GILLS
Gills are respiratory structures specialized for gas exchange in water. Uniquely structured gills are found in various animal groups including mollusks, annelids, crustaceans, echinoderms, and vertebrates. One problem that aquatic animals must deal with is that the concentration of oxygen in water is about 10,000 times less than that in air. Thus, gills have to be very efficient to meet the respiratory demands of aquatic animals. Fish gills, for example, can extract more than 80% of the oxygen dissolved in water.

Fish gills are made of numerous suspended curtains of tissue, called filaments, that increase their respiratory surface area. Gill tissue is permeated with many small water-bearing channels surrounded by capillaries. Because the water channels and capillaries are so close to one another, O₂ and CO₂ readily diffuse between the blood and water.