During molting, two successive differentiations of the midgut posterior caeca from the terrestrial crustacean Orchestia cavimana are correlated with two successive opposite phases of calcium transepithelial flux occurring in two different forms. In both cases, calcium transit (demonstrated by ultrastructural cytochemistry and microanalysis) is mainly paracellular, through a typical network of canaliculi, and implies synthesis of two types of calcified biomaterials : preexuvial intraluminal concentrations and postexuvial transepithelial spherules. A functional model has been proposed for each phase of this calcium turnover, which has been partly checked by ultrastructural cytoenzymological investigations. Basal and paracrine cells may be involved in the regulation of this calcium turnover which may be also under various hemolymphatic and hormonal controls. A possible involvement of vertebrate-related calcium regulating hormones (calcitonin and 1,25 (OH)2 vitamin D3) is discussed.
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