<P> All basal vertebrates breathe with gills . The gills are carried right behind the head, bordering the posterior margins of a series of openings from the esophagus to the exterior . Each gill is supported by a cartilagenous or bony gill arch . The gills of vertebrates typically develop in the walls of the pharynx, along a series of gill slits opening to the exterior . Most species employ a counter-current exchange system to enhance the diffusion of substances in and out of the gill, with blood and water flowing in opposite directions to each other . </P> <P> The gills are composed of comb - like filaments, the gill lamellae, which help increase their surface area for oxygen exchange . When a fish breathes, it draws in a mouthful of water at regular intervals . Then it draws the sides of its throat together, forcing the water through the gill openings, so that it passes over the gills to the outside . The bony fish have three pairs of arches, cartilaginous fish have five to seven pairs, while the primitive jawless fish have seven . The vertebrate ancestor no doubt had more arches, as some of their chordate relatives have more than 50 pairs of gills . </P> <P> Gills usually consist of thin filaments of tissue, branches, or slender tufted processes that have a highly folded surface to increase surface area . The high surface area is crucial to the gas exchange of aquatic organisms as water contains only a small fraction of the dissolved oxygen that air does . A cubic meter of air contains about 250 grams of oxygen at STP . The concentration of oxygen in water is lower than air and it diffuses more slowly . In a litre of freshwater the oxygen content is 8 cm per litre compared to 210 in the same volume of air . Water is 777 times more dense than air and is 100 times more viscous . Oxygen has a diffusion rate in air 10,000 times greater than in water . The use of sac - like lungs to remove oxygen from water would not be efficient enough to sustain life . Rather than using lungs "Gaseous exchange takes place across the surface of highly vascularised gills over which a one - way current of water is kept flowing by a specialised pumping mechanism . The density of the water prevents the gills from collapsing and lying on top of each other, which is what happens when a fish is taken out of water ." </P> <P> Higher vertebrates do not develop gills, the gill arches form during fetal development, and lay the basis of essential structures such as jaws, the thyroid gland, the larynx, the columella (corresponding to the stapes in mammals) and in mammals the malleus and incus . Fish gill slits may be the evolutionary ancestors of the tonsils, thymus gland, and Eustachian tubes, as well as many other structures derived from the embryonic branchial pouches . </P>

Where does gas exchange take place in the gills