The brain of the Nemertodermatida (Platyhelminthes) as revealed by anti-5HT and anti-FMRFamide immunostainings

The taxa Nemertodermatida and Acoela have traditionally been considered clo sely related and classified as sister groups within the Acoelomorpha Ehlers 1984 (Platyhelminthes). Recent molecular investigations have questioned th eir respective position. In this study, the 5-HT and FMRFamide immunoreacti vity (IR) in the nervous system of two nemertodermatids, Nemertoderma westb ladi and Meara stichopi, is described. The 5-HT immunoreactive pattern diff ers in the two nemertodermatids studied. In M, stichopi, two loose longitud inal bundles of 5-HT-immunoreactive fibres and an basi-epidermal nerve net were observed, In N. westbladi the 5-HT-IR shows a ring-shaped commissural structure, different from the commissural brain of acoels, In both nemertod ermatids, FMRFamide immunoreactive nerve fibres followed the 5-HT-immunorea ctive fibres, It is demonstrated that the Nemertodermatida have neither a ' commissural brain' structure similar to that of the Acoela, nor a 'true', g anglionic brain and orthogon, typical for other Platyhelminthes. The questi on of the plesiomorphic or apomorphic nature of the nervous system in Nemer todermatida cannot yet be answered. The neuroanatomy of the studied worms p rovides no synapomorphy supporting the taxon Acoelomorpha, (C) 2000 Harcour t Publishers Ltd.