View source for Biology:Cnidaria

{{short description|Aquatic animal phylum having cnydocytes}}
{{Automatic taxobox
| fossil_range ={{Fossil range|earliest=635|580|0}} [[Earth:Ediacaran|Ediacaran]]–Present
| image = Cnidaria.png
| image_upright = 1.3
| image_caption = Four examples of cnidaria (''clockwise, from top left''): 
* A jellyfish ''[[Biology:Chrysaora melanaster|Chrysaora melanaster]]'' 
* A gorgonian ''[[Biology:Annella mollis|Annella mollis]]'' 
* A sea anemone ''[[Biology:Nemanthus annamensis|Nemanthus annamensis]]''
* A stony coral ''Acropora cervicornis'' 
| display_parents = 2
| taxon = Cnidaria
| authority = Hatschek, 1888
| subdivision_ranks = Subphyla and classes
| subdivision_ref = <ref>Subphyla Anthozoa and Medusozoa based on {{cite web |url=http://www.taxonomy.nl/Taxonomicon/TaxonTree.aspx?id=11551 |title=The Taxonomicon – Taxon: Phylum Cnidaria |access-date=2007-07-10 |publisher=Universal Taxonomic Services |url-status=dead |archive-url=https://web.archive.org/web/20070929102538/http://www.taxonomy.nl/Taxonomicon/TaxonTree.aspx?id=11551 |archive-date=2007-09-29}}</ref>
| subdivision =
*Subphylum [[Biology:Anthozoa|Anthozoa]]—[[Biology:Coral|coral]]s and [[Biology:Sea anemone|sea anemone]]
** Class [[Biology:Octocorallia|Octocorallia]]
** Class [[Biology:Hexacorallia|Hexacorallia]]
** Class [[Biology:Ceriantharia|Ceriantharia]]
*Subphylum [[Biology:Medusozoa|Medusozoa]]—[[Biology:Jellyfish|jellyfish]] and hydrozoans:<ref>Classes in Medusozoa based on {{cite web|url=https://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=718920#null| title=ITIS Report – Taxon: Subphylum Medusozoa| access-date=2018-03-18| publisher=Universal Taxonomic Services}}</ref>
** Class [[Biology:Cubozoa|Cubozoa]]—[[Biology:Box jellyfish|box jellyfish]], sea wasps
** Class [[Biology:Hydrozoa|Hydrozoa]]—[[Biology:Hydroid (zoology)|hydroids]], [[Biology:Hydra (genus)|hydra]]-like animals
** Class [[Biology:Polypodiozoa|Polypodiozoa]]—parasites
** Class [[Biology:Scyphozoa|Scyphozoa]]—true jellyfish
** Class [[Biology:Staurozoa|Staurozoa]]—stalked jellyfish
* Subphylum [[Biology:Myxozoa|Myxozoa]]—[[Earth:Parasitism|parasites]]<ref name="Collins" />
|type_species=''Nematostella vectensis''<ref>{{Cite journal|last1=Steele|first1=Robert E.|last2=Technau|first2=Ulrich|date=2011-04-15|title=Evolutionary crossroads in developmental biology: Cnidaria|journal=Development|language=en|volume=138|issue=8|pages=1447–1458|doi=10.1242/dev.048959|issn=0950-1991|pmid=21389047|pmc=3062418}}</ref>}}
[[Image:Sea nettles.jpg|thumb|[[Biology:Chrysaora fuscescens|Pacific sea nettles]], ''Chrysaora fuscescens'']]

'''Cnidaria''' ({{IPAc-en|n|ᵻ|ˈ|d|ɛər|i|ə|,_|n|aɪ|-}})<ref>{{OED|cnidaria}}</ref> is a [[Biology:Phylum|phylum]] under kingdom [[Biology:Animal|Animal]]ia containing over 11,000 [[Biology:Species|species]]<ref>{{cite web|url=http://www.marinespecies.org/aphia.php?p=browser&id%5B%5D=2&id%5B%5D=1267#focus|title=WoRMS - World Register of Marine Species|website=www.marinespecies.org|access-date=2018-12-17}}</ref> of aquatic animals found both in [[Earth:Fresh water|freshwater]] and [[Biology:Marine habitats|marine environments]] (predominantly the latter), including [[Biology:Jellyfish|jellyfish]], [[Biology:Hydroid (zoology)|hydroid]]s, [[Biology:Sea anemone|sea anemone]], [[Biology:Coral|coral]]s and some of the smallest marine [[Biology:Parasite|parasite]]s. Their distinguishing features are a decentralized nervous system distributed throughout a gelatinous body and the presence of [[Biology:Cnidocyte|cnidocyte]]s or cnidoblasts, specialized cells with ejectable flagella used mainly for [[Medicine:Envenomation|envenomation]] and capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance, sandwiched between two layers of [[Biology:Epithelium|epithelium]] that are mostly one [[Biology:Cell|cell]] thick.

Cnidarians mostly have two basic body forms: swimming medusae and [[Biology:Sessility (motility)|sessile]] polyps, both of which are radially symmetrical with mouths surrounded by [[Biology:Tentacle|tentacle]]s that bear cnidocytes. Both forms have a single [[Biology:Body orifice|orifice]] and body cavity that are used for [[Biology:Digestion|digestion]] and [[Biology:Respiration (physiology)|respiration]]. Many cnidarian species produce [[Biology:Colony|colonies]] that are single organisms composed of medusa-like or polyp-like zooids, or both (hence they are [[Biology:Trimorphism|trimorphic]]). Cnidarians' activities are coordinated by a decentralized nerve net and [[Biology:Sensory neuron|simple receptors]]. Several free-swimming species of [[Biology:Cubozoa|Cubozoa]] and [[Biology:Scyphozoa|Scyphozoa]] possess balance-sensing [[Biology:Statocyst|statocyst]]s, and some have [[Biology:Simple eye in invertebrates|simple eyes]]. Not all cnidarians [[Biology:Sexual reproduction|reproduce sexually]], but many species have complex life cycles of asexual polyp stages and sexual medusae stages. Some, however, omit either the polyp or the medusa stage, and the parasitic classes evolved to have neither form.

Cnidarians were formerly grouped with ctenophores in the phylum [[Biology:Coelenterata|Coelenterata]], but increasing awareness of their differences caused them to be placed in separate phyla.<ref>{{cite journal|last1=Dunn|first1=Casey W.|last2=Leys|first2=Sally P.|last3=Haddock|first3=Steven H.D.|title=The hidden biology of sponges and ctenophores|journal=Trends in Ecology & Evolution|date=May 2015|volume=30|issue=5|pages=282–291|doi=10.1016/j.tree.2015.03.003|pmid=25840473|doi-access=free}}</ref> Cnidarians are classified into four main groups: the almost wholly sessile [[Biology:Anthozoa|Anthozoa]] ([[Biology:Sea anemone|sea anemone]]s, [[Biology:Coral|coral]]s, [[Biology:Sea pen|sea pen]]s); swimming [[Biology:Scyphozoa|Scyphozoa]] ([[Biology:Jellyfish|jellyfish]]); [[Biology:Cubozoa|Cubozoa]] (box jellies); and [[Biology:Hydrozoa|Hydrozoa]] (a diverse group that includes all the freshwater cnidarians as well as many marine forms, and which has both sessile members, such as ''[[Biology:Hydra (genus)|Hydra]]'', and colonial swimmers, such as the [[Biology:Portuguese man o' war|Portuguese man o' war]]). [[Biology:Staurozoa|Staurozoa]] have recently been recognised as a [[Biology:Class|class]] in their own right rather than a sub-group of Scyphozoa, and the highly derived parasitic [[Biology:Myxozoa|Myxozoa]] and [[Biology:Polypodiozoa|Polypodiozoa]] were firmly recognized as cnidarians only in 2007.<ref name="E.">{{cite journal|author=E. Jímenez-Guri|date=July 2007|title=''Buddenbrockia'' is a cnidarian worm|journal=Science|volume=317|issue=116|pages=116–118|doi=10.1126/science.1142024|pmid=17615357|display-authors=0|bibcode=2007Sci...317..116J|s2cid=5170702}}</ref>

Most cnidarians prey on organisms ranging in size from [[Earth:Plankton|plankton]] to animals several times larger than themselves, but many obtain much of their nutrition from [[Biology:Dinoflagellate|dinoflagellate]]s, and a few are [[Earth:Parasitism|parasites]]. Many are preyed on by other animals including [[Biology:Starfish|starfish]], [[Biology:Nudibranch|sea slugs]], [[Biology:Fish|fish]], [[Biology:Turtle|turtle]]s, and even other cnidarians. Many [[Biology:Scleractinia|scleractinia]]n corals—which form the structural foundation for [[Earth:Coral reef|coral reef]]s—possess polyps that are filled with symbiotic photo-synthetic [[Biology:Zooxanthellae|zooxanthellae]]. While reef-forming corals are almost entirely restricted to warm and shallow marine waters, other cnidarians can be found at great depths, in polar regions, and in freshwater.

Cnidarians are a very ancient phylum, with fossils having been found in rocks formed about {{ma|580}} during the [[Earth:Ediacaran|Ediacaran]] period, preceding the Cambrian Explosion. Other fossils show that corals may have been present shortly before {{ma|490}} and diversified a few million years later. [[Biology:Molecular clock|Molecular clock]] analysis of [[Biology:Mitochondria|mitochondria]]l genes suggests an even older age for the [[Biology:Crown group|crown group]] of cnidarians, estimated around {{ma|741}}, almost 200 million years before the [[Earth:Cambrian|Cambrian]] period, as well as before any fossils.<ref>{{cite journal |vauthors=Park E, Hwang D, Lee J, Song J, Seo T, Won Y |display-authors=3| title=Estimation of divergence times in cnidarian evolution based on mitochondrial protein-coding genes and the fossil record. | journal= Molecular Phylogenetics & Evolution | volume=62| issue=1| date=January 2012| pages=329–45 | doi=10.1016/j.ympev.2011.10.008 | pmid = 22040765 }}</ref> Recent phylogenetic analyses support [[Biology:Monophyly|monophyly]] of cnidarians, as well as the position of cnidarians as the [[Biology:Sister group|sister group]] of [[Biology:Bilateria|bilateria]]ns.<ref>{{cite journal |vauthors=Zapata F, Goetz FE, Smith SA, Howison M, Siebert S, Church SH |display-authors=3| title=Phylogenomic Analyses Support Traditional Relationships within Cnidaria. | journal= PLOS ONE | volume=10| issue=10| year=2015| pages=e0139068| doi=10.1371/journal.pone.0139068 | pmid=26465609 | pmc=4605497| bibcode=2015PLoSO..1039068Z| doi-access=free }}</ref>

==Distinguishing features==
{{further|Biology:Porifera|Biology:Ctenophora|Biology:Bilateria|Biology:Placozoa}}
Cnidarians form a [[Biology:Phylum|phylum]] of [[Biology:Animal|animal]]s that are more complex than [[Biology:Sponge|sponge]]s, about as complex as ctenophores (comb jellies), and less complex than [[Biology:Bilateria|bilateria]]ns, which include almost all other animals. Both cnidarians and ctenophores are more complex than sponges as they have: cells bound by inter-cell connections and carpet-like [[Biology:Basement membrane|basement membrane]]s; [[Biology:Muscle|muscle]]s; [[Biology:Nervous system|nervous system]]s; and '''some''' have sensory organs. Cnidarians are distinguished from all other animals by having [[Biology:Cnidocyte|cnidocyte]]s that fire [[Engineering:Harpoon|harpoon]]-like structures that are mainly used to capture prey. In some species, cnidocytes can also be used as anchors.<ref name="Hinde2001">{{cite book| author=Hinde, R.T.| year=1998| chapter=The Cnidaria and Ctenophora| pages=28–57| editor=Anderson, D.T.| title=Invertebrate Zoology| publisher=Oxford University Press| isbn=978-0-19-551368-4}}</ref> Cnidarians are also distinguished by the fact that they have only one opening in their body for ingestion and excretion i.e. they do not have a separate mouth and anus.

