A blood vessel, invariably thick-walled, extending the full body length behind the cerebral ganglia, running just above the alimentary canal; typically the vessel enters the ventral rhynchocoel wall near the cerebral ganglia to form either a rhynchocoelic villus or a vascular plug; in some nemerteans of the order Hoplonemertea, the vessel branches for a short distance and two vascular plugs are developed; in several nemerteans of the order Palaeonemertea and polystiliferous hoplonemertean genera the mid-dorsal vessel is either reduced or missing.