The Australian lungfish, Neoceratodus forsteri (Krefft 1870), is the sole extant member of the Ceratodontidae within the Dipnoi, a small order of sarcopterygian (lobe-finned) fishes, that is thought to be the earliest branching species of extant lungfishes, having changed little over the last 100 million years. To extend studies on anatomical adaptations associated with the fish–tetrapod transition, the ultrastructure of the cornea and iris is investigated using light and electron (transmission and scanning) microscopy to investigate structure–function relationships and compare these to other vertebrate corneas (other fishes and tetrapods). In contrast to previous studies, the cornea is found to have only three main components, comprising an epithelium with its basement membrane, a stroma with a Bowman's layer and an endothelium, and is not split into a dermal (secondary) spectacle and a scleral cornea. The epithelial cells are large, relatively low in density and similar to many species of non-aquatic tetrapods and uniquely possess numerous surface canals that contain and release mucous granules onto the corneal surface to avoid desiccation. A Bowman's layer is present and, in association with extensive branching and anastomosing of the collagen fibrils, may be an adaptation for the inhibition of swelling and/or splitting of the stroma during its amphibious lifestyle. The dorsal region of the stroma possesses aggregations of pigment granules that act as a yellow, short wavelength-absorbing filter during bright light conditions. Desçemet's membrane is absent and replaced by an incomplete basement membrane overlying a monocellular endothelium. The iris is pigmented, well-developed, vascularised and contractile containing reflective crystals anteriorly. Based upon its ultrastructure and functional adaptations, the cornea of N. forsteri is more similar to amphibians than to other bony fishes and is well-adapted for an amphibious lifestyle.

中文翻译：

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澳大利亚肺鱼 Neoceratodus forsteri (Krefft 1870) (Dipnoi) 角膜和虹膜的形态：从水生环境过渡到陆地环境的功能和进化视角


澳大利亚肺鱼 Neoceratodus forsteri (Krefft 1870) 是 Dipnoi 中角齿鱼科中唯一现存的成员，Dipnoi 是肉翅目（叶鳍）鱼的一个小目，被认为是现存肺鱼中最早的分枝物种。过去一亿年里几乎没有。为了扩展与鱼类-四足动物过渡相关的解剖适应的研究，使用光和电子（透射和扫描）显微镜研究了角膜和虹膜的超微结构，以研究结构-功能关系，并将其与其他脊椎动物角膜（其他鱼类和虹膜）进行比较。四足动物）。与之前的研究相比，角膜仅具有三个主要成分，包括上皮及其基底膜、具有鲍曼层和内皮的基质，并且不分为真皮（次级）眼镜和巩膜角膜。上皮细胞很大，密度相对较低，与许多非水生四足动物物种相似，并且独特地具有许多表面管，这些表面管包含粘液颗粒并将其释放到角膜表面以避免干燥。存在鲍曼层，并且与胶原原纤维的广泛分支和吻合相关，可能是在其两栖生活方式期间抑制基质肿胀和/或分裂的适应。基质的背侧区域具有色素颗粒聚集体，在强光条件下充当黄色、短波长吸收滤光器。 Desçemet 膜缺失，取而代之的是覆盖单细胞内皮的不完整基底膜。虹膜有色素、发育良好、有血管且可收缩，前面含有反光晶体。 基于其超微结构和功能适应性，福斯特猪笼草的角膜比其他硬骨鱼更类似于两栖动物，并且非常适合两栖生活方式。