Fig. 7. Chloride cells in the gill epithelium of an adult Geotria australis in seawater. (A) Cross section of the filament, showing a row of chloride cells on each side of the central blood space. (B) Freeze fracture of a bundle of parallel membranous tubules, showing the helicoidally arranged particles (arrowheads). (C) Zonula occludens between two chloride cells consisting of a single strand. The particles in the apical membrane of the chloride cell are clustered on the P face. (D) Clusters of particles on the E face. Scale bars, 3 µm (A), 0.5 µm (B,C) and 0.25 µm (D).