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Gills usually consist of thin filaments of tissue, branches, or slender tufted processes that have a highly folded surface to increase surface area.The high surface area is crucial to the gas exchange of aquatic organisms as water contains only a small fraction of the dissolved oxygen that air does.
Aquatic arthropods generally possess some form of gills in which gas exchange takes place by diffusing through the exoskeleton. Others may breathe atmospheric air while remaining submerged, via breathing tubes or trapped air bubbles, though some aquatic insects may remain submerged indefinitely and respire using a plastron .
The pulmonates have lost their gills [1] and adapted the mantle cavity into a pallial lung. The lung has a single opening on the right side, called the pneumostome, which either remains permanently open, or opens and closes as the animal breathes. The roof of the lung is highly vascularised, and it is through this surface that gas exchange occurs.
Most fish exchange gases using gills on either side of the pharynx (throat). Gills are tissues which consist of threadlike structures called filaments.These filaments have many functions and "are involved in ion and water transfer as well as oxygen, carbon dioxide, acid and ammonia exchange.
A current of water is maintained through the gills for gas exchange, and food particles are filtered out at the same time. These may be trapped in mucus and moved to the mouth by the beating of cilia. [16] Respiration in the echinoderms (such as starfish and sea urchins) is carried out using a very primitive version of gills called papulae.
Gas exchange is the physical process by which gases move passively by diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liquid, a gas-permeable membrane, or a biological membrane that forms the boundary between an organism and its extracellular environment.
Gas exchange takes place in the gills which consist of thin or very flat filaments and lammellae which expose a very large surface area of highly vascularized tissue to the water. Other animals, such as insects, have respiratory systems with very simple anatomical features, and in amphibians, even the skin plays a vital role in gas exchange.
In fish gills, there are two types of lamellae, primary and secondary. The primary gill lamellae (also called gill filament) extends from the gill arch, and the secondary gill lamellae extends from the primary gill lamellae. Gas exchange primarily occurs at the secondary gill lamellae, where the tissue is notably only one cell layer thick.