Lineus ruber (Müller, 1774)

Length up to 11 cm, width up to 4 mm. A long and slender, reddish brown or bright red species with somewhat paler lateral margins and distinctly paler ventral side. The typical heteronemertean lateral slits on either side of the head extend back to the brain, which is usually visible through the body wall as two diffuse reddish spots, and at least two eyes are situated along the dorsolateral margin on either side of the head. Sexually mature individuals have gonopores (visible as whitish spots) arranged in a longitudinal row on either side of the body (dorsally). A voracious predator, capable of catching prey distinctly larger than itself. Reproduction occurs in the spring, at a water temperature of 4–8 °C. Difficult to separate from Lineus viridis and Micrura varicolor.