Like sponges and ctenophores, cnidarians have two main layers of cells that sandwich a middle layer of jelly-like material, which is called the mesoglea in cnidarians; more complex [[Biology:Animal|animal]]s have three main cell layers and no intermediate jelly-like layer. Hence, cnidarians and ctenophores have traditionally been labelled diploblastic, along with sponges.<ref name="Hinde2001" /><ref name="Ruppert" /> However, both cnidarians and ctenophores have a type of [[Biology:Muscle|muscle]] that, in more complex animals, arises from the [[Biology:Mesoderm|middle cell layer]].<ref name="Seipel">{{cite journal|author1=Seipel, K. |author2=Schmid, V.| title=Evolution of striated muscle: Jellyfish and the origin of triploblasty| journal=Developmental Biology| volume=282| issue=1| date=June 2005| pages=14–26| doi=10.1016/j.ydbio.2005.03.032| pmid=15936326| doi-access=free}}</ref> As a result, some recent text books classify ctenophores as triploblastic,<ref name="Ruppert" />{{rp|182–195}} and it has been suggested that cnidarians evolved from triploblastic ancestors.<ref name="Seipel"/>

{| class="wikitable" style="margin-left:4px"
! &nbsp;!![[Biology:Sponge|Sponge]]s<ref name="Ruppert" />{{rp|76–97}}<ref>{{cite book| author=Bergquist, P.R.| year=1998| chapter=Porifera| pages=10–27| editor=Anderson, D.T.| title=Invertebrate Zoology| publisher=Oxford University Press| isbn=978-0-19-551368-4}}</ref>!!Cnidarians<ref name="Hinde2001" /><ref name="Ruppert" />!!Ctenophores<ref name="Hinde2001" /><ref name="Ruppert" />{{rp|182–195}}!![[Biology:Bilateria|Bilateria]]<ref name="Hinde2001" />
|- style="text-align:center;"
! [[Biology:Cnidocyte|Cnidocyte]]s
| No||Yes||colspan=2| No
|- style="text-align:center;"
! [[Biology:Colloblast|Colloblast]]s
| colspan=2| No||Yes||No
|- style="text-align:center;"
! Digestive and [[Biology:Circulatory system|circulatory]] [[Biology:Organ (anatomy)|organ]]s
| colspan=3| No||Yes
|- style="text-align:center;"
! Number of main cell layers
| Two, with jelly-like layer between them
|Two||Two||Three
|- style="text-align:center;"
! Cells in each layer bound together
| cell-adhesion molecules, but no basement membranes except [[Biology:Homoscleromorpha|Homoscleromorpha]].<ref>{{cite journal|author1=Exposito, J-Y. |author2=Cluzel, C. |author3=Garrone, R. |author4=Lethias, C. |name-list-style=amp | title=Evolution of collagens| journal=The Anatomical Record Part A: Discoveries in Molecular, Cellular, and Evolutionary Biology| volume=268| pages=302–316| doi=10.1002/ar.10162| year=2002|issue=3| pmid=12382326|s2cid=12376172 |doi-access=free}}</ref>||colspan=3|inter-cell connections; basement membranes
|- style="text-align:center;"
! Sensory organs
| No||colspan=3| Yes
|- style="text-align:center;"
! Number of cells in middle "jelly" layer
| Many||colspan=2| Few||(Not applicable)
|- style="text-align:center;"
! Cells in outer layers can move inwards and change functions
| Yes||colspan=2| No||(Not applicable)
|- style="text-align:center;"
! Nervous system
| No||colspan=2| Yes, simple||Simple to complex
|- style="text-align:center;"
! [[Biology:Muscle|Muscle]]s
| None||Mostly epitheliomuscular||Mostly myoepithelial||Mostly [[Biology:Myocyte|myocyte]]s
|}
{{clear}}

==Description==

===Basic body forms===
{{Annotated image| float=right| caption=Medusa (left) and polyp (right)<ref name="Ruppert" />
| image=Cnidaria medusa n polyp.png| width=300| image-width=250| image-top=5| height=240
| annotations=
{{Annotation|40|2|'''Aboral end'''}}
{{Annotation|170|2|'''Oral end'''}}
{{Annotation|53|123|Mouth}}
{{Annotation|45|155|'''Oral end'''}}
{{Annotation|165|155|'''Aboral end'''}}
{{Annotation|5|180|{{Color box|red|border=silver}} Exoderm}}
{{Annotation|140|180|
{{Color box|blue|border=silver}}Endoderm }}
{{Annotation|5|200|{{Color box|#909090|border=silver}} Mesoglea}}
{{Annotation|140|200|{{Color box|#9999ff|border=silver}}Digestive}}
{{Annotation|160|220|Cavity}}
}}
[[File:Actinodiscus macro.JPG|thumb|right|250px|Oral end of actinodiscus polyp]]
Most adult cnidarians appear as either free-swimming medusae or sessile polyps, and many [[Biology:Hydrozoa|hydrozoa]]ns species are known to alternate between the two forms.

Both are radially symmetrical, like a wheel and a tube respectively. Since these animals have no heads, their ends are described as "oral" (nearest the mouth) and "aboral" (furthest from the mouth).

Most have fringes of tentacles equipped with [[Biology:Cnidocyte|cnidocyte]]s around their edges, and medusae generally have an inner ring of tentacles around the mouth. Some hydroids may consist of colonies of zooids that serve different purposes, such as defense, reproduction and catching prey. The mesoglea of polyps is usually thin and often soft, but that of medusae is usually thick and springy, so that it returns to its original shape after muscles around the edge have contracted to squeeze water out, enabling medusae to swim by a sort of [[Physics:Jet propulsion|jet propulsion]].<ref name="Ruppert" />

===Skeletons===
In medusae the only supporting structure is the mesoglea. ''[[Biology:Hydra (genus)|Hydra]]'' and most [[Biology:Sea anemone|sea anemone]]s close their mouths when they are not feeding, and the [[Chemistry:Water|water]] in the digestive cavity then acts as a [[Biology:Hydrostatic skeleton|hydrostatic skeleton]], rather like a water-filled balloon. Other polyps such as ''[[Biology:Tubularia|Tubularia]]'' use columns of water-filled cells for support. [[Biology:Sea pen|Sea pen]]s stiffen the mesoglea with [[Chemistry:Calcium carbonate|calcium carbonate]] spicules and tough fibrous [[Biology:Protein|protein]]s, rather like [[Biology:Sponge|sponge]]s.<ref name="Ruppert" />

In some colonial polyps, a [[Chemistry:Chitin|chitin]]ous [[Biology:Epidermis (zoology)|epidermis]] gives support and some protection to the connecting sections and to the lower parts of individual polyps. [[Biology:Scleractinia|Stony coral]]s secrete massive calcium carbonate [[Biology:Exoskeleton|exoskeleton]]s. A few polyps collect materials such as sand grains and shell fragments, which they attach to their outsides. Some colonial sea anemones stiffen the mesoglea with [[Earth:Sediment|sediment]] particles.<ref name="Ruppert" />

===Main cell layers===
Cnidaria are diploblastic animals; in other words, they have two main cell layers, while more complex animals are triploblasts having three main layers. The two main cell layers of cnidarians form epithelia that are mostly one cell thick, and are attached to a fibrous [[Biology:Basement membrane|basement membrane]], which they secrete. They also secrete the jelly-like mesoglea that separates the layers. The layer that faces outwards, known as the [[Biology:Ectoderm|ectoderm]] ("outside skin"), generally contains the following types of cells:<ref name="Hinde2001" />
*Epitheliomuscular cells whose bodies form part of the epithelium but whose bases extend to form [[Biology:Muscle|muscle]] fibers in parallel rows.<ref name="Ruppert" />{{rp| 103–104}} The fibers of the outward-facing cell layer generally run at right angles to the fibers of the inward-facing one. In [[Biology:Anthozoa|Anthozoa]] (anemones, corals, etc.) and [[Biology:Scyphozoa|Scyphozoa]] (jellyfish), the mesoglea also contains some muscle cells.<ref name="Ruppert">{{cite book| author1=Ruppert, E.E.| author2=Fox, R.S.| author3=Barnes, R.D.| name-list-style=amp| title=Invertebrate Zoology| publisher=Brooks / Cole| edition=7| isbn=978-0-03-025982-1| year=2004| pages=[https://archive.org/details/isbn_9780030259821/page/111 111–124]| url=https://archive.org/details/isbn_9780030259821/page/111}}</ref>
*[[Biology:Cnidocyte|Cnidocyte]]s, the harpoon-like "nettle cells" that give the [[Biology:Phylum|phylum]] Cnidaria its name. These appear between or sometimes on top of the muscle cells.<ref name="Hinde2001" />
*[[Biology:Nerve|Nerve]] cells. [[Biology:Sensory neuron|Sensory]] cells appear between or sometimes on top of the muscle cells,<ref name="Hinde2001" /> and communicate via [[Biology:Synapse|synapse]]s (gaps across which chemical signals flow) with [[Biology:Motor nerve|motor nerve]] cells, which lie mostly between the bases of the muscle cells.<ref name="Ruppert" /> Some form a simple nerve net.
*Interstitial cells, which are unspecialized and can replace lost or damaged cells by transforming into the appropriate types. These are found between the bases of muscle cells.<ref name="Hinde2001" />

In addition to epitheliomuscular, nerve and interstitial cells, the inward-facing [[Biology:Gastrodermis|gastroderm]] ("stomach skin") contains gland cells that secrete digestive [[Biology:Enzyme|enzyme]]s. In some species it also contains low concentrations of cnidocytes, which are used to subdue prey that is still struggling.<ref name="Hinde2001" /><ref name="Ruppert" />

The mesoglea contains small numbers of [[Biology:Amoeba (genus)|amoeba]]-like cells,<ref name="Ruppert" /> and muscle cells in some species.<ref name="Hinde2001" /> However, the number of middle-layer cells and types are much lower than in sponges.<ref name="Ruppert" />

===Polymorphism===
[[Biology:Polymorphism|Polymorphism]] refers to the occurrence of structurally and functionally more than two different types of individuals within the same organism. It is a characteristic feature of Cnidarians, particularly the polyp and medusa forms, or of zooids within colonial organisms like those in [[Biology:Hydrozoa|Hydrozoa]].<ref>{{cite book|last=Ford|first=E.B.|title=Genetic polymorphism|journal=Proceedings of the Royal Society of London. Series B, Biological Sciences|volume=164|issue=995|pages=350–61|date=1965|publisher=Faber & Faber|location=London|isbn=978-0262060127|pmid=4379524|doi=10.1098/rspb.1966.0037|s2cid=202575235}}</ref> In [[Biology:Hydrozoa|Hydrozoa]]ns, colonial individuals arising from individuals zooids will take on separate tasks.<ref>{{cite journal|last=Dunn|first=Casey W.|author2=Wagner, Günter P.|title=The evolution of colony-level development in the Siphonophora (Cnidaria:Hydrozoa)|journal=Development Genes and Evolution|date=16 September 2006|volume=216|issue=12|pages=743–754|doi=10.1007/s00427-006-0101-8|pmid=16983540|s2cid=278540}}</ref> For example, in ''[[Biology:Obelia|Obelia]]'' there are feeding individuals, the gastrozooids; the individuals capable of asexual reproduction only, the gonozooids, blastostyles and free-living or sexually reproducing individuals, the medusae.

