Body thin, cylindrical and smooth, gradually tapering to the posterior end. The most posterior part , the post-abdomen lies forwardly folded in a ventral abdominal groove, with the anus facing anteriorly.
Outer opercular chaetae with 4-5 teeth; the median tooth being smooth and hardly longer than lateral teeth; median opercular chaetae geniculate, subdistally hollowed out and distally tapering; inner opercular chaetae similar to median ones, but even longer (S. alveolata-anterior end). Numerous thread-like tentacular filaments near the mouth on the ventral side. Two short, conical palps between the tentacular lappest on the ventral side.
First chaetiger with oval neuropodia, which bear fine, toothed capillaries. Second chaetiger with toothed capillaries and a small, triangular, cirrus on the neuropodia. Gills from the second chaetiger onwards, decreasing in length to the posterior end of the body.
Last 3 thoracic segments with rectangular notopodia with 8-10 large paddle-shaped chaetae. Neuropodia without cirri; chaetae like those on notopodia, but smaller. Abdomen distinguished by lateral fleshy flaps on notopodium, bearing short saw-edged uncini. Uncini with 5-6 teeth. Abdominal neuropodia papillae-like, with long feathery capillaries (S. alveolata-detail).
Up to 40 mm for 37 chaetigers.
Made of cemented sand grains, often densely aggregated, their apertures forming a honeycomb pattern (S. alveolata-tube).
Wine-red, brownish, spotted; tail brown. Mature males whitish, mature females violet.
On rock (S. alveolata-reef); eulittoral to 26 m.
North Sea, eastern North Atlantic to Senegal.