Photoreceptor cells of the compound eye were studied electron microscopically in crayfishProcambarus clarkithat had been grown in complete darkness for 11 weeks. Since the microvilli of recently differentiated retinula cells appeared uniform in size and were regularly oriented, light is not necessary for their development. Variability in size of microvilli due to enlargement and disruption of their normal orientation became progressively greater with the age of the cell, therefore, light is needed for maintenance of normal features and apparently of function. Behavioral experiments with 2 older light-deprived animals indicated that they were blind.
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