Gill
![The red gills of this common carp are visible as a result of a gill flap birth defect.](/Images/godic/202501/15/Carp_gill_defect4358.jpg")
![Freshwater Fish Gills magnified 400 times](/Images/godic/202501/15/FreshwaterFishGill400x74358.jpg")
![The red gills inside a detached tuna head (viewed from behind)](/Images/godic/202501/15/Tuna_Gills_in_Situ_cut4358.jpg")
![A live sea slug, Pleurobranchaea meckelii: The gill (or ctenidium) is visible in this view of the right-hand side of the animal.](/Images/godic/202501/15/Pleurobranchaea_meckelii4358.jpg")
A gill (/ɡɪl/) 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.