Dorsal surface with two median and two lateral rows of tiny conical papillae. Ventrally, the papillae are irregular spread.
Prostomium semicircular with four blackish eyes, with lenses, in a trapezoid arrangement, two small ocelli present; prostomium not clearly separated from the peristomium. Palps fused, together semicircular, broad and divided by a median groove. Lateral antennae flask-shaped, shorter than prostomium. Median antenna similar and hardly longer.
Tentacular cirri, flask-shaped, not much shorter than the antennae.
Dorsal cirri present on all parapodia, on anterior body flask-shaped, about as long as long antennae and always shorter than segment-width. Ventral cirri finger-shaped, shorter than parapodial lobes. Aciculae slightly hooked.
Blades of compound chaetae short, unidentate. All parapodia with one upper simple chaeta, parapodia of last chaetiger with an additional lower simple chaeta (S. tetralix-detail).
Pygidium with finger-shaped cirri, slightly longer than posterior dorsal cirri.
Proboscis with a large tooth anteriorly.
Body up to 4 mm long for 29 segments.
Eulittoral to 345 m, on soft bottoms and between algae.
North-east Atlantic up to Mediterranean, English Channel, North Sea and Skagerrak.