Body, except for parapodia and pygidium, without papillae.
Prostomium rounded quadrangular with four reddish eyes with lenses, in a trapezoid arrangement, ocelli absent. Palps longer than prostomium, longer than broad, only divided by a lateral groove. Median antenna a little longer than lateral ones. Lateral antennae flask-shaped, shorter than prostomium. Tentacular cirri and dorsal cirri like lateral antennae in size and shape, as long as parapodial lobes or shorter.
Dorsal cirri absent from the second parapodium. Ventral cirri short. Aciculae with a terminal button (S. bulbosa-parapodium).
Blades of compound chaetae short, unidentate. Middle and posterior parapodia with one more or less S-shaped upper and lower unidentate simple chaeta (S. bulbosa-chaetae).
Body up to 6 mm long for 48 segments.
Pale brownish or yellowish-white.
North Atlantic up to Mediterranean up to southern North Sea.