The transitional zone is formed of regularly arranged, columnar cells with a 40 mm maximum height and showing two different staining intensities: ligthly stained R-cells, and darker stained F-cells. Ultrastructurally both cell types are similar.in the free surface throughout the region regardless of cell type. Each microvillus, is approximately 700 nm in height and 75 nm in diameter. The apical region of all cells is joined laterally by a septate junction as noted in other Crustacea (Gilula, 1971; Hudspeth and Revel, 1971). The septate junction gives way basally to a gap junction. The cytoplasm of both cell types contains rather extensive rough endoplasmic reticulum, numerous free ribosomes, mitocondria with few cristae, golgi bodies. Basally smooth