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DescriptionLineus longissimus is an unsegmented, elongated ribbon worm. Young specimens range from dark olive brown to chocolate brown whereas adults are blackish brown to black. Epidermal cilia give the body a purplish irridescence. This species is the longest nemertean known. It is usually 5-15 m in length but can be over 30 m, usually 5 mm in width. The body is often streaked with pale longitudinal lines, especially on the anterior dorsal surface. The rectangular head has deep slits and ends in a pale colour tip. A row of up to 20 deep set reddish-brown or black eyes may be present either side of the snout. Pink or red cerebral ganglia may be seen through the epidermis.Lineus longissimus is the most commonly recorded nemertean in Britain. When handled, the animal produces large amounts of thick mucus with a faint pungent smell.