===Cnidocytes===
These "nettle cells" function as [[Engineering:Harpoon|harpoon]]s, since their payloads remain connected to the bodies of the cells by threads. Three types of [[Biology:Cnidocyte|cnidocyte]]s are known:<ref name="Hinde2001" /><ref name="Ruppert" />

[[File:Hydra nematocyst firing 01.png|thumb|right|200px| Firing sequence of the cnida in a hydra's nematocyst<ref name="Ruppert" /><br>{{Color box|blue|border=silver}} Operculum (lid)<br>{{Color box|red|border=silver}} "Finger" that turns inside out<br>/ / / Barbs<br>{{Color box|yellow|border=silver}} Venom<br>{{Color box|silver|border=silver}} Victim's skin<br>{{Color box|#ffcad0|border=silver}} Victim's tissues]]
*Nematocysts inject [[Biology:Venom|venom]] into prey, and usually have barbs to keep them embedded in the victims. Most species have nematocysts.<ref name="Hinde2001" />
*Spirocysts do not penetrate the victim or inject venom, but entangle it by means of small sticky hairs on the thread.
*Ptychocysts are not used for prey capture&nbsp;— instead the threads of discharged ptychocysts are used for building protective tubes in which their owners live. Ptychocysts are found only in the [[Biology:Order|order]] [[Biology:Ceriantharia|Ceriantharia]], [[Biology:Tube-dwelling anemone|tube anemones]].<ref name="Ruppert" />

The main components of a cnidocyte are:<ref name="Hinde2001" /><ref name="Ruppert" />
[[File:Hydra nematocyst 01.png|thumb|200px| A hydra's nematocyst, before firing.<br>{{Color box|#00b900|border=silver}} "trigger" cilium<ref name="Ruppert" />]]

*A [[Biology:Cilium|cilium]] (fine hair) which projects above the surface and acts as a trigger. Spirocysts do not have cilia.
*A tough capsule, the cnida, which houses the thread, its payload and a mixture of chemicals that may include venom or [[Chemistry:Adhesive|adhesive]]s or both. ("cnida" is derived from the Greek word κνίδη, which means "nettle"<ref>{{Cite book| title=Shorter Oxford English Dictionary| contribution=Cnida|author1=Trumble, W. |author2=Brown, L.| publisher=Oxford University Press| year=2002}}</ref>)
*A tube-like extension of the wall of the cnida that points into the cnida, like the finger of a rubber glove pushed inwards. When a cnidocyte fires, the finger pops out. If the cell is a venomous nematocyte, the "finger"'s tip reveals a set of barbs that anchor it in the prey.
*The thread, which is an extension of the "finger" and coils round it until the cnidocyte fires. The thread is usually hollow and delivers chemicals from the cnida to the target.
*An [[Biology:Operculum (animal)|operculum]] (lid) over the end of the cnida. The lid may be a single hinged flap or three flaps arranged like slices of pie.
*The cell body, which produces all the other parts.

It is difficult to study the firing mechanisms of cnidocytes as these structures are small but very complex. At least four hypotheses have been proposed:<ref name="Hinde2001" />
*Rapid contraction of fibers round the cnida may increase its internal pressure.
*The thread may be like a coiled spring that extends rapidly when released.
*In the case of ''[[Biology:Chironex|Chironex]]'' (the "sea wasp"), chemical changes in the cnida's contents may cause them to expand rapidly by [[Chemistry:Polymerization|polymerization]].
*Chemical changes in the liquid in the cnida make it a much more [[Chemistry:Concentration|concentrated]] solution, so that [[Biology:Osmotic pressure|osmotic pressure]] forces water in very rapidly to dilute it. This mechanism has been observed in nematocysts of the class [[Biology:Hydrozoa|Hydrozoa]], sometimes producing pressures as high as 140 [[Physics:Atmosphere (unit)|atmospheres]], similar to that of scuba air tanks, and fully extending the thread in as little as 2&nbsp;milliseconds (0.002&nbsp;second).<ref name="Ruppert" />

Cnidocytes can only fire once, and about 25% of a hydra's nematocysts are lost from its tentacles when capturing a [[Biology:Brine shrimp|brine shrimp]]. Used cnidocytes have to be replaced, which takes about 48&nbsp;hours. To minimise wasteful firing, two types of stimulus are generally required to trigger cnidocytes: nearby [[Biology:Sensory neuron|sensory]] cells detect chemicals in the water, and their cilia respond to contact. This combination prevents them from firing at distant or non-living objects. Groups of cnidocytes are usually connected by nerves and, if one fires, the rest of the group requires a weaker minimum stimulus than the cells that fire first.<ref name="Hinde2001" /><ref name="Ruppert" />

===Locomotion===
[[File:Chrysaora quinquecirrha-Sea nettle (jellyfish).ogg|thumb| right| 200px| A swimming sea nettle known as the purple-striped jelly (''[[Biology:Chrysaora colorata|Chrysaora colorata]]'')]]
Medusae swim by a form of jet propulsion: muscles, especially inside the rim of the bell, squeeze water out of the cavity inside the bell, and the springiness of the mesoglea powers the recovery stroke. Since the tissue layers are very thin, they provide too little power to swim against currents and just enough to control movement within currents.<ref name="Ruppert" />

[[Biology:Hydra (genus)|Hydra]]s and some [[Biology:Sea anemone|sea anemone]]s can move slowly over rocks and sea or stream beds by various means: creeping like snails, crawling like [[Biology:Geometer moth|inchworm]]s, or by somersaulting. A few can swim clumsily by waggling their bases.<ref name="Ruppert" />

===Nervous system and senses===
Cnidarians are generally thought to have no brains or even central nervous systems. However, they do have integrative areas of neural tissue that could be considered some form of centralization. Most of their bodies are innervated by decentralized nerve nets that control their swimming musculature and connect with sensory structures, though each clade has slightly different structures.<ref name=Central>{{Cite journal|last=Satterlie|first=Richard A.|date=15 April 2011|title=Do jellyfish have central nervous systems?|journal=Journal of Experimental Biology|language=en|volume=214|issue=8|pages=1215–1223|doi=10.1242/jeb.043687|issn=0022-0949|pmid=21430196|doi-access=free}}</ref> These sensory structures, usually called rhopalia, can generate signals in response to various types of stimuli such as light, pressure, and much more. Medusa usually have several of them around the margin of the bell that work together to control the motor nerve net, that directly innervates the swimming muscles. Most Cnidarians also have a parallel system. In scyphozoans, this takes the form of a diffuse nerve net, which has modulatory effects on the nervous system.<ref name=Control>{{Cite journal|last=Satterlie|first=Richard A|s2cid=18244609|date=2002-10-01|title=Neuronal control of swimming in jellyfish: a comparative story|journal=Canadian Journal of Zoology|volume=80|issue=10|pages=1654–1669|doi=10.1139/z02-132|issn=0008-4301}}</ref> As well as forming the "signal cables" between sensory neurons and motoneurons, intermediate neurons in the nerve net can also form ganglia that act as local coordination centers. Communication between nerve cells can occur by chemical synapses or gap junctions in hydrozoans, though gap junctions are not present in all groups. Cnidarians have many of the same neurotransmitters as bilaterians, including chemicals such as glutamate, GABA, and acetylcholine.<ref>{{Cite journal|last1=Kass-Simon|first1=G.|last2=Pierobon|first2=Paola|date=1 January 2007|title=Cnidarian chemical neurotransmission, an updated overview|journal=Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology|volume=146|issue=1|pages=9–25|doi=10.1016/j.cbpa.2006.09.008|pmid=17101286}}</ref> Serotonin, dopamine, noradrenaline, octopamine, histamine, and acetylcholine, on the other hand, are absent.<ref>[https://www.frontiersin.org/articles/10.3389/fcell.2022.1071961/full What is a neuron? (Ctenophores, sponges and placozoans)]</ref>

This structure ensures that the musculature is excited rapidly and simultaneously, and can be directly stimulated from any point on the body, and it also is better able to recover after injury.<ref name=Central/><ref name=Control />

Medusae and complex swimming colonies such as siphonophores and [[Biology:Chondrophore|chondrophore]]s sense tilt and acceleration by means of [[Biology:Statocyst|statocyst]]s, chambers lined with hairs which detect the movements of internal mineral grains called statoliths. If the body tilts in the wrong direction, the animal rights itself by increasing the strength of the swimming movements on the side that is too low. Most species have ocelli ("simple eyes"), which can detect sources of light. However, the agile [[Biology:Box jellyfish|box jellyfish]] are unique among Medusae because they possess four kinds of true eyes that have retinas, [[Medicine:Cornea|cornea]]s and [[Biology:Lens (anatomy)|lenses]].<ref name="live">{{cite web|url=http://www.livescience.com/7243-jellyfish-human-eyes.html|title=Jellyfish Have Human-Like Eyes|publisher=www.livescience.com|date=April 1, 2007|access-date=2012-06-12}}</ref> Although the eyes probably do not form images, Cubozoa can clearly distinguish the direction from which light is coming as well as negotiate around solid-colored objects.<ref name="Hinde2001" /><ref name="live" />

===Feeding and excretion===
Cnidarians feed in several ways: [[Earth:Predation|predation]], absorbing dissolved [[Chemistry:Organic matter|organic]] chemicals, filtering food particles out of the water, obtaining nutrients from symbiotic [[Biology:Algae|algae]] within their cells, and parasitism. Most obtain the majority of their food from predation but some, including the [[Biology:Coral|coral]]s ''Hetroxenia'' and ''[[Biology:Leptogorgia|Leptogorgia]]'', depend almost completely on their [[Biology:Endosymbiont|endosymbiont]]s and on absorbing dissolved nutrients.<ref name="Hinde2001" /> Cnidaria give their symbiotic algae [[Chemistry:Carbon dioxide|carbon dioxide]], some nutrients, and protection against predators.<ref name="Ruppert" />

Predatory species use their [[Biology:Cnidocyte|cnidocyte]]s to poison or entangle prey, and those with venomous nematocysts may start digestion by injecting digestive [[Biology:Enzyme|enzyme]]s. The "smell" of fluids from wounded prey makes the tentacles fold inwards and wipe the prey off into the mouth. In medusae the tentacles round the edge of the bell are often short and most of the prey capture is done by "oral arms", which are extensions of the edge of the mouth and are often frilled and sometimes branched to increase their surface area. Medusae often trap prey or suspended food particles by swimming upwards, spreading their tentacles and oral arms and then sinking. In species for which suspended food particles are important, the tentacles and oral arms often have rows of cilia whose beating creates currents that flow towards the mouth, and some produce nets of [[Biology:Mucus|mucus]] to trap particles.<ref name="Hinde2001" /> Their digestion is both intra and extracellular.

Once the food is in the digestive cavity, gland cells in the [[Biology:Gastrodermis|gastroderm]] release enzymes that reduce the prey to slurry, usually within a few hours. This circulates through the digestive cavity and, in colonial cnidarians, through the connecting tunnels, so that gastroderm cells can absorb the nutrients. Absorption may take a few hours, and digestion within the cells may take a few days. The circulation of nutrients is driven by water currents produced by cilia in the gastroderm or by muscular movements or both, so that nutrients reach all parts of the digestive cavity.<ref name="Ruppert" /> Nutrients reach the outer cell layer by [[Physics:Diffusion|diffusion]] or, for animals or zooids such as medusae which have thick mesogleas, are transported by mobile cells in the mesoglea.<ref name="Hinde2001" />

Indigestible remains of prey are expelled through the mouth. The main waste product of cells' internal processes is [[Chemistry:Ammonia|ammonia]], which is removed by the external and internal water currents.<ref name="Ruppert" />

===Respiration===
There are no respiratory organs, and both cell layers absorb oxygen from and expel [[Chemistry:Carbon dioxide|carbon dioxide]] into the surrounding water. When the water in the digestive cavity becomes stale it must be replaced, and nutrients that have not been absorbed will be expelled with it. Some [[Biology:Anthozoa|Anthozoa]] have ciliated grooves on their tentacles, allowing them to pump water out of and into the digestive cavity without opening the mouth. This improves respiration after feeding and allows these animals, which use the cavity as a [[Biology:Hydrostatic skeleton|hydrostatic skeleton]], to control the water pressure in the cavity without expelling undigested food.<ref name="Hinde2001" />

Cnidaria that carry photosynthetic symbionts may have the opposite problem, an excess of oxygen, which may prove [[Medicine:Oxygen toxicity|toxic]]. The animals produce large quantities of [[Biology:Antioxidant|antioxidant]]s to neutralize the excess oxygen.<ref name="Hinde2001" />

