Gills are a respiratory organ for the extraction of oxygen from water and for the excretion of carbon dioxide.
Many small aquatic animals absorb oxygen through the surface of their bodies in general, but more complex forms have localized respiratory organs formed to present an adequate surface area to the external environment. They are usually thin plates of tissue or slender tufted processes and, with the exception of some aquatic insects, they contain blood or coelomic fluid which exchange gasses through their thin walls. In the insects a unique type of respiratory organ is used, the tracheal gill, which contains air tubes. The oxygen in these tubes is renewed through the gills.
Gills are developed in starfish and sea urchins (Echinoidea) as thin protuberances on the surface of the body containing diverticula of the water vascular system. In the crustaceans, molluscs and some insects, they are tufted or plate-like structures at the surface of the body in which blood circulates. The gills of other insects are of the tracheal kind and also include both thin plates and tufted structures, and in the larval dragon fly the wall of the caudal end of the alimentary tract (rectum) is richly supplied with tracheae as a rectal gill. Water pumped into and out of the rectum provide oxygen to the closed tracheae.
Gills of vertebrates are developed in the walls of the pharynx along a series of gill slits opening to the exterior. Water taken into the mouth passes out of the slits, bathing the gills as it passes. Some fish utilize the gills for the excretion of electrolyes. In some amphibians the gills occupy the same position on the body but protrude as external tufts.