At gills (popularly known as gills) of bony fish are lateral projections of the pharynx, located in a gill chamber.
To find them one has to lift the operculum a protective bony cap located laterally near the head. Each gill consists of delicate gill filaments. In turn, these filaments contain several coverslips, richly vascularized. Through this extremely thin-walled capillary network, blood gases are exchanged.
The flow of blood on each coverslip follows toward the post towards the water that bathes it. This countercurrent flow ensures perfect oxygenation. At the same time carbon dioxide is expelled into water. After passing through the gills, the richly oxygenated blood is directed straight through the body without passing through the heart.
Most aquatic animals breathe through gills. The structure of the gills varies in complexity, from simple types, such as the echinoderms, to complexes, such as those present in crustaceans and fish.
In the echinoderm, the gas captured by the gills diffuses into the celiac fluid, from where it is distributed throughout the body. In other animals that breathe through the gills, the oxygen gas is distributed to the body's cells through the circulatory system.