The respiratory organ of an aquatic animal, often complex in form, consisting of an outgrowth from the body surface or an internal layer of modified gut, past which water flows. It provides a large surface area, well supplied with blood vessels, across whose walls oxygen and carbon dioxide diffuse. It is the chief organ of respiration in fish, in which the gills usually are supported by four or more pairs of bony or cartilaginous gill arches. In sharks each gill is more or less surrounded by a separate gill chamber, but in bony fish the gills share a common gill (branchial) chamber, one on each side of the head. Each gill arch bears rows of plate-like filaments, in turn carrying secondary lamellae, which are capable of absorbing oxygen from the water flowing over the surface.
Subjects: Zoology and Animal Sciences.