Body small, short, cylindrical, tapering at both ends. The body comprises prostomium, peristomium, 13 biannulate segments of which first and last two are achaetous, and a pygidium.
Prostomium short and rounded, broader than long. No eyes, tentacles or palps. Mouth ventrally on the 1st segment.
Segments not clearly separated.
Chaetigers with ventrolateral bundles of chaetae, arrangement of which shows little variation: chaetiger 1 with two winged chaetae with long whip-like distal extensions, two simple winged chaetae without distal extension, and two forked chaetae with inner group of 4-6 regular teeth between the strong outer prongs. This structure is difficult to see by light microscopy (S. subterranea-chaetae). Chaetiger 2 with three simple winged chaetae and two forked chaetae; two of each of these two types in chaetigers 3-10 .
Paired spermioducts open ventrally in segment 10. Females with ventral genital pores at boundary between segments 10 and 11.
Pygidium short and rounded.
1.5-2.6 mm for 13 segments.
Whitish, transparent; surface slightly iridescent.
Beaches with clean sand of medium grain size, near high tide level in tidal areas; as far as 10 m from the water line in non-tidal beaches. Down to 90 cm deep; with most of the animals at a depth of 30-60 cm in the winter and 0-20 cm in the summer.
North Sea (Sylt), Baltic (Bay of Kiel, Swedish south coast), Atlantic (Western France: Arcachon), Northern Norway, Mediterranean (French coast, Tunisian coast), Black Sea (Romanian coast), East coast of North America (Massachusetts, Maine, New Brunswick), Pacific coast of North America (California, Washington).