Gill, a respiratory organ in most animals that breathe under water. Not all underwater animals have gills; some lower forms of underwater life, such as the amoeba, breathe through thin body walls. Fish breathe by gulping water through the mouth and discharging it through the gill chamber. Many crustaceans and mollusks that live in the water have gill-like structures. Gills are also common in certain land animals, such as spiders and scorpions.

GillsGills are respiratory organs in most animals that breathe under water.

In fish, the gills are membranous, leaflike structures on either side of the body just behind the head. Each gill has two rows of gill filaments covered with thin membrane. Blood is supplied to the filaments through a network of capillaries. The blood absorbs oxygen from the water and discharges carbon dioxide. The gill chambers also contain comblike gill rakers, which serve as strainers, holding back particles of food large enough to damage the filaments.

The gills in most species of fish are protected by gill covers called opercula. Sharks and certain other species of fish have exposed gill openings. In saltwater fish, the gills serve two purposes—breathing, and ridding the body of excess salt from water taken in by the fish.