The animal possesses an elliptical shape, and may reach a length up to 10 mm and a width of 4 mm. The colouring is variable in the translucent body, derived mainly from food particles in the intestinal system, being red or orange, black, dark green, brown, yellow or white (S. ellipse-variation). The body is also patterned with a whitish latticework due to the underlying ovaries. The eyes are few, scattered above and below the bases of the marginal tentacles, there are also two divergent sets posterior to the cerebral organ; characteristic are the two pairs of eyes over the hinder margin of the cerebral organ and an isolated eye near each of its antero-lateral margins (S. ellipse-eyes). The intestinal trunk with 4-6 pairs of lateral branches; the secondary branches are not anastomosing (S. ellipse-diagrammatic). The female pore is immediately anterior to the ventral sucker. The mouth and the male aperture open together into a common atrium immediately posterior to the cerebral organ.
The male copulatory complex is anterior to the pharynx; the female complex lies beneath or posterior to the hinder end of the pharynx. the uterine canals run anteriorly and posteriorly from the vagina to form a H-shaped figure (S. ellipse-copulatory apparatus).
Common in intertidal zone, among algae, on Mytilus , Balanus and Ciona colonies. Also on shells, stones and shingle in depths to about 200 m.
From Scandinavia to western Mediterranean. Several British localities. Recorded also from South Africa, the Patagonian region and South Georgia.