Fig. 1. a: Scanning electron micrograph of a single gill arch from the gulf toadfish (Opsanus beta) showing 15 filaments that contain scores of lamellae. Reference bar is 100 mm. b: Diagram of a single filament and six lamellae showing the direction of water flow (irrigation) between the lamellae and blood flow within the filament and a single lamella. The afferent filamental artery perfuses single lamellae via a small pre-lamellar arteriole. Blood flow in the lamella is around individual pillar cells, or around the peripheral channel, through the post-lamellar arteriole and into the efferent filamental artery. Significant post-lamellar blood can be shunted into the central venous sinus (CVS) of the filament under certain conditions. The external surface of the filament and lamellae is characterized by the microridges of the PVCs, with interspersed, infrequent pores into the apical surface of MRCs (shown as small, gray circles in the figure). See text for details (redrawn from Pisam et al., ’87). c: Scanning electron micrograph of the surface of a gill filament from O. beta, showing the characteristic pattern of the apical surface of PVCs and pores leading to MRCs. Reference bar is 20 mm.