Animal with a delicate, cuneate body. It reaches a length of 12-25 mm. The body is whitish or pale ash coloured, often with a brownish tinge dorsally (L. tremellaris-variation). The cerebral eyes are arranged in two elongate clusters, each with 20-25 small eyes, on either side of the cerebral organ; there are 6-12 larger tentacular eyes, which form two round clusters. Intestinal trunk with 6-8 pairs of lateral branches, which ramify, but do not anastomose (L. tremellaris-internal). The adhesive depression between the genital pores has muscular margins and functions as a genital sucker (L. tremellaris-diagrammatic).
The vasa deferentia arise near the lateral margins of the pharynx and form a loop behind the female antrum; an arcuate seminal vesicle lies near the hinder end of the pharynx. Mass of coarse muscle-fibres invests male copulatory complex. The [t]Lang's vesicle[/t] is small.
Under stones and shells between tidemarks. Occurs in large numbers on Mytilus colonies. Also among algae in rock pools, in Laminaria holdfasts, on colonies of bryozoans and ascidians. May be dredged in depths to 100 m.
This apparently eurythermal species has a very wide distribution. It ranges from Greenland and the White Sea to the Mediterranean and the Black Sea.