Living specimens are up to 7 mm long and 1.5 mm broad, but also much smaller specimens may be mature. The body is slender and lanceolate with a narrow rounded head and broad posterior end. The two eyes are set wide apart about one-quarter to one-third of the body-length from the anterior end. Consequently there is a very long pre-ocellar duct of the intestine. The colour is usually brown, the pigment appearing mottled or flecked. Lateral to the eyes, pigment may be lacking; medial to the eyes there may be darker stripes. The body is warty or papillose, especially anteriorly and laterally.
The animal is apparently dioecious, but whether the sexes are truly separate or sequential hermaphrodite has not been determined definitively. The penis lies horizontally in the male atrium. It is broadly conical, with a large bulb and tapers to a sharp point. The vasa deferentia unite to a common duct that enters the bulb within which there is a spacious cavity or seminal vesicle. The testicular follicles are numerous and situated dorsally throughout the body. In 'male' animals the bursa is poorly developed (S. dioica-detail male). In 'female' animals the atrium is empty but posteriorly to it there is a well-developed bursa and bursal canal. The oviducts open separately into the walls of the bursal canal, as do the shell-glands between the oviducal openings and the atrium (S. dioica-detail fem.).
On marine driftweed or in sandy habitats.
This is a rare species. Originally described from Normandy. Known from a few places in the Mediterranean. Only one record off the British coasts (Plymouth) from 1893.