An immunocytochemical and ultrastructural study of the nervous and muscular systems of Xenoturbella westbladi (Bilateria inc. sed.)

The phylogenetic position of the Xenoturbellida is highly disputed. Are the y primitive flatworms? Are they related to Deuterostomia? Do they form a si ster taxon to other Bilateria? Are they bivalve molluscs? In order to provi de more data for this discussion, a study of the nervous system of Xenoturb ella westbladi and its relation to the musculature was performed, using 5-H T and FMRFamide immunocytochemistry, TRITC-conjugated phalloidin fluorescen ce for staining of F-actin filaments, confocal scanning laser microscopy an d transmission electron microscopy. The nervous system comprises solely an intraepidermal net of nerve cells and processes. No ganglia or any other in ternal nervous structures could be detected. No evidence of 5-HT- or FMRFam ide-immunoreactive innervation below the subepidermal membrane complex was obtained. The 5-HT and FMRFamide immunoreactivity occurs in separate sets o f neurones. On the ultrastructural level, three types of neurones were obse rved: (1) the predominating "light" neurones, (2) the smaller "dark" neuron es and (3) the bipolar sensory neurones bearing a single cilium with a long bipartite rootlet. Non-synaptic, paracrine, release sites are common and s ynapses are inconspicuous. In the layer of epidermal cells, close to the la teral furrow, F-actin filaments were observed. They reach from the basal me mbrane to the surface. The organisation of the nervous system appears very simple. Our results are compatible with the hypothesis of Xenoturbellida fo rming a sister taxon to Bilateria. No evidence was obtained for inclusion o f X. westbladi in either the Mollusca or Plathelminthes.