64Gills are responsible for regulating the exchange of salt and water and play a major role in the excretion of the nitrogenous waste products. The gills of most teleosts are composed of a series of arch-like structures. Each gill arch bears rows of filaments, each row constitutes a hemibranch, while a set of hemibranchs, one on each side of the arch, constitutes a holobranch. The gills of teleost fishes are composed of four holobranchs spaced between five branchial slits. Each hemibranch bears many fine primary and secondary gill lamellae. The gill arch is a curved cartilaginous or osseous structure, from which radiate the bony supports (the gill rays) of the primary lamellae. The gill arch contains the afferent and efferent brachial arteries. The gill arch is covered by epidermal tissue well endowed with mucous cells. The primary lamella has a central cartilaginous support, afferent and efferent arterioles, and other anastomosing vessels. The surface epithelium of the primary lamellae is covered with cuboidal and squamous epithelial pavement cells (PVC), pale-staining mucous cells, and salt secreting chloride cells. These chloride cells are most numerous at the basal (proximal) part of the lamellae and function in ionic transport with a possible role in detoxification.