Editing Cnidaria (section)

==Description==

===Basic body forms===
[[File:Cnidiria medusa and polip svg hariadhi.svg|thumb|250px]] 
[[File:Actinodiscus macro.JPG|thumb|right|250px|Oral end of [[actinodiscus]] polyp]]
Most adult cnidarians appear as either free-swimming [[Medusa (biology)|medusa]]e or [[Sessility (zoology)|sessile]] [[polyp (zoology)|polyp]]s, and many [[hydrozoa]]ns species are known to alternate between the two forms.

Both are [[Symmetry (biology)#Radial symmetry|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 [[cnidocyte]]s around their edges, and medusae generally have an inner ring of tentacles around the mouth. Some hydroids may consist of colonies of [[zooid]]s that serve different purposes, such as defence, 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 [[jet propulsion]].<ref name="Ruppert" />

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

In some colonial polyps, a [[chitin]]ous [[Epidermis (zoology)|epidermis]] gives support and some protection to the connecting sections and to the lower parts of individual polyps. 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 [[sediment]] particles.<ref name="Ruppert" />

A mineralized [[exoskeleton]] made of calcium carbonate is found in subphylum Anthozoa in the order [[Scleractinia]] (stony corals; class Hexacorallia) and the class [[Octocorallia]],<ref>[https://academic.oup.com/gbe/article/12/9/1623/5882021?login=false Comparative Proteomics of Octocoral and Scleractinian Skeletomes and the Evolution of Coral Calcification]</ref> and in subphylum Medusozoa in three [[hydrozoa]]n families in order [[Anthoathecata]]; [[Fire coral|Milleporidae]], [[Stylasteridae]] and [[Hydractiniidae]] (the latter with a mix of calcified and uncalcified species).<ref>[https://academic.oup.com/icb/article/50/3/428/621249?login=false Evolution of Calcium-carbonate Skeletons in the Hydractiniidae]</ref>

===Main cell layers===
Cnidaria are [[diploblastic]] animals; in other words, they have two main cell layers, while more complex animals are [[triploblast]]s 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 [[basement membrane]], which they [[secrete]]. They also secrete the jelly-like [[mesoglea]] that separates the layers. The layer that faces outwards, known as the [[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 [[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 [[Anthozoa]] (anemones, corals, etc.) and [[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>
*[[Cnidocyte]]s, the harpoon-like "nettle cells" that give the [[phylum]] Cnidaria its name. These appear between or sometimes on top of the muscle cells.<ref name="Hinde2001" />
*[[Nerve]] cells. [[Sensory neuron|Sensory]] cells appear between or sometimes on top of the muscle cells,<ref name="Hinde2001" /> and communicate via [[synapse]]s (gaps across which chemical signals flow) with [[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 [[gastrodermis|gastroderm]] ("stomach skin") contains [[gland]] cells that secrete digestive [[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 [[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===
[[Polymorphism (biology)|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 (zoology)|polyp]] and [[medusae|medusa]] forms, or of [[zooids]] within colonial organisms like those in [[Hydrozoa]].<ref>{{cite journal |doi=10.1098/rspb.1966.0037 |title=Genetic polymorphism |journal=Proceedings of the Royal Society of London. Series B. Biological Sciences |date=1966 |volume=164 |issue=995 |pages=350–361 |pmid=4379524 |bibcode=1966RSPSB.164..350F |last1=Ford |first1=E. B. }}</ref> In [[Hydrozoa]]ns, colonial individuals arising from individual 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 ''[[Obelia]]'' there are feeding individuals, the [[gastrozooid]]s; 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 [[harpoon]]s, since their [[Wikt:payload|payload]]s remain connected to the bodies of the cells by threads. Three types of [[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]]
*[[Nematocyst]]s inject [[venom]] into prey, and usually have barbs to keep them embedded in the victims. Most species have nematocysts.<ref name="Hinde2001" />
*[[Spirocyst]]s do not penetrate the victim or inject venom, but entangle it by means of small sticky hairs on the thread.
*[[Ptychocyst]]s 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 [[order (biology)|order]] [[Ceriantharia]], [[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 (genus)|hydra]]'s nematocyst, before firing.<br>{{Color box|#00b900|border=silver}} "trigger" cilium<ref name="Ruppert" />]]

*A [[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 [[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 [[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 ''[[Chironex]]'' (the "sea wasp"), chemical changes in the cnida's contents may cause them to expand rapidly by [[polymerization]].
*Chemical changes in the liquid in the cnida make it a much more [[concentration|concentrated]] solution, so that [[osmotic pressure]] forces water in very rapidly to dilute it. This mechanism has been observed in nematocysts of the class [[Hydrozoa]], sometimes producing pressures as high as 140 [[Atmosphere (unit)|atmospheres]], similar to that of [[Scuba set|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 [[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 [[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 (''[[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" />

[[hydra (genus)|Hydra]]s and some [[sea anemone]]s can move slowly over rocks and sea or stream beds by various means: creeping like snails, crawling like [[Geometer moth|inchworm]]s, or by [[Cartwheel (gymnastics)|somersault]]ing. 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|bibcode=2011JExpB.214.1215S }}</ref> These sensory structures, usually called rhopalia, can generate signals in response to various types of stimuli such as light, pressure, chemical changes, 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|bibcode=2002CaJZ...80.1654S |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 glycine.<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 [[siphonophore]]s and [[chondrophore]]s sense tilt and acceleration by means of [[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 [[box jellyfish]] are unique among Medusae because they possess four kinds of true eyes that have [[retinas]], [[cornea]]s and [[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: [[predation]], absorbing dissolved [[organic matter|organic]] chemicals, [[filter feeding|filtering]] food particles out of the water, obtaining [[nutrients]] from [[symbiotic]] [[algae]] within their cells, and parasitism. Most obtain the majority of their food from predation but some, including the [[coral]]s ''[[Hetroxenia]]'' and ''[[Leptogorgia]]'', depend almost completely on their [[endosymbiont]]s and on absorbing dissolved nutrients.<ref name="Hinde2001" /> Cnidaria give their symbiotic algae [[carbon dioxide]], some nutrients, and protection against predators.<ref name="Ruppert" />

Predatory species use their [[cnidocyte]]s to poison or entangle prey, and those with venomous [[nematocyst]]s may start digestion by injecting digestive [[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 around 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. These "oral arms" aid in cnidarians' ability to move prey towards their mouth once it has been poisoned and entangled. 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 [[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 [[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 [[diffusion]] or, for animals or zooids such as medusae which have thick [[mesoglea]]s, 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 [[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 [[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 [[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 [[hydrostatic skeleton]], to control the water pressure in the cavity without expelling undigested food.<ref name="Hinde2001" />

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

===Regeneration===
All cnidarians can [[regeneration (biology)|regenerate]], allowing them to recover from injury and to reproduce [[asexual reproduction|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" />