The lamellae are positioned for the blood flow to be
counter-current to the water flow over the gills. Despite marked differences in the gross anatomy of
the gill among the various groups, the cellular constituents of the epithelium are remarkably similar.
The lamellar gas-exchange surface is covered by squamous pavement cells, while large, mitochondriarich,
ionocytes and mucocytes are found in greatest frequency in the filament epithelium.