The annelid anterior central nervous system is often described to consist of a dorsal prostomial brain, consisting of several commissures and connected to the ventral ganglionic nerve cord via circumesophageal connectives. In the light of current molecular phylogenies, our assumptions on the primary design of the nervous system in Annelida has to be reconsidered. For that purpose we provide a detailed investigation of the adult nervous system of Magelonidae – a putatively basally branching annelid family - and studied early stages of the development of the latter. Our comparative investigation using an integrative morphological approach shows that the nervous system of Magelonidae is located inside the epidermis. The brain is composed of an anterior compact neuropil and posteriorly encircles the prostomial coelomic cavities. From the brain two lateral medullary cords branch off which fuse caudally. Prominent brain structures such as nuchal organs, ganglia or mushroom bodies are absent and the entire nervous system is medullary. Our investigations also contradict previous investigations and present an updated view on established assumptions and descriptions. The comprehensive dataset presented herein enables a detailed investigation of the magelonid anterior central nervous system for the first time. The data reveal that early in annelid evolution complexity of brains and anterior sensory structures rises. Polymorphic neurons in clusters and distinct brain parts, as well as lateral organs - all of which are not present in outgroup taxa and in the putative magelonid sister group Oweniidae - already evolved in Magelonidae. Commissures inside the brain, ganglia and nuchal organs, however, most likely evolved in the stem lineage of Amphinomidae + Sipuncula and Pleistoannelida (Errantia+ Sedentaria). The investigation demonstrates the necessity to continuously question established descriptions and interpretations of earlier publications and the need for transparent datasets. Our results also hint towards a stronger inclusion of larval morphology and developmental investigations in order to understand adult morphological features, not only in Annelida.

中文翻译：

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Magelonidae（Annelida）的神经系统的解剖结构和发育-洞悉大脑进化的见解

通常描述的前中枢神经系统由一个背侧假体脑组成，该背侧假体脑由几个连合组成，并通过食管连接体连接到腹神经节神经索。鉴于当前的分子系统发育，我们必须重新考虑对Annelida神经系统主要设计的假设。为此，我们提供了对Magelonidae的成年神经系统的详细研究，Magelonidae是一个假定的基部分支的蛛网膜家族，并研究了后者的发育早期。我们使用综合形态学方法进行的比较研究表明，Magelonidae的神经系统位于表皮内部。大脑由一个紧凑的前神经丛组成，并向后环绕假体腔腔。从大脑分支出两条外侧髓质线，尾部融合在一起。缺少诸如大脑器官，神经节或蘑菇体之类的突出大脑结构，整个神经系统都是延髓的。我们的调查还与以前的调查相矛盾，并针对已建立的假设和描述提供了最新的观点。本文介绍的综合数据集首次实现了对Magelonid前中枢神经系统的详细研究。数据显示，在早期的神经突发育过程中，大脑和前感觉结构的复杂性增加。簇状和独特的大脑部分以及侧器官中的多态神经元-所有这些都不存在于外群分类单元和推定的magelonid姐妹群Oweniidae中-已在Magelonidae中进化。大脑内部的连合，然而，神经节和颈部器官最有可能在两栖类+ Sipuncula和Pleistoannelida（Errantia + Sedentaria）的茎谱系中进化。调查表明，有必要不断质疑早期出版物的既定描述和解释，以及对透明数据集的需求。我们的研究结果还暗示，不仅在幼虫中，更强的包括幼虫形态和发育研究，以便了解成虫的形态特征。