A gill is an anatomical structure found in many aquatic organisms. It is a respiration organ whose function is the extraction of oxygen from water and the excretion of carbon dioxide. The microscopic structure of a gill is such that it presents a very large surface area to the external environment. Many microscopic aquatic animals, and those which are somewhat larger but inactive, are able to absorb adequate oxygen through the entire surface of their bodies, and thus they often can respire qui… (More on Gill)