The body is threadlike, length 10-30 cm, width only 0.5-1 mm. The head is long and bluntly pointed (C. linearis-head). The mouth is placed far behind the cerebral ganglia. The colour is white, creamish or of a pale grey translucent appearance, commonly with a yellowish tinge in the anterior regions. The intestinal track may be coloured by the gut contents.
Cephalic glands are present. The gonads, which may contain mature gametes during the period January to June, are located alongside the lateral blood vessels. When irritated, the animal may contract into a tight knot, but it does not coil spirally like species of the genus Procephalothrix , with which it may be confused.
Usually found intertidally, from the mid-tide level downwards, but may be dredged from sublittoral habitats. Borrows into clean coarse sand, muddy sand or mud to depths of 25-30 cm. Also found beneath stones and boulders, among the holdfasts of Laminaria , or crawling among small algae such as Corallina or hydroids like Tubularia .
Britain, Sweden, Mediterranean, Greenland, eastern North America, Japan.