A gill 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. <img src="https://upload.wikimedia.org/wikipedia/commons/4/47/Carp_gill_defect.jpg" width="500"> <img src="https://upload.wikimedia.org/wikipedia/commons/0/06/Tuna_Gills_in_Situ_cut.jpg" width="500"> From [Wikipedia](https://en.wikipedia.org/wiki/Gill) under [CC BY-SA 3.0](https://creativecommons.org/licenses/by-sa/3.0/) license.