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Fig. 2. Photomicrographs of transverse, 0.5 µm thick, epoxy-resin sections taken through the anterior and posterior gill lamellae of the freshwater, trichodactylid crab, Dilocarcinus pagei. (A) Microanatomy of a typical, medial lamella from anterior gill no. 4, showing the identical, very thin (2–5 µm) epithelia (e), symmetrically arranged on both sides of the lamella, separated by the ample hemolymph space (h), which contains a discontinuous septum (s) and hemocytes. Both epithelia consist of the very attenuated, lateral, apical flanges (f) that emerge from the pillar cell perikarya (pc) underlying the fine cuticle (c). Bar, 20 µm. (B) Architectural organization of the dense, osmiophilic, central region of a typical, medial lamella from posterior gill no. 7, revealing the highly asymmetrical nature of the distal (d) and proximal (p) epithelia underlying the thickened cuticle (c). The well-developed, distal epithelium measures 3–10 µm in thickness and consists exclusively of the extensive, apical flanges (f) of the pillar cells (pc), populated by numerous apical vesicles (v) and invaginations. The dense, proximal epithelial cells measure 18–20 µm in thickness and are characterized by large, spherical nuclei, numerous basal and apical invaginations (i), and a few apical vesicles. The lamellar septum is absent, and the distal pillar cell perikarya traverse the hemolymph space, abutting directly on the opposing, thick, proximal epithelial cells (arrows). Bar, 20 µm.

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