(Müller, 1776)
Description:
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; Lang's vesicle elongate.
Life-cycle direct.
Habitat:
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.
Distribution:
Ranges from Scandinavia to the English Channel and in the boreal regions of the Atlantic and Pacific coasts of North America.