Retina of the South American lungfish, Lepidosiren paradoxa Fitzinger

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Abstract
The characteristic feature of the retina of Lepidosiren paradoxa is the hugeness (20–25 μm) and sparse-ness of the cells which compose it. The epithelial layer is well pigmented. The visual cells may be classified into rods and single cones which possess a large oil droplet and a smaller paraboloid. Neither the epithelial pigment nor the visual cells show retinomotor responses. Landolt's clubs arising from the bipolar cells and terminating amidst the epithelial cell processes are present and resemble those of another Dipnoan, Protopterus. The ratio of photoreceptor, bipolar, and ganglion cells is about 2:2:1, indicating a very low degree of summation. There is no noticeable difference among the dorsal, vental, temporal, and nasal regions of the retina. The similarity of this retina to that of other dipnoans and urodeles is briefly discussed. It appears as if the growth of the eye were arrested (neoteny) during the evolution of this genus because of the insignificant role of vision in its life.

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