Editing Ctenophora (section)

=== Nervous system and senses ===

Ctenophores have no [[brain]] or [[central nervous system]], but have a subepidermal [[nerve net]] that forms a ring round the mouth and is densest near structures such as the comb rows, pharynx, tentacles and the sensory complex furthest from the mouth.<ref name="RuppertBarnes2004Ctenophora"/> Nerve cells communicate by two different methods; some of the neurons have [[synapses|synaptic connections]], but those in the nerve net are highly distinctive by being fused into a [[syncytium]].<ref name=Burkhardt2023>{{cite journal |last1=Burkhardt |first1=Pawel |last2=Colgren |first2=Jeffrey |last3=Medhus |first3=Astrid |last4=Digel |first4=Leonid |last5=Naumann |first5=Benjamin |last6=Soto-Angel |first6=Joan |last7=Nordmann |first7=Eva-Lena |last8=Sachkova |first8=Maria |last9=Kittelmann |first9=Maike |display-authors=6 |title=Syncytial nerve net in a ctenophore adds insights on the evolution of nervous systems |journal=[[Science (journal)|Science]] |date=20 April 2023 |volume=380 |issue=6642 |pages=293–297 |doi=10.1126/science.ade5645 |pmid=37079688 |bibcode=2023Sci...380..293B |s2cid=258239574 |hdl=11250/3149299 |hdl-access=free }}</ref> Fossils show that Cambrian species had a more complex nervous system, with long nerves which connected with a ring around the mouth. The only ctenophores with long nerves today is ''[[Euplokamididae|Euplokamis]]'' in the order Cydippida.<ref>{{cite journal |last1=Parry |first1=Luke A. |last2=Lerosey-Aubril |first2=Rudy |last3=Weaver |first3=James C. |last4=Ortega-Hernández |first4=Javier |title=Cambrian comb jellies from Utah illuminate the early evolution of nervous and sensory systems in ctenophores |journal=iScience |date=2021 |volume=24 |issue=9 |page=102943 |doi=10.1016/j.isci.2021.102943 |pmid=34522849 |pmc=8426560 |bibcode=2021iSci...24j2943P |doi-access=free}}</ref> Their nerve cells arise from the same [[progenitor cell]]s as the colloblasts.<ref>{{cite journal |last1=Pennisi |first1=Elizabeth |author-link=Elizabeth Pennisi |title=The gluey tentacles of comb jellies may have revealed when nerve cells first evolved |journal=[[Science (journal)|Science]] |date=10 January 2019 |doi=10.1126/science.aaw6288 |s2cid=92852830 }}</ref>

In addition, there is a less organized mesogleal nerve net consisting of single neurites. The largest single sensory feature is the [[aboral]] organ (at the opposite end from the mouth), which is underlined with its own nerve net.<ref>[http://ryanlab.whitney.ufl.edu/pdfs/doi_10.1016_j.zool.2014.06.001.pdf Did the ctenophore nervous system evolve independently?]</ref> This organ's main component is a [[statocyst]], a balance sensor consisting of a statolith, a tiny grain of calcium carbonate, supported on four bundles of [[cilia]], called "balancers", that sense its orientation. The statocyst is protected by a transparent dome of long, immobile cilia. A ctenophore does not automatically try to keep the statolith resting equally on all the balancers. Instead, its response is determined by the animal's "mood", in other words, the overall state of the nervous system. For example, if a ctenophore with trailing tentacles captures prey, it often puts some comb rows into reverse, spinning the mouth towards the prey.<ref name="RuppertBarnes2004Ctenophora" />

