Yes, the gills are a respiration organ whose function is the extraction of oxygen from water and the excretion of carbon dioxide

yes. "Gills usually consist of thin filaments of tissue, branches, or slender tufted processes which have a highly folded surface to increase surface area. A high surface area is crucial to the gas exchange of aquatic organisms as water contains only 1/20 parts dissolved oxygen compared to air. With the exception of some aquatic insects, the filaments and lamellae (folds) contain blood or coelomic fluid, from which gases are exchanged through the thin walls. Oxygen is carried by the blood to other parts of the body. Carbon dioxide passes from the blood through the thin gill tissue into the water." "Gills of vertebrates are developed in the walls of the pharynx along a series of gill slits opening to the exterior. In fish, the gills are located on both sides of the pharynx. Gills are made of filaments which help increase surface area for oxygen exchange. In bony fish, the gills are covered by a bony cover called an operculum. When a fish breathes, it opens its mouth at regular times and draws in a mouthful of water. It then draws the sides of its throat together, forcing the water through the gill openings. The water passes over the gills on the outside. Valves inside the mouth keep the water from escaping through the mouth again." "In most species, a countercurrent exchange system is employed to enhance the diffusion of substances in and out of the gill, with blood and water flowing in opposite directions to each other. Water taken into the mouth passes out of the slits, bathing the gills as it passes. In a fish, water is taken in through the mouth and passes out through the gills. Gill structure is similar to that of a fine tooth comb. On the way out, water flows through the teeth of the comb, or the gill filaments. Lemellae, similar to alveoli in the human lung, extract oxygen from the water." Source and further information: http://en.wikipedia.org/wiki/Gill "larger organisms require special structures to collect enough oxygen to sustain life. This oxygen comes from molecules of oxygen gas (O2) dissolved in the water. The oxygen atom present in the water molecule (H2O) is not suitable for respiration." Source and further information: http://en.wikipedia.org/wiki/Aquatic_respiration