The lens or cornea of many fishes contains pigments that filter out violet or ultraviolet radiation, probably improving visual acuity. Little is known about the aqueous or vitreous humors; among different species, the vitreous ranges in consistency from a liquid to a firm gel. Fish retinae are organized according to the ordinary vertebrate plan. Innermost are the various neuronal and glial elements, which are relatively transparent. Light passes through these to the photoreceptor (visual) cells. Outermost, adjacent to the choroid, is the pigment epithelium. Long processes of the pigment cells extend toward the visual cells and interdigitate with their outer segments. In the dark-adapted eye, melanin granules within these pigment cells are drawn back, away from the visual cells. A retinal tapetum occurs in the pigment epithelium of many freshwater fishes (cyprinids and percids). The epithelial cells contain particles or crystals of the reflective substance guanine. Melanin is present in the same cells and migrates normally, occluding the tapetum in bright light. A nonocclusible tapetum of the same type was said to occur in pelagic deep-sea teleosts. © 1971, Academic Press Inc.
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