Gill

1. The respiratory organ of most aquatic animals that breathe water to obtain oxygen, consisting of a filamentous structure of vascular membranes across which dissolved gases are exchanged. Sharks and rays typically have five pairs of gill slits that open directly to the outside of the body.
2. Botany. One of the thin, platelike structures on the underside of the cap of a mushroom or similar fungus. The gills are used by the mushrooms as a means of spore dispersal.