Gills

Gills, or Branchi.:, are the respiratory organs of aquatic animals. Since they occur in many classes, their form and arrangement is very various; essentially, they consist of a portion of the body wall covered by a thin membrane, beneath which there is a network of blood-vessels, the object of this arrangement being to facilitate the aeration of the blood. The surface of the gill is often thrown into complex folds, so that the surface exposed to the water is much increased.