The ciliated larvae in cnidarians and bilaterians appear to share an ancient and common origin.<ref>{{cite journal |last1=Marlow |first1=Heather |last2=Tosches |first2=Maria Antonietta |last3=Tomer |first3=Raju |last4=Steinmetz |first4=Patrick R. |last5=Lauri |first5=Antonella |last6=Larsson |first6=Tomas |last7=Arendt |first7=Detlev |date=29 January 2014 |title=Larval body patterning and apical organs are conserved in animal evolution |journal=[[BMC Biology]] |volume=12 |issue=1 |page=7 |doi=10.1186/1741-7007-12-7 |pmid=24476105 |pmc=3939940 |doi-access=free }}</ref> The larvae's apical organ is involved in the formation of the nervous system.<ref>{{cite journal |last1=Nielsen |first1=Claus |title=Larval nervous systems: true larval and precocious adult |journal=[[Journal of Experimental Biology]] |date=15 February 2015 |volume=218 |issue=4 |pages=629–636 |doi=10.1242/jeb.109603 |pmid=25696826 |s2cid=3151957 |doi-access=free }}</ref> The aboral organ of comb jellies is not homologous with the apical organ in other animals, and the formation of their nervous system has therefore a different embryonic origin.<ref>{{cite journal |last1=Nielsen |first1=Claus |title=Early animal evolution: a morphologist's view |journal=Royal Society Open Science |date=July 2019 |volume=6 |issue=7 |page=190638 |doi=10.1098/rsos.190638 |pmid=31417759 |pmc=6689584 |bibcode=2019RSOS....690638N }}</ref>

Ctenophore nerve cells and nervous system have distinctive biochemistry. They lack the genes and enzymes required to manufacture neurotransmitters like [[serotonin]], [[dopamine]], [[Biological functions of nitric oxide|nitric oxide]], [[octopamine]], [[noradrenaline]], and others, seen in all other animals with a nervous system, with the genes coding for the receptors for each of these neurotransmitters missing.<ref>{{cite web |first=Douglas |last=Fox |title=Aliens in our midst |url=https://aeon.co/essays/what-the-ctenophore-says-about-the-evolution-of-intelligence |date=1 August 2017 |work=[[Aeon (digital magazine)|Aeon]] |access-date=1 August 2017}}</ref> Monofunctional [[catalase]] (CAT), one of the three major families of antioxidant enzymes that target [[hydrogen peroxide]], an important signaling molecule for synaptic and neuronal activity, is also absent, most likely due to gene loss.<ref>{{Cite journal |last1=Hewitt |first1=Olivia H. |last2=Degnan |first2=Sandie M. |date=2023-02-13 |title=Antioxidant enzymes that target hydrogen peroxide are conserved across the animal kingdom, from sponges to mammals |journal=Scientific Reports |volume=13 |issue=1 |page=2510 |doi=10.1038/s41598-023-29304-6 |pmid=36781921 |pmc=9925728 |bibcode=2023NatSR..13.2510H |s2cid=256811787}}</ref> They use [[L-glutamate]] as a [[neurotransmitter]], and have a distinctively high number of ionotropic glutamate receptors and genes for glutamate synthesis and transport.<ref name="Norekian">{{cite journal |last1=Norekian |first1=Tigran P. |last2=Moroz |first2=Leonid L. |title=Neural system and receptor diversity in the ctenophore Beroe abyssicola |journal=[[Journal of Comparative Neurology]] |date=15 August 2019 |volume=527 |issue=12 |pages=1986–2008 |doi=10.1002/cne.24633 |pmid=30632608 |doi-access=free }}</ref> The genomic content of the nervous system is the smallest of any animal, and could represent the minimum genetic requirements for a functional nervous system.<ref>{{cite book |doi=10.1093/acprof:oso/9780199682201.003.0006 |chapter=Ctenophora |title=Structure and Evolution of Invertebrate Nervous Systems |year=2015 |last1=Simmons |first1=David K. |last2=Martindale |first2=Mark Q. |pages=48–55 |publisher=[[Oxford University Press]] |isbn=978-0-19-968220-1 }}</ref> The presence of directly fused neurons without synapses suggests that ctenophores might form a sister group to other metazoans, having developed a nervous system independently.<ref name="Burkhardt2023"/> If so, nervous systems may have either been lost in sponges and placozoans, or arisen more than once among metazoans.<ref name="Jékely2015">{{cite journal |last1=Jákely |first1=Gáspár |last2=Paps |first2=Jordi |last3=Nielsen |first3=Claus |year=2015 |title=The phylogenetic position of ctenophores and the origin(s) of nervous systems |journal=[[EvoDevo]] |volume=6 |issue=1 |page=1 |doi=10.1186/2041-9139-6-1 |pmid=25905000 |pmc=4406211 |doi-access=free }}</ref>