Gill
A gill (
With the exception of some aquatic
History
Galen observed that fish had multitudes of openings (foramina), big enough to admit gases, but too fine to give passage to water. Pliny the Elder held that fish respired by their gills, but observed that Aristotle was of another opinion.[1] The word branchia comes from the Greek βράγχια, "gills", plural of βράγχιον (in singular, meaning a fin).[2]
Function
Many microscopic aquatic animals, and some larger but inactive ones, can absorb sufficient oxygen through the entire surface of their bodies, and so can respire adequately without gills. However, more complex or more active aquatic organisms usually require a gill or gills. Many invertebrates, and even amphibians, use both the body surface and gills for gaseous exchange.[3]
Gills usually consist of thin filaments of tissue, lamellae (plates), branches, or slender, tufted processes that have a highly folded surface to increase surface area. The delicate nature of the gills is possible because the surrounding water provides support. The blood or other body fluid must be in intimate contact with the respiratory surface for ease of diffusion.[3]
A high surface area is crucial to the
Usually water is moved across the gills in one direction by the current, by the motion of the animal through the water, by the beating of cilia or other appendages, or by means of a pumping mechanism. In fish and some molluscs, the efficiency of the gills is greatly enhanced by a countercurrent exchange mechanism in which the water passes over the gills in the opposite direction to the flow of blood through them. This mechanism is very efficient and as much as 90% of the dissolved oxygen in the water may be recovered.[3]
Vertebrates
The gills of vertebrates typically develop in the walls of the pharynx, along a series of gill slits opening to the exterior. Most species employ a countercurrent exchange system to enhance the diffusion of substances in and out of the gill, with blood and water flowing in opposite directions to each other. The gills are composed of comb-like filaments, the gill lamellae, which help increase their surface area for oxygen exchange.[5]
When a fish breathes, it draws in a mouthful of water at regular intervals. Then it draws the sides of its throat together, forcing the water through the gill openings, so it passes over the gills to the outside. Fish gill slits may be the evolutionary ancestors of the
Fish
The gills of fish form a number of slits connecting the pharynx to the outside of the animal on either side of the fish behind the head. Originally there were many slits, but during evolution, the number reduced, and modern fish mostly have five pairs, and never more than eight.[8]
Cartilaginous fish
A smaller opening, the
Most sharks rely on ram ventilation, forcing water into the mouth and over the gills by rapidly swimming forward. In slow-moving or bottom-dwelling species, especially among skates and rays, the spiracle may be enlarged, and the fish breathes by sucking water through this opening, instead of through the mouth.[9]
Chimaeras differ from other cartilagenous fish, having lost both the spiracle and the fifth gill slit. The remaining slits are covered by an operculum, developed from the septum of the gill arch in front of the first gill.[9]
Bony fish
In bony fish, the gills lie in a branchial chamber covered by a bony operculum. The great majority of bony fish species have five pairs of gills, although a few have lost some over the course of evolution. The operculum can be important in adjusting the pressure of water inside of the pharynx to allow proper ventilation of the gills, so bony fish do not have to rely on ram ventilation (and hence near constant motion) to breathe. Valves inside the mouth keep the water from escaping.[9]
The gill arches of bony fish typically have no septum, so the gills alone project from the arch, supported by individual gill rays. Some species retain gill rakers. Though all but the most primitive bony fish lack spiracles, the pseudobranch associated with them often remains, being located at the base of the operculum. This is, however, often greatly reduced, consisting of a small mass of cells without any remaining gill-like structure.[9]
Marine
Lampreys and hagfish do not have gill slits as such. Instead, the gills are contained in spherical pouches, with a circular opening to the outside. Like the gill slits of higher fish, each pouch contains two gills. In some cases, the openings may be fused together, effectively forming an operculum. Lampreys have seven pairs of pouches, while hagfishes may have six to fourteen, depending on the species. In the hagfish, the pouches connect with the pharynx internally and a separate tube which has no respiratory tissue (the pharyngocutaneous duct) develops beneath the pharynx proper, expelling ingested debris by closing a valve at its anterior end.[9] Lungfish larvae also have external gills, as does the primitive ray-finned fish Polypterus, though the latter has a structure different from amphibians.[9]
Amphibians
.jpg)
Still, some extinct tetrapod groups did retain true gills. A study on Archegosaurus demonstrates that it had internal gills like true fish.[12]
Invertebrates
Aquatic arthropods usually have gills which are in most cases modified appendages. In some crustaceans these are exposed directly to the water, while in others, they are protected inside a gill chamber.[14] Horseshoe crabs have book gills which are external flaps, each with many thin leaf-like membranes.[15]
Many marine invertebrates such as bivalve molluscs are filter feeders. 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
The gills of aquatic insects are tracheal, but the air tubes are sealed, commonly connected to thin external plates or tufted structures that allow diffusion. The oxygen in these tubes is renewed through the gills. In the larval dragonfly, the wall of the caudal end of the alimentary tract (rectum) is richly supplied with tracheae as a rectal gill, and water pumped into and out of the rectum provides oxygen to the closed tracheae.
Plastrons
A plastron is a type of structural adaptation occurring among some aquatic arthropods (primarily insects), a form of inorganic gill which holds a thin film of atmospheric oxygen in an area with small openings called
The inorganic gill mechanism allows aquatic arthropods with plastrons to remain constantly submerged. Examples include many
See also
References
- ^
This article incorporates text from a publication now in the public domain: Chambers, Ephraim, ed. (1728). Cyclopædia, or an Universal Dictionary of Arts and Sciences (1st ed.). James and John Knapton, et al. {{cite encyclopedia}}: Missing or empty|title=(help) - ^ "Branchia". Oxford English Dictionary. Oxford University Press. 2nd Ed. 1989.
- ^ ISBN 978-0-03-030504-7.
- ^ a b c d e M. b. v. Roberts; Michael Reiss; Grace Monger (2000). Advanced Biology. London, UK: Nelson. pp. 164–165.
- ^ Andrews, Chris; Adrian Exell; Neville Carrington (2003). Manual Of Fish Health. Firefly Books.
- ISSN 0531-5131. Retrieved 18 February 2022.
- PMID 23020903.
- ISBN 978-0-674-15250-2.
- ^ ISBN 0-03-910284-X.
- ^ Laurin M. (1998): The importance of global parsimony and historical bias in understanding tetrapod evolution. Part I-systematics, middle ear evolution, and jaw suspension. Annales des Sciences Naturelles, Zoologie, Paris, 13e Série 19: pp 1–42.
- ^ PMID 15618479.
- ^ Florian Witzmann; Elizabeth Brainerd (2017). "Modeling the physiology of the aquatic temnospondyl Archegosaurus decheni from the early Permian of Germany". Fossil Record. 20 (2): 105–127. doi:10.5194/fr-20-105-2017.
- ISBN 978-81-7648-524-1.
- ISBN 978-81-8356-016-0.
- ISBN 978-4-915572-25-8.
- ISBN 978-0-17-448019-8.
- .
- .
- S2CID 90340235.
External links
- Fish Dissection - Gills exposed Australian Museum. Updated: 11 June 2010. Retrieved 16 January 2012.