Many annelids, including well-studied species such as Platynereis, show similar structured segments along their body axis (homonomous segmentation). However, numerous annelid species diverge from this pattern and exhibit specialised segments or body regions (heteronomous segmentation). Recent phylogenomic studies and paleontological findings suggest that a heteronomous body architecture may represent an ancestral condition in Annelida. To better understand the segmentation within heteronomous species we describe the myogenesis and mesodermal delineation of segments in Siboglinum fiordicum during development. Employing confocal and transmission electron microscopy we show that the somatic longitudinal musculature consists of four separate strands, among which ventrolateral one is the most prominent and is proposed to drive the search movements of the head of the late metatrochophore. The somatic circular musculature lies inside the longitudinal musculature and is predominantly developed at the anterior end of the competent larva to support the burrowing behaviour. Our application of transmission electron microscopy allows us to describe the developmental order of the non-muscular septa. The first septum to form is supported by thick bundles of longitudinal muscles and separates the body into an anterior and a posterior region. The second group of septa to develop further divides the posterior body region (opisthosoma) and is supported by developing circular muscles. At the late larval stage, a septum reinforced by circular muscles divides the anterior body region into a forepart and a trunk segment. The remaining septa and their circular muscles form one by one at the very posterior end of the opisthosoma. The heteronomous Siboglinum lacks the strict anterior to posterior sequence of segment formation as it is found in the most studied annelid species. Instead, the first septum divides the body into two body regions before segments are laid down in first the posterior opisthosoma and then in the anterior body, respectively. Similar patterns of segment formation are described for the heteronomous chaetopterid Chaetopterus variopedatus and serpulid Hydroides elegans and may represent an adaptation of these annelids to the settlement and transition to the sedentarian-tubiculous mode of life.

中文翻译：

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Siboglinum fiordicum 的肌发生揭示了胡须蠕虫（Siboglinidae、Annelida）的身体区域化

许多环节动物，包括经过充分研究的物种，如扁肠动物，沿其体轴显示出相似的结构节段（同源节段）。然而，许多环节动物物种偏离了这种模式，并表现出专门的节段或身体区域（异质节段）。最近的系统发育学研究和古生物学发现表明，异律的身体结构可能代表环节动物的祖先状况。为了更好地理解异律物种内的分割，我们描述了 Siboglinum fiordicum 发育过程中节段的肌发生和中胚层轮廓。采用共聚焦和透射电子显微镜，我们发现体细胞纵向肌肉组织由四个独立的肌束组成，其中腹外侧肌组织是最突出的，并且被认为驱动晚期后元细胞头部的搜索运动。体细胞圆形肌肉组织位于纵向肌肉组织内部，主要在有能力的幼虫的前端发育，以支持挖洞行为。我们对透射电子显微镜的应用使我们能够描述非肌肉隔膜的发育顺序。第一个形成的隔膜由厚的纵向肌肉束支撑，并将身体分为前部和后部区域。第二组隔膜进一步发育，将后体区域（后体）进一步分开，并由发育中的圆形肌肉支撑。在幼虫后期，由圆形肌肉加固的隔膜将身体前部区域分为前部和躯干部分。剩余的隔膜及其环肌在后体的最后端一一形成。异律的 Siboglinum 缺乏节段形成的严格的前后顺序，正如在大多数研究的环节动物中所发现的那样。相反，第一隔膜将身体分为两个身体区域，然后分别将节段放置在后体，然后放置在前体。异律毛蕨类 Chaetopterus variopedatus 和蛇类 Hydroides elegans 也有类似的节段形成模式，可能代表了这些环节动物对定居的适应以及向久坐管状生活方式的过渡。