Among bryozoans, cyclostome anatomy is the least studied by modern methods. New data on the nervous system fill the gap in our knowledge and make morphological analysis much more fruitful to resolve some questions of bryozoan evolution and phylogeny. The nervous system of cyclostome Crisia eburnea was studied by transmission electron microscopy and confocal laser scanning microscopy. The cerebral ganglion has an upper concavity and a small inner cavity filled with cilia and microvilli, thus exhibiting features of neuroepithelium. The cerebral ganglion is associated with the circumoral nerve ring, the circumpharyngeal nerve ring, and the outer nerve ring. Each tentacle has six longitudinal neurite bundles. The body wall is innervated by thick paired longitudinal nerves. Circular nerves are associated with atrial sphincter. A membranous sac, cardia, and caecum all have nervous plexus. The nervous system of the cyclostome C. eburnea combines phylactolaemate and gymnolaemate features. Innervation of tentacles by six neurite bundles is similar of that in Phylactolaemata. The presence of circumpharyngeal nerve ring and outer nerve ring is characteristic of both, Cyclostomata and Gymnolaemata. The structure of the cerebral ganglion may be regarded as a result of transformation of hypothetical ancestral neuroepithelium. Primitive cerebral ganglion and combination of nerve plexus and cords in the nervous system of C. eburnea allows to suggest that the nerve system topography of C. eburnea may represent an ancestral state of nervous system organization in Bryozoa. Several scenarios describing evolution of the cerebral ganglion in different bryozoan groups are proposed.

中文翻译：

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透射电子和共焦激光扫描显微镜揭示了环口苔藓虫 Crisia eburnea 的神经系统

在苔藓虫中，环口动物解剖学的现代方法研究最少。关于神经系统的新数据填补了我们知识的空白，并使形态学分析在解决苔藓虫进化和系统发育的一些问题方面更加富有成效。采用透射电子显微镜和共焦激光扫描显微镜研究了圆口虫 Crisia eburnea 的神经系统。大脑神经节有一个上部凹面和一个充满纤毛和微绒毛的小内腔，从而表现出神经上皮的特征。脑神经节与口周神经环、咽周神经环、外神经环相关。每个触手有六个纵向神经突束。体壁由粗的成对纵向神经支配。环神经与心房括约肌相关。膜囊、贲门和盲肠都有神经丛。环口虫 C. eburnea 的神经系统结合了 phylactolaemate 和 gynolaemate 的特征。触手的六个神经突束的神经支配与Phylactolaemata 相似。环口目和裸口目均具有咽周神经环和外神经环的特征。大脑神经节的结构可以被认为是假设的祖先神经上皮转化的结果。原始脑神经节以及 C. eburnea 神经系统中神经丛和索的组合表明 C. eburnea 的神经系统拓扑可能代表了苔藓虫神经系统组织的祖先状态。提出了几种描述不同苔藓虫类群中脑神经节进化的方案。