This species may reach a length of 1 m and width of 25 mm or more. The worms, however, are exceptionally contractile and may be less than half their extended length (C. marginatus). The head is sharply tapered and bluntly pointed, with wide cephalic furrows (C. marginatus-detail). Although it is stated that eyes are absent from this species, pigmented eyes are present. Their small size, however, makes them easily overlooked in living individuals. The body behind the mouth is dorsoventrally compressed, with sharp lateral margins. Transverse epidermal folds of varying sizes often give the nemerteans an irregularly wrinkled appearance, particularly when the body is contracted. The transparent caudal cirrus is slender.
The colour is typically a uniform greyish-brown with whitish or transparent lateral margins through which the pink coloured lateral nerve cords are often visible. Younger individuals are generally paler, and the dorsal pigmentation of older animals may be somewhat darker than that of the underside. Variations in colour reported include dark greyish green, slate-blue or dull brown. Both the mouth and cephalic slits are pale on their inner margins. In young specimens the cerebral ganglia and proboscis apparatus may show through the epidermis. Cerebratulus marginatus is able to swim actively with strong dorsoventral undulatory movements. When swimming it often rotates about its own longitudinal axis and frequently projects its head above the water surface. It also readily fragments when handled.
Rarely found on the lower shore, Cerebratulus marginatus is usually obtained when dredging in sandy or muddy sediments at depths of 20-150 m or more, although it may be caught on fishing lines when mussel flesh is used as bait.
The species has a wide geographic range in the northern hemisphere. In European waters it extends from Scandinavia and the British Isles to the Mediterranean and southwards to Madeira; it is widespread in the Arctic (King Charles Land, Bremer Sound, Hinlopen Strait, east Spitzbergen), and is found in the western North Altantic from Greenland, Labrador and Cape Cod southwards beneath the off-shore Arctic current, and in the Pacific from Alaska to San Diego, California on the eastern seaboard and Japan in the west.