リファレンス
The sensory epithelium of the tentacles and the rhinophore of Nautilus pompilius L.(Cephalopoda, Nautiloidea).
Ruth, P., Schmidtberg, H., Westermann, B., & Schipp, R. (2002). Journal of Morphology, 251(3), 239-255.
https://onlinelibrary.wiley.com/doi/abs/10.1002/jmor.1086
コメント
オウムガイ Nautilus pompiliusの眼周辺にあるrhinophoreという感覚器官やocular tentacles，そして多数あるdigital tentaclesなどの触手にある感覚性の上皮細胞について，SEMや組織切片を用いて詳細に調べた論文．繊毛を持つ感覚性上皮細胞をタイプ分けし，その分布などの違いが器官や部位ごとに記述されている．
餌を採る際の行動などと合わせて機能的な面での考察もなされていた．以前より感覚器官とみなされていたrhinophoreやocular tentaclesに加え，digital tentaclesの一部も感覚器官として機能しているのではないかということが明らかにされている．また，digital tentaclesの中でも外側にあるlong digital tentaclesと，口周辺のbuccal tentaclesを取り囲むmedial digital tentaclesでは形態的，機能的に大きく違いがみられるとのことである．このような違いは異なる種のオウムガイ (オオベソオウムガイ Nautilus macromphalus) では見られないようであり，種特異的にdigital tentaclesの機能分化が起きているかもしれないというのはとても興味深い．この論文ではある程度成長した幼若個体を観察しているので，さらに発生過程ごとに形態にどのような変化がみられ，どのように二つのタイプに分化していくのか，ocular tentales との違いがどのように生じるのかというのも非常に気になるところ．
要旨
Nine intraepithelial ciliated cell types that are presumed to be sensory cells were identified in the epithelium of the pre- and postocular tentacles, the digital tentacles, and the rhinophore of the juvenile tetrabranchiate cephalopod Nautilus pompilius L. The morphological diversity and specialization in distribution of the different ciliated cell types analyzed by SEM methods suggest that these cells include receptors of several sensory functions. Ciliated cell types in different organs that show similar surface features were combined in named groups. The most striking cell, type I, is characterized by a tuft of long and numerous cilia. The highest density of this cell type occurs in ciliary fields in the epithelium of the lamellae of the pre- and postocular tentacles, in the olfactory pits of the rhinophores, and in the lamellae of four pairs of lateral digital tentacles, but not in the epithelium of the medial digital tentacles. The similar morphological data, together with behavioral observations on feeding habits, suggest that this cell type may serve in long-distance chemosensory function. The other ciliated cell types are solitary cells with specific spatial distributions in the various organs. Cell types with tufts of relatively short, stiff cilia (types III, IV, VIII), which are distributed in the lateral and aboral areas of the tentacles and at the base of the tentacle-like process of the rhinophore, are considered to be employed in mechanosensory transduction, while the solitary cells with bristle-like cilia at the margin of the ciliary fields (type II) and at the base of the rhinophore (type IX) may be involved in chemoreception. Histological investigation of the epithelium and the nerve structures of the different organs shows the proportion and distribution of the sensory pathways.
Two different types of digital tentacles can be distinguished according to their putative functions: lateral slender digital tentacles in four pairs, of which the lowermost are the so-called long digital tentacles, participate in distance chemoreception, and the medial digital tentacles, whose terminal axial nerve cord may represent a specialized neuromechanosensory structure, appear to have contact chemoreceptive abilities.
