Many microscopic aquatic animals, and those which are somewhat larger but inactive, are able to absorb adequate oxygen through the entire surface of their bodies, and thus they often can respire quite adequately without a gill. However, more complex or more active aquatic organisms usually require a gill or gills.
Gills usually consist of thin plates of tissue, branches, or slender tufted processes. With the exception of some aquatic insects, they contain blood or coelomic fluid, from which gases are exchanged through the thin walls. Oxygen is carried by the blood to other parts of the body. Carbon dioxide passes from the blood through the thin gill tissue into the water.
Respiration in the Echinodermata (includes starfish and sea urchins) is carried out using a very primitive version of gills called papulli. These are thin protuberances on the surface of the body containing diverticula of the water vascular system.
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. In the aquatic 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.
The physical gill mechanism allows aquatic insects with plastrons to remain constantly submerged. Examples include many beetles in the family Elmidae, aquatic weevils, and true bugs in the family Aphelocheiridae.
Gills of vertebrates are developed in the walls of the pharynx along a series of gill slits opening to the exterior. In fish, the gills are located on both sides of the pharynx. Gills are made of filaments which help increase surface area for oxygen exchange. In bony fish, the gills are covered by a bony cover called an operculum. When a fish breathes, it opens its mouth at regular times and draws in a mouthful of water. It then draws the sides of its throat together, forcing the water through the gill openings. The water passes over the gills on the outside. Valves inside the mouth keep the water from escaping through the mouth again. The operculum can be very important in adjusting the pressure of water inside of the pharynx to allow proper ventilation of the gills. Lampreys and sharks lack an operculum, they have multiple gill openings. Also, they must use different methods to force water over the gills. In sharks and rays, this ventilation of the gills is achieved either by the use of spiracles or ram ventilation (ventilation by constantly swimming).
In most species, a countercurrent exchange system is employed to enhance the diffusion of substances in and out of the gill, with blood and water flowing in opposite directions to each other. 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 electrolytes. Gills' large surface area tends to create a problem for fish seeking to regulate the osmolarity of their internal fluids. Saltwater is less dilute than these internal fluids; as a consequence, saltwater fish lose large quantities of water osmotically through their gills. To regain the water, they drink large amounts of seawater and excrete the salt. Freshwater is more dilute than the internal fluids of fish, however, so freshwater fish gain water osmotically through their gills.
The gill slits of fish are believed to be the evolutionary ancestors of the tonsils, thymus gland, and Eustachian tubes, as well as many other structures derived from the embryonic branchial pouches. In some amphibians, the gills occupy the same position on the body but protrude as external tufts.