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Evolution of the nervous system in Paraphanostoma (Acoela). -Zoologica Scripta, 33 , 71-88. According to recent molecular studies, the Acoela are the earliest extant bilaterian group. Their nervous system displays a striking variety of patterns. The aim of the present investigation was to study the variability of the nervous system in a monophyletic group of the Acoela. Six species of Paraphanostoma were chosen for the study. Using immunocytochemical methods and confocal scanning laser microscopy, the immunoreactive patterns of serotonin (5-HT) and the neuropeptide GYIRFamide were described in detail. The study has demonstrated that the brains in Paraphanostoma species, although diverse in detail, still follow the same general pattern. 18S rDNA sequences were used to generate a hypothesis of the phylogeny within the group. Characters of the nervous system revealed in this study were coded and analysed together with 18S rDNA data. Several synapomorphies in the nervous system characters were identified. However, numerous parallelisms in the nervous system evolution have occurred. Data obtained demonstrate that the genus Paraphanostoma is closely related to Childia and should belong to the same family, Childiidae.

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

The phenomenon of asexual multiplication is rare in the animal kingdom, but it occurs in all main flatworm taxa. In the present paper, we review data regarding the presence of different forms of asexual multiplication in flatworms and argue that the presence of a population of totipotent or pluripotent stem cells, "neoblasts", is a primitive feature of decisive importance for the developing potential of flatworms. Next we present information on the role of stem cells in fission, head regeneration, and pharynx regeneration of planarians. Furthermore, the tracing of neoblasts in lower flatworms and cestodes is presented, and the results indicating heterogeneity of the neoblast pool are discussed. Finally, the mode by which the neoblasts are stimulated to divide, migrate, and differentiate and the nature of the interactions are discussed. We focus on (i) biogenic amines and neuropeptides, (ii) the role of neuropeptides in the early stage of regeneration, (iii) the evidence for the influences of growth factors and nitric oxide, and (iv) the influence of weak electromagnetic fields. We discuss the pattern in which a gradient system of morphogens and (or) a hierarchical system of inductions is expressed in development.

In order to solve the question whether Plathelminthes belonging to the taxon Acoela have a brain and an orthogon of the common flatworm type, an immunocytochemical study of the pattern of serotonin (5-HT) in four species was performed. In all species the 5-HT immunoreactivity revealed no ganglionic cell mass typical for other Plathelminthes, only a symmetrical brain-like structure composed of commissural fibres associated with a few cell bodies. 5-HT immunoreactivity was detected in three to five pairs of longitudinal nerve cords, connected by an irregular network of immunoreactive transverse fibres. No regular orthogon was visualised. All the surface sensilla were strongly immunoreactive. The unique commissural brain and cordal nervous system found in Acoela support the view of a deep gap lying between Acoela and other Plathelminthes.& b d y :