How gills work?

   Gills are respiratory organs of animals breathing oxygen under water. Not all such animals have gills, for the lower forms of life breathe through the entire body-covering, it being thin enough for that purpose. But when some water animals developed thick coverings, such as scales, special organs for breathing became necessary, and gills developed. They are found always in pairs, and are simply outgrowths of the body wall, consisting of a thin membrane shot with blood vessels and bathed constantly in water. The blood vessels absorb oxygen from the water and carry it to the tissues, probably taking back carbonic acid gas to be excreted by the gills.

Gills - How gills work?