The organization of the body cavities is an important morphological trait that can be used for establishing the phylogenetic relationships between different groups of animals. In the present study, the hemocoel and coelomic systems of 10‐hr‐old juveniles and adults of the hermaphroditic oikopleurid Oikopleura gracilis were examined using light and transmission electron microscopy. The trunk hemocoel in 10‐hr‐old juveniles was represented by small clefts containing layers of extracellular matrix of adjacent tissues or interstices with migrating primordial germ syncytium. The wide hemocoel in the tail contained extracellular strands, subdividing the hemocoel into hemal sinuses. In adults, a large hemocoel appeared in the trunk and tail, and also contained extracellular strands. The hermaphroditic gonad was surrounded by its own lining, separating it from the hemocoel. The gamete‐filled cavity in the ovary and testis appeared only at late‐stage gonadogenesis, when the pre‐spawning reduction of syncytium occurred in the gonads. The true coelom in 10‐hr‐old juveniles and adults was represented by the pericardium. The lining of the pericardium consisted of myoepithelial and peritoneal cells. In the myoepithelial cells of 10‐hr‐old juveniles, myofibrils had been formed. The myoepithelial cells of adults had several parallel rows of completely differentiated myofibrils. The substantial reduction of the coelomic and circulatory systems in O. gracilis evidently results from the extreme shortening of ontogeny in appendicularians. Development in O. gracilis from early juvenile to adult involves the following steps, which also suggest how the tunicate heart may have evolved: a single‐layered coelomic sac gives rise to a grooved pericardium with an open hemal sinus (simple heart). In ascidians, this simple heart in turn gives rise to a closed tubular, double‐layered heart–pericardial complex, with a separate pericardial cavity and a closed heart, whose wall is formed by specialized myocardium.

中文翻译：

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附肢动物Oikopleura gracilis（Tunicata，Chordata）的少年和成年成年人的体腔超微结构表明心脏可能在被膜中演化

体腔的组织是重要的形态特征，可用于建立不同动物群之间的系统发育关系。在本研究中，雌雄同体的Oikopleurid Ocilpleura gracilis的10小时龄少年和成年人的血小管和腔速系统使用光和透射电子显微镜检查。10小时龄少年的躯干血小管以小裂口为代表，这些小裂口含有邻近组织或间隙的细胞外基质层，并伴有原始原始细菌合胞体的迁移。尾巴中宽阔的血co包含细胞外链，将血co细分为血窦。在成年人中，躯干和尾巴上出现了大的血co，并且还含有细胞外链。雌雄同体的性腺被其自身的衬里包围，将其与血细胞隔离开。当性腺的产卵前合胞体减少时，卵巢和睾丸中的配子填充腔才出现在生殖腺的晚期。10小时大的青少年和成年人的真正体腔以心包为代表。心包内膜由肌上皮和腹膜细胞组成。在10小时大的少年的肌上皮细胞中，形成了肌原纤维。成人的肌上皮细胞具有几排完全分化的肌原纤维。大幅减少了人体的循环系统和循环系统O.纤细显然从个体发育的appendicularians极端缩短的结果。从幼年幼虫到成年鼠的O. gracilis的发展涉及以下步骤，这也表明被膜心脏可能如何进化：单层腔囊形成心包膜开裂的心包膜，并带有开放性窦静脉（简单的心脏）。在海鞘中，这种简单的心脏反过来会形成一个封闭的管状双层心脏-心包复合体，具有一个独立的心包腔和一个封闭的心脏，其壁由特殊的心肌形成。