Wanninger A
Department of Biology, University of Copenhagen, Universitetsparken 15, DK-2100 Copenhagen, Denmark.
Acta Biol Hung. 2008;59 Suppl:127-36. doi: 10.1556/ABiol.59.2008.Suppl.21.
Recently, a number of neurodevelopmental studies of hitherto neglected taxa have become available, contributing to questions relating to the evolution of the nervous system of Lophotrochozoa (Spiralia + Lophophorata). As an example, neurogenesis of echiurans showed that these worm-shaped spiralians, which as adults do not exhibit any signs of segmentation, do show such traits during ontogeny, e.g. by segmentally arranged perikarya and commissures. Similarly, sipunculan worms, which have a single ventral nerve cord in the adult stage, develop this nerve cord by gradual fusion of a paired larval nerve during metamorphosis, and show transitional stages of segmentation. These findings indicate that echiurans, annelids and sipunculans stem from a segmented ancestor. By contrast, no traces of body segmentation are present during neurogenesis of basal molluscs. However, a tetraneurous condition (i.e. one pair of ventral and one pair of lateral nerve cords), as is typical for Mollusca, and a serotonergic larval apical organ that matches the complexity of polyplacophoran apical organs, were found in larval entoprocts, thus strongly supporting a mollusc-entoproct clade. Within the Lophophorata (Ectoprocta + Phoronida + Brachiopoda), data on nervous system development for any of the 3 lophophorate phyla are as of yet too scarce for profound phylogenetic inferences. Taking into account the most recent advances in molecular phylogenetics and developmental neurobiology, a scenario emerges that proposes a clade comprising Sipuncula + Annelida (including Echiura) on the one hand and a monophyletic assemblage of Entoprocta + Mollusca on the other.
最近，一些针对此前被忽视类群的神经发育研究已经开展，这有助于解答与冠轮动物（螺旋动物 + 触手冠动物）神经系统进化相关的问题。例如，螠虫的神经发生表明，这些蠕虫状的螺旋动物，成年时没有任何分节迹象，但在个体发育过程中确实表现出此类特征，比如通过分段排列的神经核和神经连合。同样，星虫类蠕虫在成年阶段有一条单一的腹侧神经索，在变态过程中通过成对幼虫神经的逐渐融合发育出这条神经索，并显示出分节的过渡阶段。这些发现表明，螠虫、环节动物和星虫起源于一个分节的祖先。相比之下，在基部软体动物的神经发生过程中没有身体分节的痕迹。然而，在幼虫内肛动物中发现了软体动物典型的四对神经索状态（即一对腹侧和一对外侧神经索），以及与多板纲动物顶器复杂性相匹配的5-羟色胺能幼虫顶器，从而有力地支持了软体动物 - 内肛动物进化枝。在触手冠动物（外肛动物 + 帚虫动物 + 腕足动物）中，关于这三个触手冠动物门中任何一门神经系统发育的数据都非常稀少，无法进行深入的系统发育推断。考虑到分子系统发育学和发育神经生物学的最新进展，出现了一种设想，即一方面提出一个包括星虫动物门 + 环节动物门（包括螠虫纲）的进化枝，另一方面提出一个内肛动物门 + 软体动物门的单系类群。
