Editing Gas exchange (section)

==Invertebrates==

The mechanism of gas exchange in invertebrates depends their size, feeding strategy, and habitat (aquatic or terrestrial).

[[File:Porifera body structures 01.png|thumb|'''Fig. 13.'''  Diagram representing the body structure of Porifera. The diagram shows the mechanism of water uptake for sponges. Yellow: [[pinacocytes]], red: choanocytes, grey: [[mesohyl]], pale blue: water flow]]

The [[sponge]]s (Porifera) are sessile creatures, meaning they are unable to move on their own and normally remain attached to their [[Substrate (marine biology)|substrate]]. They obtain nutrients through the flow of water across their cells, and they exchange gases by simple diffusion across their cell membranes. Pores called [[Ostium (sponges)|ostia]] draw water into the sponge and the water is subsequently circulated through the sponge by cells called [[choanocyte]]s which have [[flagellum|hair-like structures]] that move the water through the sponge.<ref>Anderson, D. (2001) ''Invertebrate Zoology'' Oxford University Press</ref>

[[File:Coral reef at palmyra.jpg|thumb|'''Fig. 14.'''  Cnidarians are always found in aquatic environments, meaning that their gas exchange involves absorbing oxygen from water.]]

The [[cnidarian]]s include [[corals]], [[sea anemones]], [[jellyfish]] and [[hydras]]. These animals are always found in aquatic environments, ranging from fresh water to salt water. They do not have any dedicated [[respiratory organs]]; instead, every cell in their body can absorb oxygen from the surrounding water, and release waste gases to it. One key disadvantage of this feature is that cnidarians can die in environments where water is [[water stagnation|stagnant]], as they deplete the water of its [[oxygen]] supply.<ref name="cnidarians">{{cite web |url=http://study.com/academy/lesson/cnidaria-respiratory-system.html |title= Cnidarian Respiratory System|author=<!--Not stated--> |website=study.com |access-date=20 March 2017}}</ref> Corals often form symbiosis with other organisms, particularly photosynthetic [[dinoflagellate]]s. In this [[symbiosis]], the [[coral]] provides shelter and the other organism provides nutrients to the coral, including oxygen.{{citation needed|date=November 2023}}

[[File:Giant roundworm (265 11) Cross-section.jpg|thumb|180 px|left|'''Fig. 15.''' Cross section of a nematode.]]

The [[nematode|roundworms]] (Nematoda), [[flatworm]]s (Platyhelminthes), and many other small invertebrate animals living in aquatic or otherwise wet habitats do not have a dedicated gas-exchange surface or circulatory system. They instead rely on [[diffusion]] of {{Chem|CO|2}} and {{Chem|O|2}} directly across their cuticle.<ref>{{cite web |url=http://study.com/academy/lesson/nematode-respiratory-system.html |title= Nematode Respiratory System|author=<!--Not stated--> |website=study.com |access-date=20 March 2017}}</ref><ref>{{cite web |url=http://rspp.weebly.com/platyhelminthes.html |title= Platyhelminthes Respiratory System|author=<!--Not stated--> |website=rspp.weebly.com|access-date=20 March 2017}}</ref> The cuticle is the [[semi-permeable]] outermost layer of their bodies.{{citation needed|date=November 2023}}

Other aquatic invertebrates such as most [[mollusc]]s (Mollusca) and larger [[crustacean]]s (Crustacea) such as [[lobster]]s, have gills analogous to those of fish, which operate in a similar way.

[[File:Actias selene 5th instar spiracles sjh.jpg|thumb|left|200 px|'''Fig. 16.''' Photographic representation of spiracles.]]
Unlike the invertebrates groups mentioned so far, [[insect]]s are usually terrestrial, and exchange gases across a moist surface in direct contact with the atmosphere, rather than in contact with surrounding water. The insect's [[exoskeleton]] is impermeable to gases, including water vapor, so they have a more specialised gas exchange system, requiring gases to be directly transported to the tissues via a complex network of tubes. This respiratory system is separated from their circulatory system. Gases enter and leave the body through openings called [[Spiracle (arthropods)|spiracle]]s, located laterally along the [[thorax]] and [[abdomen]]. Similar to plants, insects are able to control the opening and closing of these spiracles, but instead of relying on [[turgor pressure]], they rely on [[muscle contraction]]s.<ref>{{cite journal |last1=Lane |first1=N. J. |last2=Harrison |first2= J. B. |date=1986 |title= Junctions and the cytoskeleton in insect tissues |journal=Journal of Cell Biology |volume=103 |issue=5 |pages= A69 }}</ref> These [[muscle contractions|contractions]] result in an insect's abdomen being pumped in and out. The spiracles are connected to tubes called [[tracheae]], which branch repeatedly and ramify into the insect's body. These branches terminate in specialised [[tracheole|tracheole cells]] which provides a thin, moist surface for efficient gas exchange, directly with cells.<ref>Klowden, M. J. 2007. Physiological systems in insects. Elsevier/Academic Press. pp. 440–442</ref>

The other main group of terrestrial [[arthropod]], the [[arachnid]]s ([[spider]]s, [[scorpion]], [[mite]]s, and their relatives) typically perform gas exchange with a [[book lung]].<ref name="Garwood">{{cite journal |last1=Garwood |first1=Russell J. |last2=Edgecombe |first2=Gregory D. |date=September 2011 |title=Early Terrestrial Animals, Evolution, and Uncertainty |journal=Evolution: Education and Outreach |volume=4 |issue=3 |pages=489–501 |doi=10.1007/s12052-011-0357-y  |name-list-style=amp|doi-access=free }}</ref>