Slender, uniformly shaped animals with no division into regions except, perhaps, by means of colouring. They have the ability, unusual for the Caudofoveata, to coil up their bodies, enabled by the strengthened longitudinal layer of the ventromedially interrupted body-wall musculature at both edges of the suture. Anterior portion of middle body with a distinct midventral line, this suture of mantle edges corresponding with muscle interruption. Adjacent scales of proper shape accompany this suture. The almost clinging scales in general more slenderly formed and lanceolate in anterior body; those in posterior body show a basically rounded, tapering form. Sometimes the stroke of the scales is regionally reversed (S. ventrolineatus-scales). Foot shield situated only behind mouth opening which, however, assumes a dorsofrontal position, so that the foot plate in living animals can be stated to be in a frontal position; semi-contracted foot shield, however, simulates a divided plate. Diameter of pallial region corresponds with that of body, and has no special colour. Ctenidia small, medially bent, and have 6-8 distinct lamellae merely at outer edges.
Animals with ctenidia range from 3.5 to 35 mm, usually, however, they measure 5-20 mm. Retraction or extension may cause an alternation of 50 % in length.
Young or smaller specimens colourless, viz. with a whitish or pale brownish tinge. Larger individuals more definitively brown, at least in digestive gland region. The short prepallial region always seems to be light in colour and more or less transparent. Ctenidia and sometimes even the anterior body end with pinkish glimmer.
Seemingly predaceously formed teeth present in 10-16 transverse rows ending in an inwardly curved hook, medially provided with 9-11 bent denticles. Even in large individuals teeth hardly sclerotized, at most only frontal 2-4 pairs show a distinct strengthening. Consequently, radula generally difficult to observe in preparations.
Soft, muddy bottoms in depths of 40 to 1248 m. The species represents a typical archibenthal form, rising upwards only at the edges of these regions. Locally very high population densities may be found.
West coast of Sweden, Skagerrak, the Norwegian Channel, the Norwegian coast north to Vengsöyfjorden (near Tromsö). Seems to be a typical East-Atlantic species (Distr. S. ventrolineatus), distributed in Western Scotland, Loch Linnhe, Loch Creran and Firth of Lorne; the Irish Sea (off the Isle of Man); Bay of Biscay; Western Mediterranean; SE Africa.