The body is elliptical, somewhat rounded anteriorly and tapering posteriorly. It may be up to 50 mm long and 25 mm wide. The dorsal surface is yellowish, yellowish-white or whitish-cream. The dark median longitudinal line runs from the anterior end to near posterior region. Several similar lines run from the anterior region to unite posteriorly to the median longitudinal line. The well-developed marginal tentacles are speckled with numerous eyes. The cerebral eyes are arranged in two small elongate groups. The mouth is situated close behind the cerebral organ. The intestinal trunk extends to the posterior end of the body, trifurcates anteriorly and gives off several pairs of lateral branches, whose secondary branches anastomose freely.
The male copularory complex is complex, situated midway between the mouth and the ventral sucker. The female genital pore is close behind the male pore; the uterine canals are confluent posteriorly with several pairs of lateral vesicles (P. vittatus-diagrammatic).
Among fouling organisms on piles, under stones in the intertidal zone, in rock crevices, among seaweeds and tunicate colonies (P. vittatus) in depths to 20 m or more.
Ranges from Scandinavia to western Mediterranean. There are several British records.