Shape characteristic, posterior body being tail-like slender and terminating in an orange-coloured tassel, sometimes coiled posteriorly; bodyform is unique for the species, enabling easy identification. Characteristic, clinging scales in the middle of body, whereas the scales of second region are similarly represented in other, non-Scandinavian species (F. crossotus-scales). Foot shield flanks mouth opening laterally, but does not close preorally (F. crossotus-detail). Digestive midgut gland is short, not entering the slendered posterior body. Ctenidia large and up to 11-12 alternating lamellae at each side.
Animals with ctenidia and typical shape measure from 3 to 25 mm, usually only up to 15-20 mm. Retractions or extension causes an alteration of up to 100 % in length (fully contracted 10 mm, fully extended 20 mm).
Body brownish or even blackish veiled by distinct silver lustre. Slendered posterior body lighter, terminating in the orange-coloured pallial tassel. This orange-brown colour originates from secretion which also helps glue the spines. Ctenidia glimmer pinkishly.
Two sickle-like radular teeth basally pointed and not showing a symphyse. Lateral supports simple, and like the basal plate, only strengthened distally.
Soft, muddy bottoms in depths of 30 to 470 m.
Skagerrak, the Norwegian Channel, Scandinavian fjords from Bohuslän to Lofoten. Until now there are no records from the Norwegian Sea and accordingly it is not known whether F. crossotus or a congener is native to these bottoms. One record from the west Scottish Firth of Lorne (Distr. F. crossotus).