The operculum may be divided into the proximal, the distal and the intermediate regions. The epithelium lining the inner surface of the operculum (EISO) and the opercular epidermis (OE) of these regions inClarias batrachus show significant differences in their thickness, in the density, in the dimensions of mucous cells and club cells, and in the distribution of lymphocytes, melanocytes, taste buds and ampullary organs. These differences in structural organization are associated with the different conditions prevailing at these locations. Rich vascularization observed in the tissues underlying the OE has been correlated with assisting the fish in accessory respiration. In contrast, the tissues underlying the EISO are poorly vascularized. Accessory respiration in this region may not be so advantageous.