Muramatsu Banri, Suzuki Daichi G, Suzuki Masakazu, Higashiyama Hiroki
Department of Biological Science, Graduate School of Science, Shizuoka University, Shizuoka, 422-8529, Japan.
Faculty of Life and Environmental Sciences, University of Tsukuba, Tennodai, Tsukuba, 305-8572, Japan.
Anat Rec (Hoboken). 2024 Jan;307(1):155-171. doi: 10.1002/ar.25208. Epub 2023 Mar 23.
Hagfish (Myxinoidea) are a deep-sea taxon of cyclostomes, the extant jawless vertebrates. Many researchers have examined the anatomy and embryology of hagfish to shed light on the early evolution of vertebrates; however, the diversity within hagfish is often overlooked. Hagfish have three lineages, Myxininae, Eptatretinae, and Rubicundinae. Usually, textbook illustrations of hagfish anatomy reflect the morphology of the Myxininae lineage, especially Myxine glutinosa, with its single pair of external branchial pores. Here, we instead report the gross anatomy of an Eptatretinae, Eptatretus burgeri, which has six pairs of branchial pores, especially focusing on the coelomic organs. Dissections were performed on fixed and unfixed specimens to provide a guide for those doing organ- or tissue-specific molecular experiments. Our dissections revealed that the ventral aorta is Y-branched in E. burgeri, which differs from the unbranched morphology of Myxine. Otherwise, there were no differences in the morphology of the lingual apparatus or heart in the pharyngeal domain. The thyroid follicles were scattered around the ventral aorta, as has been reported for adult lampreys. The hepatobiliary system more closely resembled those of jawed vertebrates than those of adult lampreys, with the liver having two lobes and a bile duct connecting the gallbladder to each lobe. Overall, the visceral morphology of E. burgeri does not differ significantly from that of the known Myxine at the level of gross anatomy, although the branchial morphology is phylogenetically ancestral compared to Myxine.
盲鳗（黏盲鳗目）是圆口纲的一个深海分类群，属于现存的无颌脊椎动物。许多研究人员研究了盲鳗的解剖结构和胚胎学，以阐明脊椎动物的早期进化；然而，盲鳗内部的多样性常常被忽视。盲鳗有三个谱系，即黏液盲鳗亚科、七鳃鳗亚科和红盲鳗亚科。通常，教科书上盲鳗解剖结构的插图反映的是黏液盲鳗亚科谱系的形态，尤其是单对外部鳃孔的黏盲鳗。在这里，我们转而报告七鳃鳗亚科的日本七鳃鳗的大体解剖结构，它有六对鳃孔，特别关注体腔器官。对固定和未固定的标本进行了解剖，为那些进行器官或组织特异性分子实验的人提供指导。我们的解剖显示，日本七鳃鳗的腹主动脉呈Y形分支，这与黏盲鳗的非分支形态不同。否则，在咽区的舌器或心脏形态上没有差异。甲状腺滤泡散布在腹主动脉周围，这与成年七鳃鳗的情况相同。与成年七鳃鳗相比，肝胆系统更类似于有颌脊椎动物，肝脏有两叶，胆管将胆囊连接到每个叶。总体而言，在大体解剖水平上，日本七鳃鳗的内脏形态与已知的黏盲鳗没有显著差异，尽管鳃的形态在系统发育上比黏盲鳗更原始。
