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 they primitive flatworms? Are they related to Deuterostomia? Do they form a sister taxon to other Bilateria? Are they bivalve molluscs? In order to provide more data for this discussion, a study of the nervous system of Xenoturbella westbladi and its relation to the musculature was performed, using 5-HT and FMRFamide immunocytochemistry, TRITC-conjugated phalloidin fluorescence for staining of F-actin filaments, confocal scanning laser microscopy and transmission electron microscopy. The nervous system comprises solely an intraepidermal net of nerve cells and processes. No ganglia or any other internal nervous structures could be detected. No evidence of 5-HT- or FMRFamide-immunoreactive innervation below the subepidermal membrane complex was obtained. The 5-HT and FMRFamide immunoreactivity occurs in separate sets of neurones. On the ultrastructural level, three types of neurones were observed: (1) the predominating light neurones, (2) the smaller dark neurones and (3) the bipolar sensory neurones bearing a single cilium with a long bipartite rootlet. Non-synaptic, paracrine, release sites are common and synapses are inconspicuous. In the layer of epidermal cells, close to the lateral furrow, F-actin filaments were observed. They reach from the basal membrane to the surface. The organisation of the nervous system appears very simple. Our results are compatible with the hypothesis of Xenoturbellida forming a sister taxon to Bilateria. No evidence was obtained for inclusion of X. westbladi in either the Mollusca or Plathelminthes.