Some ultrastructural features of the nuchal organ of Daphnia magna Straus (Crustacea: Branchiopoda)

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Abstract
The ultrastructure of the nuchal organ of 1st instar of D. magna is similar to that described in the literature for presumed ion-transporting tissues in other crustaceans. The apical and basal plasma membranes are extensively amplified through microvilli and infoldings; mitochondria are abundant and distributed unequally between the 2 types of cells (dark and light) composing the organ, more being present in the dark cells. The 2 cell types differ also in the type of microvilli projecting from their apical surface. In proecdysis these microvilli are replaced by much smaller ones which are associated with the deposition of the new cuticle.