The living specimens are up to 9 mm long and 1.75 mm wide. This species is noteworthy for it's rounded head, slender appearance and the characteristic arrangement of the two eyes: these are close-set, distant from the anterior margin and about level with the narrow neck region. The dorsal surface is variable in colour: milky-white, yellow-orange or pale brown. In the more heavily pigmented forms the areas over the pharynx and copulatory organs appear pale. Frequently the testes and intestinal branches are visible and have a remarkably regular, or pseudometametric, arrangement along the axis of the body. The ventral surface lacks pigment, and in mature specimens two gonopores are visible posterior to the mouth.
The penis is large and conical and it projects downwards into the male atrium, which opens ventrally via a gonopore. The vasa deferentia open separately into the penis bulb. Anterior to the penis lies the bursa copulatrix, which opens anteriorly to, and independently of the common gonopore. The female genital duct (vagina and primary bursal canal) extends posteriorly from the rear wall of the atrium. It receives numerous shell-glands and the two oviducts. Paired receptacular ducts connect the bursa with the oviducts at their point of entry into the primary bursal canal and may here be swollen to form accessory seminal vesicles (U. vulgaris-detail).
In the eulittoral zone beneath rocks and other objects. May also be common in salt marshes among algae or under foreign objects.
Known from northern Europe and Atlantic North America.