Mayer Georg, Harzsch Steffen
Department of Anatomy and Cell Biology, University of Melbourne, Victoria, Australia.
BMC Evol Biol. 2007 Jul 15;7:118. doi: 10.1186/1471-2148-7-118.
Onychophora (velvet worms) represent the most basal arthropod group and play a pivotal role in the current discussion on the evolution of nervous systems and segmentation in arthropods. Although there is a wealth of information on the immunolocalization of serotonin (5-hydroxytryptamine, 5-HT) in various euarthropods, as yet no comparable localization data are available for Onychophora. In order to understand how the onychophoran nervous system compares to that of other arthropods, we studied the distribution of serotonin-like immunoreactive neurons and histological characteristics of ventral nerve cords in Metaperipatus blainvillei (Onychophora, Peripatopsidae) and Epiperipatus biolleyi (Onychophora, Peripatidae).
We demonstrate that paired leg nerves are the only segmental structures associated with the onychophoran nerve cord. Although the median commissures and peripheral nerves show a repeated pattern, their arrangement is independent from body segments characterized by the position of legs and associated structures. Moreover, the somata of serotonin-like immunoreactive neurons do not show any ordered arrangement in both species studied but are instead scattered throughout the entire length of each nerve cord. We observed neither a serially iterated nor a bilaterally symmetric pattern, which is in contrast to the strictly segmental arrangement of serotonergic neurons in other arthropods.
Our histological findings and immunolocalization experiments highlight the medullary organization of the onychophoran nerve cord and argue against segmental ganglia of the typical euarthropodan type being an ancestral feature of Onychophora. These results contradict a priori assumptions of segmental ganglia being an ancestral feature of arthropods and, thus, weaken the traditional Articulata hypothesis, which proposes a sistergroup relationship of Annelida and Arthropoda.
有爪动物（天鹅绒虫）代表了最基础的节肢动物类群，在当前关于节肢动物神经系统和体节进化的讨论中起着关键作用。尽管在各种真节肢动物中关于血清素（5-羟色胺，5-HT）的免疫定位有大量信息，但目前尚无关于有爪动物的可比定位数据。为了了解有爪动物的神经系统与其他节肢动物的神经系统相比如何，我们研究了布莱因维尔后栉蚕（有爪动物，栉蚕科）和比奥莱伊上栉蚕（有爪动物，栉蚕科）中血清素样免疫反应性神经元的分布以及腹神经索的组织学特征。
我们证明成对的腿神经是与有爪动物神经索相关的唯一节段性结构。尽管正中连合和外周神经呈现出重复模式，但其排列独立于以腿和相关结构的位置为特征的身体节段。此外，在所研究的两个物种中，血清素样免疫反应性神经元的胞体均未表现出任何有序排列，而是散布在每条神经索的整个长度上。我们既未观察到连续重复模式，也未观察到双侧对称模式，这与其他节肢动物中血清素能神经元的严格节段性排列形成对比。
我们的组织学发现和免疫定位实验突出了有爪动物神经索的髓质组织，并反对典型真节肢动物类型的节段性神经节是有爪动物的祖先特征这一观点。这些结果与节段性神经节是节肢动物祖先特征的先验假设相矛盾，从而削弱了提出环节动物和节肢动物为姐妹群关系的传统关节动物假说。
