, the rest kidney-shaped. All scales have short papillae on the outer margins and tubercles on the surface. Three ctenidial cushions attached to the dorsal surface of notopodia appear on all segments, except the 1st and 2nd segments which have two. Branchial cirri occur above the ctenidial cushions on all chaetigers from the 3rd. Ctenidia appear on the posterior base of elytrophores and dorsal tubercles. Ventral cirri with a basal bulge present on all segments and a small ctenidium occurs, between the cirri and the body wall. Notopodia bulbous with numerous chaetae arranged in a semicircle and several finger-like projections. Notopodial chaetae spinose capillaries with bifid tips. Neuropodia extend as far as notopodia with a smooth, bilobed posterior lobe, a ventral lobe with finger-like projections sometimes with small papillae, a finger-like projection near the acicula, and a sinuous row of chaetae (S. boa-parapodium). Neuropodial chaetae of three kinds: 1) a few simple with whorls of spines; 2) compound, with short blades, bidentate tips and smooth shafts; (3) compound with multiarticulate blades, bidentate tips and spinose shafts (S. boa-neuropodial chaetae).
Up to 200 mm for 200 chaetigers.
Scales mottled grey.
Littoral to sublittoral in sand.
North Sea, Skagerrak. World-wide except Arctic and Antarctic.