BACKGROUND The Chaetognatha (arrow worms) are a group of marine carnivores whose phylogenetic relationships are still vigorously debated. Molecular studies have as yet failed to come up with a stable hypothesis on their phylogenetic position. In a wide range of metazoans, the nervous system has proven to provide a wealth of characters for analysing phylogenetic relationships (neurophylogeny). Therefore, in the present study we explored the structure of the ventral nerve centre ("ventral ganglion") in Sagitta setosa with a set of histochemical and immunohistochemical markers. RESULTS In specimens that were immunolabeled for acetylated-alpha tubulin the ventral nerve centre appeared to be a condensed continuation of the peripheral intraepidermal nerve plexus. Yet, synapsin immunolocalization showed that the ventral nerve centre is organized into a highly ordered array of ca. 80 serially arranged microcompartments. Immunohistochemistry against RFamide revealed a set of serially arranged individually identifiable neurons in the ventral nerve centre that we charted in detail. CONCLUSION The new information on the structure of the chaetognath nervous system is compared to previous descriptions of the ventral nerve centre which are critically evaluated. Our findings are discussed with regard to the debate on nervous system organisation in the last common bilaterian ancestor and with regard to the phylogenetic affinities of this Chaetognatha. We suggest to place the Chaetognatha within the Protostomia and argue against hypotheses which propose a deuterostome affinity of Chaetognatha or a sister-group relationship to all other Bilateria.

中文翻译：

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对人马座腹神经中枢的新观察：对Chaetognatha（箭头蠕虫）的系统发育位置和双侧神经系统进化的影响。

背景技术Chaetognatha（箭头蠕虫）是一组海洋食肉动物，其系统进化关系仍在激烈争论中。分子研究尚未就其系统发生位置提出稳定的假设。在广泛的后生动物中，神经系统已被证明具有丰富的特征，可用于分析系统发育关系（神经系统发育）。因此，在本研究中，我们利用一组组织化学和免疫组织化学标记物探索了箭叶腹中腹神经中枢（“腹神经节”）的结构。结果在用乙酰化α微管蛋白免疫标记的标本中，腹神经中枢似乎是周围表皮内神经丛的浓缩延续。然而，突触素的免疫定位显示，腹神经中枢组织成高度排列的ca。80个连续排列的微型隔间。针对RFamide的免疫组织化学揭示了一组详细排列在腹神经中枢的可依次排列的可单独识别的神经元。结论将有关舌颌神经系统结构的新信息与先前对腹神经中枢的描述进行了比较，并对其进行了严格评估。我们的发现是关于最后一位共同的双边祖先关于神经系统组织的辩论，以及关于该龟足目的系统亲缘关系的讨论。