===Regeneration===
All cnidarians can [[Biology:Regeneration|regenerate]], allowing them to recover from injury and to reproduce asexually. Medusae have limited ability to regenerate, but polyps can do so from small pieces or even collections of separated cells. This enables corals to recover even after apparently being destroyed by predators.<ref name="Hinde2001" />

==Reproduction==
{{Annotated image| float=right| caption=Life cycle of a [[Biology:Jellyfish|jellyfish]]:<ref name="Hinde2001" /><ref name="Ruppert" /><br>'''1–3''' Larva searches for site<br>'''4–8''' Polyp grows<br>'''9–11''' Polyp [[Biology:Strobilation|strobilates]]<br>'''12–14''' Medusa grows| image=Schleiden-meduse-2.jpg| width=193| image-width=200| height=292| annot-font-weight=bold| annot-background-color=yellow| annot-font-size=10
| annotations=
{{Annotation|100|231|&nbsp;1&nbsp;|background-color=#ffcccc}}
{{Annotation|60|232|&nbsp;2&nbsp;|background-color=#ffcccc}}
{{Annotation|9|232|&nbsp;3&nbsp;|background-color=#ffcccc}}
{{Annotation|4|260|&nbsp;4&nbsp;|background-color=#ffcccc}}
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{{Annotation|78|260|&nbsp;6&nbsp;|background-color=#ffcccc}}
{{Annotation|120|260|&nbsp;7&nbsp;|background-color=#ffcccc}}
{{Annotation|151|216|&nbsp;8&nbsp;|background-color=#ffcccc}}
{{Annotation|166|147|&nbsp;9&nbsp;|background-color=#ffcccc}}
{{Annotation|104|159|&nbsp;10&nbsp;|background-color=#ffcccc}}
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{{Annotation|87|60|&nbsp;13&nbsp;|background-color=#ffcccc}}
{{Annotation|161|62|&nbsp;14&nbsp;|background-color=#ffcccc}}
}}

===Sexual===
Cnidarian [[Biology:Sexual reproduction|sexual reproduction]] often involves a complex life cycle with both polyp and medusa stages. For example, in [[Biology:Scyphozoa|Scyphozoa]] (jellyfish) and [[Biology:Cubozoa|Cubozoa]] (box jellies) a [[Biology:Larva|larva]] swims until it finds a good site, and then becomes a polyp. This grows normally but then absorbs its tentacles and splits horizontally into a series of disks that become juvenile medusae, a process called [[Biology:Strobilation|strobilation]]. The juveniles swim off and slowly grow to maturity, while the polyp re-grows and may continue strobilating periodically. The adult medusae have gonads in the [[Biology:Gastrodermis|gastroderm]], and these release ova and [[Biology:Sperm|sperm]] into the water in the breeding season.<ref name="Hinde2001" /><ref name="Ruppert" />

This phenomenon of succession of differently organized generations (one asexually reproducing, sessile polyp, followed by a free-swimming medusa or a sessile polyp that reproduces sexually)<ref>{{cite book|author=Vernon A. Harris |year=1990 |chapter=Hydroids |title=Sessile animals of the sea shore |publisher=Springer |page=223 |isbn=9780412337604 |chapter-url=https://books.google.com/books?id=DbaBQm_ZQkIC&q=alternation+of+generation+&pg=PA223}}</ref> is sometimes called "alternation of asexual and sexual phases" or "metagenesis", but should not be confused with the [[Biology:Alternation of generations|alternation of generations]] as found in plants.

Shortened forms of this life cycle are common, for example some oceanic scyphozoans omit the polyp stage completely, and cubozoan polyps produce only one medusa. [[Biology:Hydrozoa|Hydrozoa]] have a variety of life cycles. Some have no polyp stages and some (e.g. ''[[Biology:Hydra (genus)|hydra]]'') have no medusae. In some species, the medusae remain attached to the polyp and are responsible for sexual reproduction; in extreme cases these reproductive zooids may not look much like medusae. Meanwhile, life cycle reversal, in which polyps are formed directly from medusae without the involvement of sexual reproduction process, was observed in both Hydrozoa (''Turritopsis dohrnii''<ref>{{Cite journal|url = http://www.icm.csic.es/scimar/pdf/56/sm56n2137.pdf|title = Bi-directional conversion in Turritopsis nutricula (Hydrozoa)|last = Bavestrello|date = 1992|journal = Scientia Marina|access-date = 2015-12-31|display-authors = etal|archive-date = 2014-12-14|archive-url = https://web.archive.org/web/20141214014457/http://www.icm.csic.es/scimar/pdf/56/sm56n2137.pdf|url-status = dead}}</ref> and ''Laodicea undulata''<ref>{{Cite journal|title = Evidence of reverse development in Leptomedusae (Cnidaria, Hydrozoa): the case of Laodicea undulata (Forbes and Goodsir 1851)|last = De Vito|date = 2006|journal = Marine Biology|doi = 10.1007/s00227-005-0182-3|display-authors=0|volume=149|issue = 2|pages=339–346| bibcode=2006MarBi.149..339D |s2cid = 84325535}}</ref>) and Scyphozoa (''Aurelia'' sp.1<ref>{{Cite journal|title = Life Cycle Reversal in Aurelia sp.1 (Cnidaria, Scyphozoa)|last = He|date = 21 December 2015|journal = PLOS ONE|doi = 10.1371/journal.pone.0145314|pmid = 26690755|display-authors=0|pmc=4687044|volume=10|issue = 12|pages=e0145314|bibcode=2015PLoSO..1045314H|doi-access = free}}</ref>). [[Biology:Anthozoa|Anthozoa]] have no medusa stage at all and the polyps are responsible for sexual reproduction.<ref name="Hinde2001" />

Spawning is generally driven by environmental factors such as changes in the water temperature, and their release is triggered by lighting conditions such as sunrise, sunset or the [[Astronomy:Lunar phase|phase of the moon]]. Many species of Cnidaria may spawn simultaneously in the same location, so that there are too many ova and sperm for predators to eat more than a tiny percentage&nbsp;— one famous example is the Great Barrier Reef, where at least 110 [[Biology:Coral|coral]]s and a few non-cnidarian [[Biology:Invertebrate|invertebrate]]s produce enough gametes to turn the water cloudy. These mass spawnings may produce [[Biology:Hybrid|hybrids]], some of which can settle and form polyps, but it is not known how long these can survive. In some species the ova release chemicals that attract sperm of the same species.<ref name="Hinde2001" />

The fertilized eggs develop into larvae by dividing until there are enough cells to form a hollow sphere ([[Biology:Blastula|blastula]]) and then a depression forms at one end ([[Biology:Gastrulation|gastrulation]]) and eventually becomes the digestive cavity. However, in cnidarians the depression forms at the end further from the yolk (at the animal pole), while in bilaterians it forms at the other end (vegetal pole).<ref name="Ruppert" /> The larvae, called [[Biology:Planula|planula]]e, swim or crawl by means of cilia.<ref name="Hinde2001" /> They are cigar-shaped but slightly broader at the "front" end, which is the aboral, vegetal-pole end and eventually attaches to a substrate if the species has a polyp stage.<ref name="Ruppert" />

Anthozoan larvae either have large yolks or are capable of feeding on [[Earth:Plankton|plankton]], and some already have endosymbiotic [[Biology:Algae|algae]] that help to feed them. Since the parents are immobile, these feeding capabilities extend the larvae's range and avoid overcrowding of sites. Scyphozoan and hydrozoan larvae have little yolk and most lack endosymbiotic algae, and therefore have to settle quickly and metamorphose into polyps. Instead, these species rely on their medusae to extend their ranges.<ref name="Ruppert" />

===Asexual===
All known cnidaria can reproduce asexually by various means, in addition to regenerating after being fragmented. [[Biology:Hydrozoa|Hydrozoa]]n polyps only bud, while the medusae of some hydrozoans can divide down the middle. [[Biology:Scyphozoa|Scyphozoa]]n polyps can both bud and split down the middle. In addition to both of these methods, [[Biology:Anthozoa|Anthozoa]] can split horizontally just above the base. Asexual reproduction makes the daughter cnidarian a clone of the adult.<ref name="Hinde2001" /><ref name="Ruppert" />

===DNA repair===

Two classical [[Biology:DNA repair|DNA repair]] pathways, [[Biology:Nucleotide excision repair|nucleotide excision repair]] and [[Biology:Base excision repair|base excision repair]], are present in [[Biology:Hydra (genus)|hydra]],<ref name = Barve2021>{{cite journal|pmid=33995496 |date=2021 |last1=Barve |first1=A. |last2=Galande |first2=A. A. |last3=Ghaskadbi |first3=S. S. |last4=Ghaskadbi |first4=S. |title=DNA Repair Repertoire of the Enigmatic Hydra |journal=Frontiers in Genetics |volume=12 |page=670695 |doi=10.3389/fgene.2021.670695 |pmc=8117345 |doi-access=free }}</ref> and these repair pathways facilitate unhindered reproduction.  The identification of these pathways in hydra is based, in part, on the presence in the hydra [[Biology:Genome|genome]] of genes homologous to genes in other genetically well studied species that have been demonstrated to play key roles in these DNA repair pathways.<ref name = Barve2021/>

==Classification==
Cnidarians were for a long time grouped with Ctenophores in the phylum [[Biology:Coelenterata|Coelenterata]], but increasing awareness of their differences caused them to be placed in separate phyla. Modern cnidarians are generally classified into four main [[Biology:Class|classes]]:<ref name="Hinde2001" /> sessile [[Biology:Anthozoa|Anthozoa]] ([[Biology:Sea anemone|sea anemone]]s, [[Biology:Coral|coral]]s, [[Biology:Sea pen|sea pen]]s); swimming [[Biology:Scyphozoa|Scyphozoa]] (jellyfish) and [[Biology:Cubozoa|Cubozoa]] (box jellies); and [[Biology:Hydrozoa|Hydrozoa]], a diverse group that includes all the freshwater cnidarians as well as many marine forms, and has both sessile members such as ''[[Biology:Hydra (genus)|Hydra]]'' and colonial swimmers such as the Portuguese Man o' War. [[Biology:Staurozoa|Staurozoa]] have recently been recognised as a [[Biology:Class|class]] in their own right rather than a sub-group of Scyphozoa, and the parasitic [[Biology:Myxozoa|Myxozoa]] and [[Biology:Polypodiozoa|Polypodiozoa]] are now recognized as highly derived cnidarians rather than more closely related to the [[Biology:Bilateria|bilateria]]ns.<ref name="E."/><ref name="haaretz">{{cite news|last=Schuster|first=Ruth|date=20 November 2015|url=http://www.haaretz.com/israel-news/science/1.687159|title=Microscopic parasitic jellyfish defy everything we know, astonish scientists|work=[[Social:Haaretz|Haaretz]]|access-date=4 April 2018}}</ref>

{| class="wikitable"
|-
! !![[Biology:Hydrozoa|Hydrozoa]]!![[Biology:Scyphozoa|Scyphozoa]]!![[Biology:Cubozoa|Cubozoa]]!![[Biology:Anthozoa|Anthozoa]]!![[Biology:Myxozoa|Myxozoa]]
|-
! Number of species<ref>{{cite journal|author=Zhang, Z.-Q.|year=2011|title=Animal biodiversity: An introduction to higher-level classification and taxonomic richness|url=http://mapress.com/zootaxa/2011/f/zt03148p012.pdf |archive-url=https://web.archive.org/web/20120112043434/http://mapress.com/zootaxa/2011/f/zt03148p012.pdf |archive-date=2012-01-12 |url-status=live|journal=Zootaxa|volume=3148|pages=7–12|doi=10.11646/zootaxa.3148.1.3}}</ref>
||3,600||228||42||6,100||1300
|-
! Examples
| ''[[Biology:Hydra (genus)|Hydra]]'', siphonophores||[[Biology:Jellyfish|Jellyfish]]||Box jellies||[[Biology:Sea anemone|Sea anemone]]s, [[Biology:Coral|coral]]s, [[Biology:Sea pen|sea pen]]s||''[[Biology:Myxobolus cerebralis|Myxobolus cerebralis]]''
|-
! Cells found in mesoglea
| No||Yes||Yes||Yes||
|-
! Nematocysts in [[Biology:Exodermis|exodermis]]
| No||Yes||Yes||Yes||
|-
! Medusa phase in life cycle
| In some species||Yes||Yes||No||
|-
! Number of medusae produced per polyp
| Many||Many||One||(not applicable)||
|}

