An order of the class Anopla in the phylum Rhynchocoela, with an unarmed proboscis, a thick partly fibrous dermis, and a three-layered body musculature composed of outer longitudinal, median circular, and inner longitudinal strata. Cerebral organs, cephalic grooves and slits, and eyes are generally present. The alimentary system consists of a mouth, foregut, intestine with regular lateral diverticula but no cecum, and anus. The principal family, Lineidae, contains some of the most common and best-known rhynchocoelan genera of temperate seashores, such as Lineus (“bootlace worms”), Micrura, and Cerebratulus (“ribbon worms”); the last one widely used in studies on regeneration, embryology, and nutrition. Heteronemertini are mainly marine littoral in habit.