(Ström, 1768)
Description:
The living specimens may reach a length of up to 9 mm and width of 1.5 mm, but they are usually smaller. The species have a broad, plump body with a rounded head. The marginal tentacles are distinct and the two eyes are widely set. The body is constricted laterally near the eyes and gradually widens so that it is broadest posteriorly. The dorsal surface is usually in some shade of brown, mostly with a greyish or olive tinge. The alimentary and reproductive systems are often visible through the surface (P. littoralis-internal). The pigmentation of the head is characteristic to this species: laterally there are two large white patches, which contain the eyes; the pigmented area between the white patches is broken by two small wedges of unpigmented tissue and occasionally these may narrow posteriorly to form weak longitudinal stripes. The ventral surface is pale.
The penis is conical, unarmed and set in a spacious male atrium. The vasa deferentia open separately into the penis bulb, but do not form a well-developed seminal vesicle. The testicular follicles are numerous and dorsally situated throughout the body. The bursa copulatrix is large and the bursal canal runs anteriorly into the atrium and its ventral diverticulum receives both the shell-glands and the oviducts, the latter having united to form a common oviduct.
Habitat:
A typical inhabitant of freshwater streams running through the intertidal zone, where it may be found under stones and other submerged objects. It also occurs in rock pools and under stones embedded in damp sand of the intertidal zone. This is a very active organism and when disturbed it will indulge in a characteristic rapid looping, leech-like method of locomotion.
Distribution:
Known from Europe and Atlantic North America. This is the most common of the British marine planarians.