Stauromedusae, small sessile cnidarians with stalks and no medusa stage, have traditionally been classified as members of the Scyphozoa, but recent research suggests they should be regarded as a separate class, Staurozoa.<ref>{{cite journal|author1=Collins, A.G. |author2=Cartwright, P. |author3=McFadden, C.S. |author4=Schierwater, B. |name-list-style=amp | title=Phylogenetic Context and Basal Metazoan Model Systems| journal=Integrative and Comparative Biology| year=2005| volume=45| issue=4| pages=585–594| doi=10.1093/icb/45.4.585| pmid=21676805| doi-access=free}}</ref>

The [[Biology:Myxozoa|Myxozoa]], microscopic [[Biology:Parasite|parasite]]s, were first classified as [[Biology:Protozoa|protozoa]]ns.<ref>{{cite journal|author=Štolc, A.|title=Actinomyxidies, nouveau groupe de Mesozoaires parent des Myxosporidies|year=1899|journal= Bull. Int. l'Acad. Sci. Bohème|volume=12|pages=1–12}}</ref> Research then found that ''[[Biology:Polypodium hydriforme|Polypodium hydriforme]]'', a non-Myxozoan parasite ''within'' the egg cells of [[Biology:Sturgeon|sturgeon]], is closely related to the Myxozoa and suggested that both ''Polypodium'' and the Myxozoa were intermediate between cnidarians and [[Biology:Bilateria|bilateria]]n animals.<ref name="ZrzavýHypša2003MyxozoaPolypodium">{{cite journal|author1=Zrzavý, J. |author2=Hypša, V.| date=April 2003| title=Myxozoa, ''Polypodium'', and the origin of the Bilateria: The phylogenetic position of "Endocnidozoa" in light of the rediscovery of ''Buddenbrockia''| journal=Cladistics| volume=19| issue=2| pages=164–169| doi=10.1111/j.1096-0031.2003.tb00305.x
|s2cid=221583517}}</ref> More recent research demonstrates that the previous identification of bilaterian genes reflected contamination of the Myxozoan samples by material from their host organism, and they are now firmly identified as heavily derived cnidarians, and more closely related to Hydrozoa and Scyphozoa than to Anthozoa.<ref name="E."/><ref name="haaretz"/><ref>{{cite journal|author=E. Jímenez-Guri|date=July 2007|title=''Buddenbrockia'' is a cnidarian worm|journal=Science|volume=317|issue=116|pages=116–118|doi=10.1126/science.1142024|pmid=17615357|last2=Philippe|first2=H|last3=Okamura|first3=B|last4=Holland|first4=PW|bibcode = 2007Sci...317..116J |s2cid=5170702}}</ref><ref>{{cite journal|last1=Chang|first1=E. Sally|last2=Neuhof|first2=Moran|last3=Rubinstein|first3=Nimrod D.|last4=Diamant|first4=Arik|last5=Philippe|first5=Hervé|last6=Huchon|first6=Dorothée|last7=Cartwright|first7=Paulyn|display-authors=3|title=Genomic insights into the evolutionary origin of Myxozoa within Cnidaria|journal=Proceedings of the National Academy of Sciences|date=1 December 2015|volume=112|issue=48|pages=14912–14917|doi=10.1073/pnas.1511468112|pmid=26627241|pmc=4672818|bibcode=2015PNAS..11214912C|doi-access=free}}</ref>

Some researchers classify the extinct [[Biology:Conulariida|conulariid]]s as cnidarians, while others propose that they form a completely separate [[Biology:Phylum|phylum]].<ref>{{cite web| url=http://www.ucmp.berkeley.edu/cnidaria/conulariida.html| access-date=2008-11-27| title=The Conulariida| publisher=University of California Museum of Paleontology}}</ref>

Current classification according to the [[Organization:World Register of Marine Species|World Register of Marine Species]]: 
* class [[Biology:Anthozoa|Anthozoa]] <small>Ehrenberg, 1834</small>
** subclass [[Biology:Ceriantharia|Ceriantharia]] <small>Perrier, 1893</small> — Tube-dwelling anemones
** subclass [[Biology:Hexacorallia|Hexacorallia]] <small>Haeckel, 1896</small> — stony corals
** subclass [[Biology:Octocorallia|Octocorallia]] <small>Haeckel, 1866</small> — soft corals and sea fans
* class [[Biology:Cubozoa|Cubozoa]] <small>Werner, 1973</small> — box jellies
* class [[Biology:Hydrozoa|Hydrozoa]] <small>Owen, 1843</small> — hydrozoans (fire corals, hydroids, hydroid jellyfishes, siphonophores...)
* class [[Biology:Myxozoa|Myxozoa]] <small>Grassé, 1970</small> — obligate parasites
* class [[Biology:Polypodiozoa|Polypodiozoa]] <small>Raikova, 1994</small> — (uncertain status)
* class [[Biology:Scyphozoa|Scyphozoa]] <small>Goette, 1887</small> — "true" jellyfishes
* class [[Biology:Staurozoa|Staurozoa]] <small>Marques & Collins, 2004</small> — stalked jellyfishes

<gallery style="text-align:center;" mode="packed">
Image:Cerianthus filiformis.jpg|''[[Biology:Cerianthus filiformis|Cerianthus filiformis]]'' ([[Biology:Ceriantharia|Ceriantharia]])
Image:Haeckel Actiniae.jpg|Sea anemones (Actinaria, part of [[Biology:Hexacorallia|Hexacorallia]])
Image:Hertshoon.jpg|Coral ''[[Biology:Acropora muricata|Acropora muricata]]'' ([[Biology:Scleractinia|Scleractinia]], part of [[Biology:Hexacorallia|Hexacorallia]])
Image:Gorgonia ventalina, Bahamas.jpg|Sea fan ''[[Biology:Gorgonia ventalina|Gorgonia ventalina]]'' ([[Biology:Alcyonacea|Alcyonacea]], part of [[Biology:Octocorallia|Octocorallia]])
Image:Carybdea branchi9.jpg|Box jellyfish ''[[Biology:Carybdea branchi|Carybdea branchi]]'' ([[Biology:Cubozoa|Cubozoa]])
Image:Portuguese Man-O-War (Physalia physalis).jpg|Siphonophore ''Physalia physalis'' ([[Biology:Hydrozoa|Hydrozoa]])
Image:Fdl17-9-grey.jpg|''[[Biology:Myxobolus cerebralis|Myxobolus cerebralis]]'' ([[Biology:Myxozoa|Myxozoa]])
Image:Polypodium hydriforme.jpg|''[[Biology:Polypodium hydriforme|Polypodium hydriforme]]'' ([[Biology:Polypodiozoa|Polypodiozoa]])
Image:Phyllorhiza punctata macro II.jpg|Jellyfish ''[[Biology:Phyllorhiza punctata|Phyllorhiza punctata]]'' ([[Biology:Scyphozoa|Scyphozoa]])
Image:Haliclystus antarcticus 1B.jpg|Stalked jelly ''[[Biology:Haliclystus antarcticus|Haliclystus antarcticus]]'' ([[Biology:Staurozoa|Staurozoa]])
</gallery>

==Ecology==
Many cnidarians are limited to shallow waters because they depend on endosymbiotic [[Biology:Algae|algae]] for much of their nutrients. The life cycles of most have polyp stages, which are limited to locations that offer stable substrates. Nevertheless, major cnidarian groups contain species that have escaped these limitations. Hydrozoans have a worldwide range: some, such as ''[[Biology:Hydra (genus)|Hydra]]'', live in freshwater; ''[[Biology:Obelia|Obelia]]'' appears in the coastal waters of all the oceans; and ''[[Biology:Liriope (cnidarian)|Liriope]]'' can form large shoals near the surface in mid-ocean. Among [[Biology:Anthozoa|anthozoa]]ns, a few [[Biology:Scleractinia|scleractinia]]n [[Biology:Coral|coral]]s, [[Biology:Sea pen|sea pen]]s and [[Biology:Gorgonia|sea fan]]s live in deep, cold waters, and some sea anemones inhabit polar seabeds while others live near [[Earth:Hydrothermal vent|hydrothermal vent]]s over {{convert|10|km|ft|abbr=on}} below sea-level. [[Earth:Reef|Reef]]-building corals are limited to tropical seas between 30°N and 30°S with a maximum depth of {{convert|46|m|ft|abbr=on}}, temperatures between {{convert|20|and|28|C}}, high [[Earth:Salinity|salinity]], and low [[Chemistry:Carbon dioxide|carbon dioxide]] levels. [[Biology:Stauromedusae|Stauromedusae]], although usually classified as jellyfish, are stalked, sessile animals that live in cool to Arctic waters.<ref name="Shostak">{{Cite book| author=Shostak, S.| contribution=Cnidaria (Coelenterates)| title=Encyclopedia of Life Sciences| publisher= John Wiley & Sons| year=2006| doi=10.1038/npg.els.0004117| isbn=978-0470016176}}</ref> Cnidarians range in size from a mere handful of cells for the parasitic myxozoans<ref name="haaretz"/> through ''Hydra'''s length of {{convert|5|-|20|mm|in|frac=8|abbr=on}},<ref>{{cite book| title=Small-scale Freshwater Toxicity Investigations: Toxicity Test Methods|author1=Blaise, C. |author2=Férard, J-F.| publisher=Springer| year=2005|isbn=978-1-4020-3119-9| page=398| url=https://books.google.com/books?id=Ibew5SLx2oMC&q=hydra+size+length| access-date=2008-11-21}}</ref> to the [[Biology:Lion's mane jellyfish|Lion's mane jellyfish]], which may exceed {{convert|2|m|ftin|abbr=on}} in diameter and {{convert|75|m|ft|abbr=on}} in length.<ref name="Safina" />

Prey of cnidarians ranges from plankton to animals several times larger than themselves.<ref name="Shostak" /><ref>{{cite book| author=Cowen, R.| title=History of Life| publisher=Blackwell| year=2000| edition=3| isbn=978-0-632-04444-3| page=54| url=https://books.google.com/books?id=qvyBS4gwPF4C&q=cnidaria+prey&pg=PA54| access-date=2008-11-21}}</ref> Some cnidarians are [[Biology:Parasite|parasite]]s, mainly on jellyfish but a few are major pests of fish.<ref name="Shostak" /> Others obtain most of their nourishment from endosymbiotic algae or dissolved nutrients.<ref name="Hinde2001" /> Predators of cnidarians include: [[Biology:Nudibranch|sea slugs]], [[Biology:Flatworm|flatworm]]s and [[Biology:Ctenophora|comb jellies]], which can incorporate nematocysts into their own bodies for self-defense (nematocysts used by cnidarian predators are referred to as kleptocnidae);<ref>{{cite journal | url=https://www.sciencedirect.com/science/article/abs/pii/S0171933518300268 | doi=10.1016/j.ejcb.2018.04.002 | title=Organelle survival in a foreign organism: Hydra nematocysts in the flatworm Microstomum lineare | year=2018 | last1=Krohne | first1=Georg | journal=European Journal of Cell Biology | volume=97 | issue=4 | pages=289–299 | pmid=29661512 }}</ref><ref>{{cite journal | pmc=2783962 | year=2009 | last1=Greenwood | first1=P. G. | title=Acquisition and Use of Nematocysts by Cnidarian Predators | journal=Toxicon | volume=54 | issue=8 | pages=1065–1070 | doi=10.1016/j.toxicon.2009.02.029 | pmid=19269306 }}</ref><ref>{{cite journal|author=Frick, K|title=Predator Suites and Flabellinid Nudibranch Nematocyst Complements in the Gulf of Maine|journal=In: SF Norton (Ed). Diving for Science...2003.|volume=Proceedings of the American Academy of Underwater Sciences|issue=22nd Annual Scientific Diving Symposium|year=2003|url=http://archive.rubicon-foundation.org/4744|access-date=2008-07-03|archive-date=2011-01-06|archive-url=https://web.archive.org/web/20110106062453/http://archive.rubicon-foundation.org/4744|url-status=usurped}}</ref> [[Biology:Starfish|starfish]], notably the crown of thorns starfish, which can devastate corals;<ref name="Shostak" /> butterfly fish and parrot fish, which eat corals;<ref>{{cite book|editor=Paxton, J.R. |editor2=Eschmeyer, W.N. |author1=Choat, J.H. |author2=Bellwood, D.R.|year=1998|title=Encyclopedia of Fishes|publisher= Academic Press|location=San Diego|pages= 209–211|isbn= 978-0-12-547665-2}}</ref> and marine [[Biology:Turtle|turtle]]s, which eat jellyfish.<ref name="Safina">{{cite book| author=Safina, C.| title=Voyage of the Turtle: In Pursuit of the Earth's Last Dinosaur| publisher=Macmillan| year=2007| isbn=978-0-8050-8318-7| page=154| url=https://books.google.com/books?id=dQD883dAv6YC&q=cnidaria+turtle&pg=PA154| access-date=2008-11-21}}</ref> Some sea anemones and jellyfish have a symbiotic relationship with some fish; for example [[Biology:Clownfish|clownfish]] live among the tentacles of sea anemones, and each partner protects the other against predators.<ref name="Shostak" />

