<P> A gill (/ ɡɪl / (listen)) is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water and excretes carbon dioxide . The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are kept moist . The microscopic structure of a gill presents a large surface area to the external environment . Branchia (pl . branchiae) is the zoologists' name for gills . </P> <P> 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 . The blood carries oxygen to other parts of the body . Carbon dioxide passes from the blood through the thin gill tissue into the water . Gills or gill - like organs, located in different parts of the body, are found in various groups of aquatic animals, including mollusks, crustaceans, insects, fish, and amphibians . Marine animals such as crabs and mud skippers have gill chambers in which they store water, enabling them to use the dissolved oxygen when they are on land . </P> <P> Galen observed that fish had multitudes of openings (foramina), big enough to admit gases, but too fine to give passage to water . Pliny the Elder held that fish respired by their gills, but observed that Aristotle was of another opinion . The word branchia comes from the Greek βράγχια, "gills", plural of βράγχιον (in singular, meaning a fin). </P> <P> Many microscopic aquatic animals, and some larger but inactive ones, can absorb sufficient oxygen through the entire surface of their bodies, and so can respire adequately without gills . However, more complex or more active aquatic organisms usually require a gill or gills . Many invertebrates, and even amphibians, use both the body surface and gills for gaseous exchange . </P>

When did you find gill slits in a tadpole