Editing Gill (section)

== Invertebrates ==
[[File:Pleurobranchaea meckelii.jpg|thumb|A [[sea slug]], ''[[Pleurobranchaea meckelii]]'': The gill (or [[Ctenidium (mollusc)|ctenidium]]) is visible in this view of the right-hand side of the animal.]]
[[Crustacean]]s, [[mollusc]]s, and some aquatic insects have tufted gills or plate-like structures on the surfaces of their bodies. Gills of various types and designs, simple or more elaborate, have evolved independently in the past, even among the same class of animals. The segments of [[Polychaete|polychaete worms]] bear [[Parapodium|parapodia]] many of which carry gills.<ref name=Dorit/> Sponges lack specialised respiratory structures, and the whole of the animal acts as a gill as water is drawn through its spongy structure.<ref>{{cite book|author=Choudhary, S.|title=Teaching of Biology |url=https://books.google.com/books?id=oIfw9sxcxDIC&pg=PA269 |publisher=APH Publishing |isbn=978-81-7648-524-1 |page=269}}</ref>

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 [[Branchial chamber|gill chamber]].<ref>{{cite book|author=Saxena, Amita|title=Text Book of Crustacea|url=https://books.google.com/books?id=uH7t82WQ1J0C&pg=PA180 |year=2005 |publisher=Discovery Publishing House|isbn=978-81-8356-016-0 |page=180}}</ref> [[Horseshoe crab]]s have [[Book lung#Book gills|book gills]] which are external flaps, each with many thin leaf-like membranes.<ref>{{cite book|author=Sekiguchi, K.|title=Biology of Horseshoe Crabs|url=https://books.google.com/books?id=SXk_AAAAYAAJ|year=1988 |publisher=サイエンスハウス|isbn=978-4-915572-25-8|page=91}}</ref>

Many marine invertebrates such as [[Bivalvia|bivalve molluscs]] are [[filter feeder]]s. 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.<ref name=Roberts>{{cite book|author=Roberts, M.B.V.|title=Biology: A Functional Approach|url=https://books.google.com/books?id=ASADBUVAiDUC&pg=PA139 |year=1986 |publisher=Nelson Thornes |isbn=978-0-17-448019-8 |page=139}}</ref>

Respiration in the [[echinoderm]]s (such as [[starfish]] and [[sea urchin]]s) is carried out using a very primitive version of gills called [[papulae]]. These thin protuberances on the surface of the body contain [[diverticulum|diverticula]] of the [[water vascular system]].

[[File:Hermit Crab Gills.jpg|thumb|right|[[Caribbean hermit crab]]s have modified gills that allow them to live in humid conditions.]]
The gills of [[aquatic insect]]s are [[Trachea#Invertebrates|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 [[larva]]l [[dragonfly]], the wall of the caudal end of the [[Gastrointestinal tract|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.

=== {{anchor|Plastron}} 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 [[Spiracle (vertebrates)|spiracle]]s that connect to the tracheal system. The plastron typically consists of dense patches of [[Hydrophobe|hydrophobic]] [[setae]] on the body, which prevent water entry into the spiracles, but may also involve scales or microscopic ridges projecting from the cuticle. The physical properties of the interface between the trapped air film and surrounding water allow gas exchange through the spiracles, almost as if the insect were in atmospheric air. [[Carbon dioxide]] diffuses into the surrounding water due to its high [[solubility]], while oxygen diffuses into the film as the concentration within the film has been reduced by [[Respiration (physiology)|respiration]], and nitrogen also diffuses out as its tension has been increased. Oxygen diffuses into the air film at a higher rate than nitrogen diffuses out. However, water surrounding the insect can become oxygen-depleted if there is no water movement, so many such insects in still water actively direct a flow of water over their bodies.

The inorganic gill mechanism allows aquatic arthropods with plastrons to remain constantly submerged. Examples include many [[beetle]]s in the family [[Elmidae]], aquatic [[weevil]]s, and [[true bug]]s in the family [[Aphelocheiridae]], as well as at least one species of [[Ricinulei|ricinuleid arachnid]]<ref>{{cite journal |author=Joachim Adis, Benjamin Messner & [[Norman Platnick]] |year=1999 |title=Morphological structures and vertical distribution in the soil indicate facultative plastron respiration in ''Cryptocellus adisi'' (Arachnida, Ricinulei) from Central Amazonia |journal=[[Studies on Neotropical Fauna and Environment]] |volume=34 |issue=1 |pages=1–9 |doi=10.1076/snfe.34.1.1.8915|bibcode=1999SNFE...34....1A }}</ref> and various mites.<ref>{{Cite journal |last=Hinton |first=H.E. |date=1971 |title=Plastron respiration in the mite, Platyseius italicus |url=https://linkinghub.elsevier.com/retrieve/pii/0022191071901843 |journal=Journal of Insect Physiology |language=en |volume=17 |issue=7 |pages=1185–1199 |doi=10.1016/0022-1910(71)90184-3|bibcode=1971JInsP..17.1185H }}</ref><ref>{{Cite journal |last=Pfingstl |first=Tobias |date=2017-09-30 |title=The marine-associated lifestyle of ameronothroid mites (Acari, Oribatida) and its evolutionary origin: a review |url=https://www1.montpellier.inrae.fr/CBGP/acarologia/article.php?id=4197 |journal=Acarologia |volume=57 |issue=3 |pages=693–721 |doi=10.24349/acarologia/20174197|s2cid=90340235 |doi-access=free }}</ref> A somewhat similar mechanism is used by the [[diving bell spider]], which maintains an underwater bubble that exchanges gas like a plastron. Other diving insects (such as [[backswimmer]]s, and [[Hydrophilidae|hydrophilid beetles]]) may carry trapped air bubbles, but deplete the oxygen more quickly, and thus need constant replenishment.