[[Earth:Coral reef|Coral reef]]s form some of the world's most productive ecosystems. Common coral reef cnidarians include both Anthozoans (hard corals, octocorals, anemones) and Hydrozoans (fire corals, lace corals). The endosymbiotic algae of many cnidarian species are very effective primary producers, in other words converters of inorganic chemicals into [[Chemistry:Organic compound|organic]] ones that other organisms can use, and their coral hosts use these organic chemicals very efficiently. In addition, reefs provide complex and varied habitats that support a wide range of other organisms.<ref name="Barnes">{{cite book| title=Fundamentals of Aquatic Ecology|author1=Barnes, R.S.K. |author2=Mann, K.H.| publisher=Blackwell Publishing| year=1991| isbn=978-0-632-02983-9| pages=217–227| url=https://books.google.com/books?id=mOZZlzgdTrwC&q=%22Coral+Reef%22+productivity&pg=PA227| access-date=2008-11-26}}</ref> [[Earth:Fringing reef|Fringing reef]]s just below low-[[Astronomy:Tide|tide]] level also have a mutually beneficial relationship with [[Earth:Mangrove|mangrove]] forests at high-tide level and [[Biology:Seagrass meadow|seagrass meadow]]s in between: the reefs protect the mangroves and seagrass from strong currents and waves that would damage them or [[Earth:Erosion|erode]] the sediments in which they are rooted, while the mangroves and seagrass protect the coral from large influxes of [[Earth:Silt|silt]], fresh water and pollutants. This additional level of variety in the environment is beneficial to many types of coral reef animals, which for example may feed in the sea grass and use the reefs for protection or breeding.<ref>{{cite book|author=Hatcher, B.G. |author2=Johannes, R.E. |author3=Robertson, A.J. |name-list-style=amp| chapter=Conservation of Shallow-water Marine Ecosystems| title=Oceanography and Marine Biology: An Annual Review: Volume 27| publisher=Routledge| year=1989| isbn=978-0-08-037718-6| chapter-url=https://books.google.com/books?id=XpmNqFaDZ7cC&q=%22Coral+Reef%22+mangrove+%22seagrass%22&pg=PA320| access-date=2008-11-21|page=320}}</ref>

==Evolutionary history==
[[File:Stranded Cambrian scyphozoans.jpg|right|thumb|Stranded scyphozoans on a Cambrian tidal flat in Blackberry Hill, Wisconsin.]]
[[File:Cladocora.jpg|right|thumb|The fossil coral ''[[Biology:Cladocora|Cladocora]]'' from Pliocene rocks in Cyprus]]

===Fossil record===
The earliest widely accepted animal fossils are rather modern-looking cnidarians, possibly from around {{ma|580}}, although fossils from the [[Earth:Doushantuo Formation|Doushantuo Formation]] can only be dated approximately.<ref>{{cite journal|last1=Chen|first1=J-Y. |date=25 April 2000 |title=Putative phosphatized embryos from the Doushantuo Formation of China| journal=Proceedings of the National Academy of Sciences |volume=97|pages=4457–4462|doi=10.1073/pnas.97.9.4457|pmid=10781044|pmc=18256|issue=9|last2=Oliveri|first2=P|last3=Li|first3=CW|last4=Zhou|first4=GQ|last5=Gao|first5=F|last6=Hagadorn|first6=JW|last7=Peterson|first7=KJ|last8=Davidson|first8=EH|display-authors=3|bibcode = 2000PNAS...97.4457C |doi-access=free}}</ref> The identification of some of these as embryos of animals has been contested, but other fossils from these rocks strongly resemble tubes and other [[Earth:Biomineralization|mineralized]] structures made by [[Biology:Coral|coral]]s.<ref>{{cite journal|author1=Xiao, S. |author2=Yuan, X. |author3=Knoll, A.H. |name-list-style=amp | date=5 December 2000| title=Eumetazoan fossils in terminal Proterozoic phosphorites?| journal=Proceedings of the National Academy of Sciences| volume=97| pages=13684–13689| doi=10.1073/pnas.250491697| pmid=11095754| issue=25| pmc=17636|bibcode = 2000PNAS...9713684X |doi-access=free}}</ref> Their presence implies that the cnidarian and [[Biology:Bilateria|bilateria]]n lineages had already diverged.<ref>{{cite journal |author=Chen, J.-Y. |author2=Oliveri, P. |author3=Gao, F. |author4=Dornbos, S.Q. |author5=Li, C-W. |author6=Bottjer, D.J. |author7=Davidson, E.H. |display-authors=3|name-list-style=amp |title=Precambrian Animal Life: Probable Developmental and Adult Cnidarian Forms from Southwest China |journal=Developmental Biology |volume=248 |issue=1 |date=August 2002 |doi=10.1006/dbio.2002.0714 |url=http://www.uwm.edu/~sdornbos/PDF's/Chen%20et%20al.%202002.pdf |access-date=2008-09-03 |pages=182–196 |pmid=12142030 |archive-url=https://web.archive.org/web/20080911075353/http://www.uwm.edu/~sdornbos/PDF%27s/Chen%20et%20al.%202002.pdf |archive-date=2008-09-11 |url-status=dead }}</ref> Although the Ediacaran fossil ''[[Biology:Charnia|Charnia]]'' used to be classified as a [[Biology:Jellyfish|jellyfish]] or [[Biology:Sea pen|sea pen]],<ref>{{cite journal|author1=Donovan, Stephen K. |author2=Lewis, David N. | year = 2001| title = Fossils explained 35. The Ediacaran biota| journal = Geology Today| volume =17| issue =3| pages =115–120| doi =10.1046/j.0266-6979.2001.00285.x|s2cid=128395097 | type = abstract}}</ref> more recent study of growth patterns in ''Charnia'' and modern cnidarians has cast doubt on this hypothesis,<ref>{{cite journal| author = Antcliffe, J.B.| year = 2007| title = Charnia and sea pens are poles apart| journal = Journal of the Geological Society| volume = 164| issue = 1| pages = 49–51| doi = 10.1144/0016-76492006-080| last2 = Brasier| first2 = M. D.| bibcode = 2007JGSoc.164...49A| s2cid = 130602154}}</ref><ref>{{cite journal| author = Antcliffe, J.B.| last2 = Brasier| year = 2007| first2 = Martin D.| title = Charnia At 50: Developmental Models For Ediacaran Fronds| journal = Palaeontology| volume = 51| issue = 1| pages = 11–26| doi = 10.1111/j.1475-4983.2007.00738.x| s2cid = 83486435| doi-access = free}}</ref> leaving the Canadian polyp ''[[Biology:Haootia|Haootia]]'' and the British ''[[Biology:Auroralumina|Auroralumina]]'' as the only recognized cnidarian body fossils from the Ediacaran. ''Auroralumina'' is the earliest known animal [[Earth:Predation|predator]].<ref>{{cite web |url=https://www.bbc.co.uk/news/science-environment-62291954 |title=Ancient fossil is earliest known animal predator |last=Amos |first=Jonathan |date=25 July 2022 |website=bbc.co.uk |publisher=BBC News |access-date=25 July 2022}}</ref> Few fossils of cnidarians without mineralized [[Biology:Skeleton|skeleton]]s are known from more recent rocks, except in lagerstätten that preserved soft-bodied animals.<ref name="UCMP">{{cite web| url=http://www.ucmp.berkeley.edu/cnidaria/cnidariafr.html| access-date=2008-11-27| title=Cnidaria: Fossil Record| publisher=University of California Museum of Paleontology}}</ref>

A few mineralized fossils that resemble [[Biology:Coral|coral]]s have been found in rocks from the [[Earth:Cambrian|Cambrian]] period, and corals diversified in the Early [[Earth:Ordovician|Ordovician]].<ref name="UCMP" /> These corals, which were wiped out in the [[Earth:Permian–Triassic extinction event|Permian–Triassic extinction event]] about {{ma|252}},<ref name="UCMP" /> did not dominate reef construction since [[Biology:Sponge|sponge]]s and algae also played a major part.<ref>{{cite journal| author=Copper, P.| title=Ancient reef ecosystem expansion and collapse| journal=Coral Reefs| volume=13| issue=1|date=January 1994| pages=3–11| doi=10.1007/BF00426428| bibcode=1994CorRe..13....3C| s2cid=42938715}}</ref> During the [[Earth:Mesozoic|Mesozoic]] era rudist bivalves were the main reef-builders, but they were wiped out in the [[Earth:Cretaceous–Paleogene extinction event|Cretaceous–Paleogene extinction event]] {{ma|66}},<ref>{{cite web| url=http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/rudists.php| access-date=2008-11-27| title=The Rudists| publisher=University of California Museum of Paleontology}}</ref> and since then the main reef-builders have been [[Biology:Scleractinia|scleractinia]]n corals.<ref name="UCMP" />

===Family tree===
{{further|Biology:Phylogeny}}

It is difficult to reconstruct the early stages in the [[Biology:Evolution|evolution]]ary "family tree" of animals using only [[Biology:Morphology|morphology]] (their shapes and structures), because the large differences between [[Biology:Porifera|Porifera]] (sponges), Cnidaria plus [[Biology:Ctenophora|Ctenophora]] (comb jellies), [[Biology:Placozoa|Placozoa]] and [[Biology:Bilateria|Bilateria]] (all the more complex animals) make comparisons difficult. Hence reconstructions now rely largely or entirely on [[Biology:Molecular phylogenetics|molecular phylogenetics]], which groups organisms according to similarities and differences in their [[Biology:Biochemistry|biochemistry]], usually in their [[Biology:DNA|DNA]] or [[Biology:RNA|RNA]].<ref>{{cite journal| author=Halanych, K.M.| title=The New View of Animal Phylogeny| journal=Annual Review of Ecology, Evolution, and Systematics| volume=35| pages=229–256| date=December 2004| doi=10.1146/annurev.ecolsys.35.112202.130124| url=http://gump.auburn.edu/halanych/lab/Pub.pdfs/Halanych2004.pdf| access-date=2008-11-27| archive-url=https://web.archive.org/web/20081007035514/http://gump.auburn.edu/halanych/lab/Pub.pdfs/Halanych2004.pdf| archive-date=2008-10-07| url-status=dead}}</ref>

[[File:Radiata diversity.jpg|thumb|left|750px|Illustrated tree of cnidarians and their closest relatives]]

