The oval body is rounded anteriorly and slightly tapering posteriorly. Mature specimens are 9-28 mm long and 4-13 mm wide. The worm is dorsally whitish, brown or greyish-brown, spotted with reddish-brown, or reddish-brown with scattered darker spots; ventrally greyish-white. There are no tentacles. The cerebral eyes are arranged in two elongate clusters extending anteriorly from the cerebral organ, 10-20 eyes in each cluster; the larger tentacular eyes form a paired cluster laterally to the hinder ends of cerebral clusters with 6-10 eyes in each. The mouth is centrally placed.
The genital pores are well separated. The seminal vesicle is arcuate or bulbous; the prostatic organ is bulbous with a thick muscular wall and lined with tall epithelium, containing 6-7 longitudinal chambers; the penis-papilla is small, bearing a long stylet lying coiled in a long penis-pocket and sometimes projecting through the male pore. The vagina externa is long and strongly muscular; [t]Lang's vesicle[/t] elongate.
Lives under stones and on weeds in the intertidal zone, on tunicate and oyster colonies; occurs in large numbers on Mytilus colonies. Dredged on muddy sand, gravel and shells in depths to 200 m.
Ranges from Scandinavia to the English Channel and in the boreal regions of the Atlantic and Pacific coasts of North America.