Consists mostly of the traditional group Heteronemertea plus one small former palaeonemertean group, all having a ventral mouth located immediately posterior of the brain. They have in common the presence of a specialized pelagic larval form called a Pilidium, which in some cases has become modified so that it no longer is pelagic. The heteronemerteans consist of two groups, the Baseodiscidae and the Lineidae. The Lineidae comprises the most species (about 450), all of which have an elongate head with a more or less conspicuous pair of horizontal cephalic slits, one along each lateral margin, running from the tip to the back of the head. Usually, if eyes are present they are in one or both of the dorsal and ventral lips of these slits. The baseodiscids lack these slits and have a more rounded, lobe-shaped, head, with eyes if they are visible clustered near the anterior margin.