It is now generally thought that the [[Biology:Calcarea|Calcarea]] (sponges with [[Chemistry:Calcium carbonate|calcium carbonate]] spicules) are more closely related to Cnidaria, [[Biology:Ctenophora|Ctenophora]] (comb jellies) and [[Biology:Bilateria|Bilateria]] (all the more complex animals) than they are to the other groups of sponges.<ref>{{cite journal| author=Borchiellini, C. |author2=Manuel, M. |author3=Alivon, E. |author4=Boury-Esnault, N. |author5=Vacelet J. |author6=Le Parco, Y. |display-authors=3|name-list-style=amp| title=Sponge paraphyly and the origin of Metazoa| journal=Journal of Evolutionary Biology| volume=14| issue=1| pages=171–179| date=January 2001| doi=10.1046/j.1420-9101.2001.00244.x | pmid=29280585|s2cid=25119754 | doi-access=free}}</ref><ref>{{cite journal|author1=Medina, M. |author2=Collins, A.G. |author3=Silberman, J.D. |author4=Sogin, M.L. |name-list-style=amp | title=Evaluating hypotheses of basal animal phylogeny using complete sequences of large and small subunit rRNA| journal=Proceedings of the National Academy of Sciences|date=August 2001| volume=98| issue=17| pages=9707–9712| doi=10.1073/pnas.171316998| pmid=11504944| pmc=55517|bibcode = 2001PNAS...98.9707M |doi-access=free }}</ref><ref>{{cite journal|author1=Müller, W.E.G. |author2=Li, J. |author3=Schröder, H.C. |author4=Qiao, L. |author5=Wang, X. |display-authors=3|name-list-style=amp | title=The unique skeleton of siliceous sponges (Porifera; Hexactinellida and Demospongiae) that evolved first from the Urmetazoa during the Proterozoic: a review| journal=Biogeosciences| volume=4|pages=219–232| year=2007| doi=10.5194/bg-4-219-2007| issue=2|bibcode=2007BGeo....4..219M |url=https://hal.archives-ouvertes.fr/hal-00297608/file/bg-4-219-2007.pdf |archive-url=https://web.archive.org/web/20180726115528/https://hal.archives-ouvertes.fr/hal-00297608/file/bg-4-219-2007.pdf |archive-date=2018-07-26 |url-status=live|doi-access=free}}</ref> In 1866 it was proposed that Cnidaria and Ctenophora were more closely related to each other than to Bilateria and formed a group called [[Biology:Coelenterata|Coelenterata]] ("hollow guts"), because Cnidaria and Ctenophora both rely on the flow of water in and out of a single cavity for feeding, excretion and respiration. In 1881, it was proposed that Ctenophora and Bilateria were more closely related to each other, since they shared features that Cnidaria lack, for example muscles in the middle layer (mesoglea in Ctenophora, [[Biology:Mesoderm|mesoderm]] in Bilateria). However more recent analyses indicate that these similarities are rather vague, and the current view, based on molecular phylogenetics, is that Cnidaria and Bilateria are more closely related to each other than either is to Ctenophora. This grouping of Cnidaria and Bilateria has been labelled "Planulozoa" because it suggests that the earliest Bilateria were similar to the [[Biology:Planula|planula]] larvae of Cnidaria.<ref name="Collins">{{cite journal| author=Collins, A.G.| date=May 2002| title=Phylogeny of Medusozoa and the Evolution of Cnidarian Life Cycles| journal=Journal of Evolutionary Biology| volume=15| issue=3| pages=418–432| doi=10.1046/j.1420-9101.2002.00403.x| s2cid=11108911| doi-access=free}}</ref><ref>{{cite journal| author=Wallberg, A. | author2=Thollesson, M. | author3=Farris, J.S. | author4=Jondelius, U. |name-list-style=amp| title=The phylogenetic position of the comb jellies (Ctenophora) and the importance of taxonomic sampling| journal=Cladistics| volume=20| year=2004| pages=558–578| doi=10.1111/j.1096-0031.2004.00041.x| issue=6| pmid=34892961 |s2cid=86185156| doi-access=free}}</ref>

Within the Cnidaria, the Anthozoa (sea anemones and corals) are regarded as the sister-group of the rest, which suggests that the earliest cnidarians were sessile polyps with no medusa stage. However, it is unclear how the other groups acquired the medusa stage, since [[Biology:Hydrozoa|Hydrozoa]] form medusae by budding from the side of the polyp while the other Medusozoa do so by splitting them off from the tip of the polyp. The traditional grouping of [[Biology:Scyphozoa|Scyphozoa]] included the [[Biology:Staurozoa|Staurozoa]], but morphology and molecular phylogenetics indicate that Staurozoa are more closely related to [[Biology:Cubozoa|Cubozoa]] (box jellies) than to other "Scyphozoa". Similarities in the double body walls of Staurozoa and the extinct [[Biology:Conulariida|Conulariida]] suggest that they are closely related.<ref name="Collins" /><ref>{{cite journal|author1=Marques, A.C. |author2=Collins, A.G.| year=2004| title=Cladistic analysis of Medusozoa and cnidarian evolution| journal=Invertebrate Biology| volume=123| issue=1| pages=23–42| doi=10.1111/j.1744-7410.2004.tb00139.x| url=http://www.marinespecies.org/aphia.php?p=sourceget&id=38492| access-date=2008-11-27}}</ref>

However, in 2005 Katja Seipel and Volker Schmid suggested that cnidarians and ctenophores are simplified descendants of triploblastic animals, since ctenophores and the medusa stage of some cnidarians have striated muscle, which in bilaterians arises from the [[Biology:Mesoderm|mesoderm]]. They did not commit themselves on whether bilaterians evolved from early cnidarians or from the hypothesized triploblastic ancestors of cnidarians.<ref name="Seipel" />

In molecular phylogenetics analyses from 2005 onwards, important groups of developmental genes show the same variety in cnidarians as in [[Biology:Chordate|chordate]]s.<ref>{{cite journal|author1=Miller, D.J. |author2=Ball, E.E. |author3=Technau, U. |name-list-style=amp | title=Cnidarians and ancestral genetic complexity in the animal kingdom| journal=Trends in Genetics| volume=21| issue=10|date=October 2005| pages=536–539| doi=10.1016/j.tig.2005.08.002| pmid=16098631}}</ref> In fact cnidarians, and especially [[Biology:Anthozoa|anthozoa]]ns (sea anemones and corals), retain some genes that are present in [[Biology:Bacteria|bacteria]], [[Biology:Protist|protist]]s, [[Biology:Plant|plant]]s and [[Biology:Fungus|fungi]] but not in bilaterians.<ref>{{cite journal|author1=Technau, U. |author2=Rudd, S. |author3=Maxwell, P |name-list-style=amp | title=Maintenance of ancestral complexity and non-metazoan genes in two basal cnidarians| journal=Trends in Genetics| volume=21| issue=12|date=December 2005| pages=633–639| doi=10.1016/j.tig.2005.09.007| pmid=16226338}}</ref>

The mitochondrial genome in the medusozoan cnidarians, unlike those in other animals, is linear with fragmented genes.<ref>{{Cite journal | doi = 10.1093/gbe/evr127| pmid = 22117085| pmc = 3268669| title = First Complete Mitochondrial Genome Sequence from a Box Jellyfish Reveals a Highly Fragmented Linear Architecture and Insights into Telomere Evolution| journal = Genome Biology and Evolution| volume = 4| issue = 1| pages = 52–58| year = 2011| last1 = Smith | first1 = D. R.| last2 = Kayal | first2 = E.| last3 = Yanagihara | first3 = A. A.| last4 = Collins | first4 = A. G.| last5 = Pirro | first5 = S.| last6 = Keeling | first6 = P. J.|display-authors=3}}</ref> The reason for this difference is unknown.

==Interaction with humans==
[[File:Cubozoas.JPG|thumb|The dangerous ''[[Biology:Carukia barnesi|Carukia barnesi]]'', one of the known species of box jellyfish which can cause Irukandji syndrome.]]

Jellyfish stings killed about 1,500 people in the 20th century,<ref>{{cite book|author1=Williamson, J.A. |author2=Fenner, P.J. |author3=Burnett, J.W. |author4=Rifkin, J. |name-list-style=amp | title=Venomous and Poisonous Marine Animals: A Medical and Biological Handbook| publisher=UNSW Press| year=1996| isbn=978-0-86840-279-6| pages=65–68| url=https://books.google.com/books?id=YsZ3GryFIzEC&q=mollusc+venom+fatal&pg=PA75| access-date=2008-10-03}}</ref> and cubozoans are particularly dangerous. On the other hand, some large jellyfish are considered a delicacy in [[Place:East Asia|East]] and [[Place:Southeast Asia|Southeast Asia]]. [[Earth:Coral reef|Coral reef]]s have long been economically important as providers of fishing grounds, protectors of shore buildings against currents and tides, and more recently as centers of tourism. However, they are vulnerable to over-fishing, mining for construction materials, pollution, and damage caused by tourism.

{{annotated image| float=left| caption=The dangerous "sea wasp" ''[[Biology:Chironex fleckeri|Chironex fleckeri]]''
| image=Avispa marina.jpg| width=130| image-width=150| height=160| image-top=-90
| annotations=
}}

Beaches protected from tides and storms by coral reefs are often the best places for housing in tropical countries. Reefs are an important food source for low-technology fishing, both on the reefs themselves and in the adjacent seas.<ref name="Clark">{{cite book| title=Coastal Seas: The Conservation Challenge| author=Clark, J.R.| publisher=Blackwell| year=1998| isbn=978-0-632-04955-4| pages=[https://archive.org/details/coastalseasconse0000clar/page/8 8]–9| url=https://archive.org/details/coastalseasconse0000clar| url-access=registration| quote=Coral Reef productivity.| access-date=2008-11-28}}</ref> However, despite their great [[Earth:Primary production|productivity]], reefs are vulnerable to over-fishing, because much of the organic carbon they produce is exhaled as [[Chemistry:Carbon dioxide|carbon dioxide]] by organisms at the middle levels of the food chain and never reaches the larger species that are of interest to fishermen.<ref name="Barnes" /> Tourism centered on reefs provides much of the income of some tropical islands, attracting photographers, divers and sports fishermen. However, human activities damage reefs in several ways: mining for construction materials; pollution, including large influxes of fresh water from [[Earth:Storm drain|storm drain]]s; commercial fishing, including the use of dynamite to stun fish and the capture of young fish for aquariums; and tourist damage caused by boat anchors and the cumulative effect of walking on the reefs.<ref name="Clark" /> Coral, mainly from the [[Earth:Pacific Ocean|Pacific Ocean]] has long been used in jewellery, and demand rose sharply in the 1980s.<ref>{{cite book| title=Marine Minerals in Exclusive Economic Zones| author=Cronan, D.S.| year=1991| publisher=Springer| pages=63–65| isbn=978-0-412-29270-5| url=https://books.google.com/books?id=4g4nhd8USO8C&q=coral+jewellery&pg=PA63| access-date=2008-11-28}}</ref>

Some large [[Biology:Jellyfish|jellyfish]] species of the [[Biology:Rhizostomeae|Rhizostomeae]] order are commonly consumed in {{wipe|Japan}}, [[Place:Korea|Korea]] and Southeast Asia.<ref>{{Cite journal|author1=Kitamura, M. |author2=Omori, M.| title = Synopsis of edible jellyfishes collected from Southeast Asia, with notes on jellyfish fisheries| journal = Plankton and Benthos Research| volume = 5| issue = 3| pages = 106–118| year = 2010| issn = 1880-8247| doi=10.3800/pbr.5.106| doi-access = free}}</ref><ref>{{Cite journal|author1=Omori, M. |author2=Kitamura, M.| title = Taxonomic review of three Japanese species of edible jellyfish (Scyphozoa: Rhizostomeae)| journal = Plankton Biol. Ecol.| volume = 51| issue = 1| pages = 36–51| year = 2004}}</ref><ref name=Omori2001/> In parts of the range, fishing industry is restricted to daylight hours and calm conditions in two short seasons, from March to May and August to November.<ref name=Omori2001>{{cite journal | doi = 10.1023/A:1011879821323 | last1 = Omori | first1 = M. | last2 = Nakano | first2 = E. | date = May 2001 | title = Jellyfish fisheries in southeast Asia | journal = Hydrobiologia | volume = 451 | pages = 19–26 | s2cid = 6518460 }}</ref> The commercial value of jellyfish food products depends on the skill with which they are prepared, and "Jellyfish Masters" guard their [[Social:Trade secret|trade secret]]s carefully. Jellyfish is very low in [[Chemistry:Cholesterol|cholesterol]] and [[Chemistry:Sugar|sugar]]s, but cheap preparation can introduce undesirable amounts of heavy metals.<ref>{{Cite journal|author1=Y-H. Peggy Hsieh |author2=Fui-Ming Leong |author3=Jack Rudloe | title = Jellyfish as food| journal = Hydrobiologia| volume = 451| issue = 1–3| pages = 11–17| date = May 2001| doi = 10.1023/A:1011875720415|s2cid=20719121 }}</ref>

