Of or relating to the gills. Most fish possess five pairs of branchial arches, skeletal supports for the gills, which are contained in a cavity known as the branchial chamber. Water is drawn into the branchial chamber through narrow gill slits (or branchial clefts) in the wall of the pharynx and passes over the gills, where gaseous exchange takes place. Blood is transported to and from the gills in afferent and efferent branchial arteries, respectively.