Vestimentiferans (Siboglinidae, Polychaeta) live as juveniles and adults in an obligate mutualistic association with thiotrophic bacteria. Since their development is aposymbiotic, metatrochophores of vestimentiferans from the East Pacific Rise colonizing deep-sea hydrothermal vents are infected with the specific symbiont, develop the trophosome, and reduce their digestive system. To gain insight into the anatomy and ultrastructure and to compare this stage with metatrochophores from other siboglinids, we serial sectioned and reconstructed three specimens using light and transmission electron microscopy. The metatrochophore was composed of a prostomium, a small peristomium, two chaetigers (or two chaetigers and one additional segment without chaetae), and a minute pygidium. A digestive system and an intraepidermal nervous system were developed. Larval organs such as the prototroch, the neurotroch, and an apical organ were present, along with juvenile/adult organs such as tentacles, uncini, pyriform glands, and the anlage of the nephridial organ. We propose that in vestimentiferans, the vestimentum is the head arising from the prostomium, peristomium, and the anterior part of the first chaetiger. In frenulates, in contrast, the head is composed on the one hand of the cephalic lobe arising from the prostomium and on the other of the forepart developing from the peristomium and the anterior part of the first chaetiger. In frenulates the muscular septum between the forepart and trunk develops later than the first two chaetigers. Since this septum has no counterpart in vestimentiferans, the forepart-trunk border of frenulates is not considered homologous with the vestimentum-trunk border in vestimentiferans. The obturacular region in vestimentiferans does not appear to be a body region but rather the head appendages arising from the first chaetiger. In contrast, the tentacles in frenulates are prostomial head appendages. In both taxa, the trunk is the posterior part of the first chaetiger, and the opisthosoma is the following chaetigers and the pygidium. Comparisons with other polychaetes suggest that two larval segments are autapomorphic for the monophyletic Siboglinidae.

中文翻译：

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深海热液喷口前肢动物（多毛纲：Siboglinidae）的metatrochophore

Vestimentiferans（Siboglinidae，Polychaeta）作为幼鱼和成虫生活在与硫营养细菌的专性互惠关系中。由于它们的发育是非共生的，来自东太平洋海隆的前卫藻的metatrochophores 殖民深海热液喷口被特定的共生体感染，发展滋养体，并减少它们的消化系统。为了深入了解解剖结构和超微结构，并将这个阶段与来自其他 sibogliids 的 metatrochophores 进行比较，我们使用光学和透射电子显微镜连续切片和重建了三个标本。metatrochophore 由一个 prostomium、一个小的 peristomium、两个 chaetigers（或两个 chaetigers 和一个没有 chaetae 的附加部分）和一个微小的 pygidium 组成。消化系统和表皮内神经系统得到发展。存在幼虫器官，例如原滑车、神经滑车和顶端器官，以及幼虫/成虫器官，例如触手、钩爪、梨状腺和肾器官的原位。我们提出，在前肢动物中，前肢是由前体、口周和第一毛囊的前部产生的头部。相比之下，在系带动物中，头部一方面由从前口产生的头叶组成，另一方面由从口周和第一毛囊前部发育的前部组成。在系带中，前部和躯干之间的肌肉隔膜比前两个毛囊发育晚。由于这个隔膜在前卫体中没有对应物，frenulates 的前躯干边界不被认为与前肢动物的外衣躯干边界同源。前肢动物的闭孔区域似乎不是身体区域，而是来自第一个 chaetiger 的头部附属物。相比之下，系带中的触手是前叶的头部附属物。在这两个分类群中，躯干是第一毛毛虫的后部，后躯体是下面的毛毛虫和尾毛虫。与其他多毛类动物的比较表明，单系 Siboglinidae 的两个幼虫片段是自体变形的。躯干是第一毛毛虫的后部，后躯体是下面毛毛虫和尾甲。与其他多毛类动物的比较表明，单系 Siboglinidae 的两个幼虫片段是自体变形的。躯干是第一毛毛虫的后部，后躯体是下面毛毛虫和尾甲。与其他多毛类动物的比较表明，单系 Siboglinidae 的两个幼虫片段是自体变形的。