The "sea wasp" ''[[Biology:Chironex fleckeri|Chironex fleckeri]]'' has been described as the world's most venomous jellyfish and is held responsible for 67&nbsp;deaths, although it is difficult to identify the animal as it is almost transparent. Most stingings by ''C. fleckeri'' cause only mild symptoms.<ref>{{cite book| title=Greenberg's Text-atlas of Emergency Medicine
|author1=Greenberg, M.I. |author2=Hendrickson, R.G. |author3=Silverberg, M. |author4=Campbell, C. |author5=Morocco, A. |display-authors=3|name-list-style=amp | publisher=Lippincott Williams & Wilkins| year=2004|isbn=978-0-7817-4586-4| page=875| chapter=Box Jellyfish Envenomation}}</ref> Seven other [[Biology:Cubozoa|box jellies]] can cause a set of symptoms called [[Medicine:Irukandji syndrome|Irukandji syndrome]],<ref name="Little">{{cite journal|author1=Little, M. |author2=Pereira, P. |author3=Carrette, T. |author4=Seymour, J. |name-list-style=amp | title = Jellyfish Responsible for Irukandji Syndrome| journal = QJM| volume=99| issue = 6| pages = 425–427| date = June 2006| doi = 10.1093/qjmed/hcl057| pmid = 16687419| doi-access = free}}</ref> which takes about 30&nbsp;minutes to develop,<ref>{{cite journal| author = Barnes, J.| title = Cause and effect in Irukandji stingings| journal = Medical Journal of Australia| volume = 1| issue = 24| pages = 897–904| year = 1964| pmid = 14172390| doi=10.5694/j.1326-5377.1964.tb114424.x}}</ref> and from a few hours to two&nbsp;weeks to disappear.<ref>{{cite journal|vauthors=Grady J, Burnett J | title = Irukandji-like syndrome in South Florida divers| journal = Annals of Emergency Medicine| volume = 42| issue = 6| pages = 763–6| date = December 2003| pmid = 14634600| doi = 10.1016/S0196-0644(03)00513-4}}</ref> Hospital treatment is usually required, and there have been a few deaths.<ref name="Little" />

A number of the parasitic [[Biology:Myxozoa|Myxozoa]]ns are commercially important pathogens in salmonid aquaculture.<ref>{{Cite journal |last1=Braden |first1=Laura M. |last2=Rasmussen |first2=Karina J. |last3=Purcell |first3=Sara L. |last4=Ellis |first4=Lauren |last5=Mahony |first5=Amelia |last6=Cho |first6=Steven |last7=Whyte |first7=Shona K. |last8=Jones |first8=Simon R. M. |last9=Fast |first9=Mark D. |display-authors=3|date=December 19, 2017 |editor-last=Appleton |editor-first=Judith A. |title=Acquired Protective Immunity in Atlantic Salmon Salmo salar against the Myxozoan Kudoa thyrsites Involves Induction of MHIIβ + CD83 + Antigen-Presenting Cells |journal=Infection and Immunity |language=en |volume=86 |issue=1 |pages=e00556–17 |doi=10.1128/IAI.00556-17 |issn=0019-9567 |pmc=5736826 |pmid=28993459}}</ref> A [[Biology:Scyphozoa|Scyphozoa]] species – ''[[Biology:Pelagia noctiluca|Pelagia noctiluca]]'' – and a [[Biology:Hydrozoa|Hydrozoa]] – ''[[Biology:Muggiaea atlantica|Muggiaea atlantica]]'' – have caused repeated mass mortality in salmon farms over the years around Ireland.<ref name="IrishTimes">{{cite web | last=O'Sullivan | first=Kevin | title=Stinger jellyfish swarms wipe out farmed salmon in west of Ireland | website=The Irish Times | date=2017-10-06 | url=http://www.irishtimes.com/news/environment/stinger-jellyfish-swarms-wipe-out-farmed-salmon-in-west-of-ireland-1.3247245 | access-date=2022-04-23}}</ref> A loss valued at £1 million struck in November 2007, 20,000 died off Clare Island in 2013 and four fish farms collectively lost tens of thousands of salmon in September 2017.<ref name="IrishTimes" />
{{clear}}

==Notes==
{{reflist|colwidth=30em}}

==Further reading==

===Books===
*Arai, M.N. (1997). ''A Functional Biology of Scyphozoa.'' London: Chapman & Hall [p.&nbsp;316]. {{ISBN|0-412-45110-7}}.
*Ax, P. (1999). ''Das System der Metazoa I. Ein Lehrbuch der phylogenetischen Systematik.'' Gustav Fischer, Stuttgart-Jena: Gustav Fischer. {{ISBN|3-437-30803-3}}.
*Barnes, R.S.K., P. Calow, P. J. W. Olive, D. W. Golding & J. I. Spicer (2001). ''The invertebrates—a synthesis.'' Oxford: Blackwell. 3rd edition [chapter 3.4.2, p.&nbsp;54]. {{ISBN|0-632-04761-5}}.
*Brusca, R.C., G.J. Brusca (2003). ''Invertebrates.'' Sunderland, Mass.: Sinauer Associates. 2nd edition [chapter 8, p.&nbsp;219]. {{ISBN|0-87893-097-3}}.
*Dalby, A. (2003). ''Food in the Ancient World: from A to Z.'' London: Routledge.
*Moore, J.(2001). ''An Introduction to the Invertebrates.'' Cambridge: Cambridge University Press [chapter 4, p.&nbsp;30]. {{ISBN|0-521-77914-6}}.
*Schäfer, W. (1997). Cnidaria, Nesseltiere. In Rieger, W. (ed.) ''Spezielle Zoologie. Teil 1. Einzeller und Wirbellose Tiere.'' Stuttgart-Jena: Gustav Fischer. Spektrum Akademischer Verl., Heidelberg, 2004. {{ISBN|3-8274-1482-2}}.
*Werner, B. 4. Stamm Cnidaria. In: V. Gruner (ed.) ''Lehrbuch der speziellen Zoologie.'' Begr. von Kaestner. 2 Bde. Stuttgart-Jena: Gustav Fischer, Stuttgart-Jena. 1954, 1980, 1984, Spektrum Akad. Verl., Heidelberg-Berlin, 1993. 5th edition. {{ISBN|3-334-60474-8}}.
<!-- The following are now cited inline ****
*Anderson, D.T. (2001). ''Invertebrate Zoology.'' Oxford: Oxford University Press. 2nd edition [chapter 3, p.31]. ISBN 0-19-551368-1.
*Ruppert, E.E., R.S. Fox & R.P. Barnes (2004). ''Invertebrate Zoology—a Functional Evolutionary Approach.'' Belmont: Brooks-Cole [chapter 7, p.111]. ISBN 0-03-025982-7.
****** -->

===Journal articles===
* D. Bridge, B. Schierwater, C. W. Cunningham, R. DeSalle R, L. W. Buss: ''Mitochondrial DNA structure and the molecular phylogeny of recent cnidaria classes.'' in: ''Proceedings of the Academy of Natural Sciences of Philadelphia.'' Philadelphia USA 89.1992, p.&nbsp;8750. {{ISSN|0097-3157}}
* D. Bridge, C. W. Cunningham, R. DeSalle, L. W. Buss: ''Class-level relationships in the phylum Cnidaria—Molecular and morphological evidence.'' in: ''Molecular biology and evolution.'' Oxford University Press, Oxford 12.1995, p.&nbsp;679. {{ISSN|0737-4038}}
* D. G. Fautin: ''[http://www.nrcresearchpress.com/doi/abs/10.1139/z02-133 Reproduction of Cnidaria]{{closed access}}.'' in: ''Canadian Journal of Zoology.'' Ottawa Ont. 80.2002, p.&nbsp;1735. ([[PDF]], online) {{ISSN|0008-4301}}
* G. O. Mackie: ''[http://www.nrcresearchpress.com/doi/abs/10.1139/z02-138 What's new in cnidarian biology?] {{Webarchive|url=https://web.archive.org/web/20190218062823/http://www.nrcresearchpress.com/doi/abs/10.1139/z02-138 |date=2019-02-18 }}{{closed access}}'' in: ''Canadian Journal of Zoology.'' Ottawa Ont. 80.2002, p.&nbsp;1649. (PDF, online) {{ISSN|0008-4301}}
* P. Schuchert: ''Phylogenetic analysis of the Cnidaria.'' in: ''Zeitschrift für zoologische Systematik und Evolutionsforschung.'' Paray, Hamburg-Berlin 31.1993, p.&nbsp;161. {{ISSN|0044-3808}}
* G. Kass-Simon, A. A. Scappaticci Jr.: ''[http://www.nrcresearchpress.com/doi/abs/10.1139/z02-135 The behavioral and developmental physiology of nematocysts.]{{closed access}}'' in: ''Canadian Journal of Zoology.'' Ottawa Ont. 80.2002, p.&nbsp;1772. (PDF, online) {{ISSN|0044-3808}}
* {{Cite journal
| author=J. Zrzavý| year=2001| journal=Folia Parasitologica| volume=48| issue=2| pages=81–103
| title=The interrelationships of metazoan parasites: a review of phylum- and higher-level hypotheses from recent morphological and molecular phylogenetic analyses
| pmid=11437135| doi=10.14411/fp.2001.013
| doi-access=free}}
<!-- The following are now cited inline ****
*{{cite journal | last1 = Marques | first1 = A. C. | last2 = Collins | first2 = A. G. | year = 2004 | title = Cladistic analysis of Medusozoa and cnidarian evolution | journal = Invertebrate Biology | volume = 123 | issue = 1| pages = 23–42 | doi=10.1111/j.1744-7410.2004.tb00139.x|doi-access=free}}
******** -->

==External links==
*[https://www.youtube.com/watch?v=6zJiBc_N1Zk YouTube: Nematocysts Firing]
*[https://www.youtube.com/watch?v=WmvjGc2bojk YouTube:My Anemone Eat Meat] Defensive and feeding behaviour of sea anemone
* [https://web.archive.org/web/20120119003639/http://www.tafi.org.au/zooplankton/imagekey/cnidaria/index.html Cnidaria - Guide to the Marine Zooplankton of south eastern Australia], [https://web.archive.org/web/20080930064242/http://www.tafi.org.au/ Tasmanian Aquaculture & Fisheries Institute]
* [http://www.biochem.uci.edu/steele/default.html A Cnidaria homepage maintained by University of California, Irvine]
* [http://tolweb.org/tree?group=Cnidaria&contgroup=Animals Cnidaria page at ''Tree of Life'']
* [http://www.paleoportal.org/index.php?globalnav=fossil_gallery&sectionnav=search&taxon_id=55&state_id=&period_id=&assemblage_id=&last_section=search&p=0 Fossil Gallery: Cnidarians]
* [http://www.ville-ge.ch/musinfo/mhng/hydrozoa/hydrozoa-directory.htm The Hydrozoa Directory]
* [https://web.archive.org/web/20180908223624/http://hercules.kgs.ku.edu/Hexacoral/Anemone2/ Hexacorallians of the World]

{{Eukaryota classification}}
{{Animalia}}
{{Cnidaria}}
{{Life on Earth}}

{{Taxonbar|from=Q25441}}

[[Category:Animal phyla]]
[[Category:Marine animals]]
[[Category:Ediacaran first appearances]]

{{Sourceattribution|Cnidaria}}