The jaw apparatus in several annelid families represents a powerful tool for systematic approaches and evolutionary investigations. Nevertheless, for several taxa, this character complex has scarcely been investigated, and complete comparative analyses of all annelid jaws are lacking. In our comprehensive study, we described the fine structure of the jaw apparatus and the ventral pharyngeal organ (VPO) in Histriobdella homari – a minute ectocommensal of lobsters putatively belonging to the Eunicida – using different comparative morphological approaches, including SEM, TEM, CLSM and subsequent 3D reconstruction. The H. homari jaw apparatus is composed of ventral paired mandibles and dorsal symmetrical maxillae consisting of numerous dental plates, ventral carriers and an unpaired dorsal rod, and the general assemblage and arrangement of the different parts are highly comparable to those of other eunicid families. The jaw ultrastructure of histriobdellids resembles that of the families Dorvilleidae and (juvenile) Onuphidae. Furthermore, our data reveal that in the process of development of the jaw apparatus, the mandibles, maxillae II and unpaired dorsal rod are formed first, and the remaining maxillae and ventral carriers appear later. Notably, the muscular apparatus differs from that in Dorvilleidae and Onuphidae in terms of the number and arrangement of muscle fibers encompassing the jaws – not only because of the very small size of Histriobdella but also because histriobdellid maxillary protraction occurs due to straightening of the dorsal rod and thus requires a different muscular scaffold. Based on our investigations, the arrangement of the muscular apparatus of the jaws, the presence of paired ventral carriers and the dorsal rod, and the morphology of the ventral pharyngeal organ represent a histriobdellid autapomorphy. Our datasets form a basis for further comparative analyses to elucidate the evolution of Eunicida and jaw-bearing Annelida.

中文翻译：

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洞into科（Eunicida，Annelida）腹咽和颚的形态和演变的新见解

多个无节肢动物科的颌骨器械是进行系统方法和进化研究的有力工具。然而，对于几个分类单元，几乎没有研究过这种性状复合体，并且缺乏对所有无核颌骨的完整比较分析。在我们的综合研究中，我们使用不同的比较形态学方法，包括SEM，TEM，CLSM和随后的3D重建。H. homari颌骨器械由成对的下颌骨和对称的上颌骨组成，上颌骨由许多牙板，腹侧支架和不成对的背杆组成，而且各个部分的总体组装和布置与其他真人世家的家族具有高度的可比性。纤毛虫的下颌超微结构类似于Dorvilleidae和（少年）Onuphidae的下颌超微结构。此外，我们的数据表明，在颌骨器械的发展过程中，下颌骨，上颌II和不成对的背杆首先形成，而其余的上颌和腹侧支架则随后出现。值得注意的是，在围绕颌骨的肌肉纤维的数量和排列方面，肌肉器具与Dorvilleidae和Onuphidae的肌肉器具不同-不仅是因为Histriobdella的尺寸非常小，而且还因为Histriobdellid的上颌伸出是由于背杆挺直而发生的因此需要不同的肌肉支架。根据我们的调查，颌骨肌肉装置的排列，腹侧运载体和背杆成对存在以及腹咽器官的形态学表现为类三叶形的共形。我们的数据集为进一步的比较分析奠定了基础，从而阐明了真叶虫和下颚jaw鱼的进化。