Fig. 1. (A–F) Anatomical and histological features of the gills of the hololimnetic trichodactylid crab Dilocarcinus pagei. (A) Macroscopic view of the eight gills (g) in situ in the left branchial chamber after removal of the carapace. Scale bar, 5 mm. (B) Phyllobranchiate gills 1–8 (1–8) arranged according to insertion sequence along the antero-posterior body axis. Scale bar, 5 mm. (C,D) Epoxy-embedded whole mounts of paraformaldehyde/glutaraldehyde/OsO₄-fixed lamellae from anterior gill 4 (C) and posterior gill 7 (D). The dense osmiophilic areas (*) in gill 7 reflect an underlying, thick transporting epithelium (see E,F). A clearly less-dense area is also seen in anterior gill 4. Afferent epibranchial (e) and efferent hypobranchial (h) vessels and haemolymph channels (c) originating on either side of the gill shaft (s) are visible. Scale bar, 1 mm. (E) Micrograph of a 0.5 µm thick epoxy section taken transversely across a lamella from posterior gill 7. An intralamellar septum (s) in the haemolymph space (h) separates the two thin epithelia (arrowheads) near the gill shaft (left side, not visible). Approximately 80 µm from the shaft, the epithelial layers become asymmetrical: 3–10 µm thick on the distal side (d) and 18–20 µm on the proximal side (p). Scale bar, 20 µm. (F) Micrograph of a 0.5 µm thick section taken transversely through the osmiophilic region of a lamella from posterior gill 7. The dense, thick proximal epithelium is characterised by numerous basal invaginations (i) and a few apical vesicles. The thin distal epithelium consists of the extensive apical flanges (f) of the pillar cells (pc), populated by numerous vesicles (v) and apical invaginations. h, haemolymph space. Scale bar, 20 µm.