Editing Ctenophora (section)

== Taxonomy ==

The number of known living ctenophore species is uncertain since many of those named and formally described have turned out to be identical to species known under other scientific names. Claudia Mills estimates that there about 100–150&nbsp;valid species that are not duplicates, and that at least another&nbsp;25, mostly deep-sea forms, have been recognized as distinct but not yet analyzed in enough detail to support a formal description and naming.<ref name=MillsCtenoList/>

=== Early classification ===

Early writers combined ctenophores with [[cnidarians]] into a single phylum called [[Coelenterata]] on account of morphological similarities between the two groups. Like cnidarians, the bodies of ctenophores consist of a mass of jelly, with one layer of [[cell (biology)|cells]] on the outside and another lining the internal cavity. In ctenophores, however, these layers are two cells deep, while those in cnidarians are only a single cell deep. Ctenophores also resemble cnidarians in relying on water flow through the body cavity for both digestion and respiration, as well as in having a decentralized [[nerve net]] rather than a brain.<ref name="Dunn2015">{{cite journal |last1=Dunn |first1=Casey W. |last2=Leys |first2=Sally P. |last3=Haddock |first3=Steven H.D. |author3-link=Steven Haddock |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 |doi-access=free |pmid=25840473 |bibcode=2015TEcoE..30..282D }}</ref> Genomic studies have suggested that the [[neuron]]s of Ctenophora, which differ in many ways from other animal neurons, evolved independently from those of the other animals.<ref>{{cite news |title=Comb jelly nseurons spark evolution debate |magazine=[[Quanta Magazine]] |date=2015-03-25 |url=https://www.quantamagazine.org/20150325-did-neurons-evolve-twice/ |access-date=2015-06-12}}</ref>

===Modern taxonomy===

[[File:Lobate ctenophore.jpg|thumb| [[Lobata]] sp., with paired thick lobes]]

The traditional classification divides ctenophores into two [[class (biology)|classes]], those with tentacles ([[Tentaculata]]) and those without ([[Nuda]]). The Nuda contains only one [[order (biology)|order]] ([[Beroida]]) and [[family (biology)|family]] ([[Beroidae]]), and two [[genus|genera]], ''Beroe'' (several species) and ''[[Neis]]'' (one species).<ref name=MillsCtenoList/>

The [[Tentaculata]] are divided into the following eight [[order (biology)|orders]]:<ref name=MillsCtenoList/>

* [[Cydippida]], egg-shaped animals with long tentacles<ref name=RuppertBarnes2004Ctenophora/>
* [[Lobata]], with paired thick lobes<ref name=RuppertBarnes2004Ctenophora/>
* [[Platyctenida]], flattened animals that live on or near the sea-bed; most lack combs as adults, and use their [[pharynx|pharynges]] as suckers to attach themselves to surfaces<ref name=RuppertBarnes2004Ctenophora/>
* [[Ganeshida]], with a pair of small lobes round the mouth, but an extended [[pharynx]] like that of platyctenids<ref name=RuppertBarnes2004Ctenophora/>
* [[Cambojiida]]
* [[Cryptolobiferida]]
* [[Thalassocalycida]], with short tentacles and a jellyfish-like "umbrella"<ref name=RuppertBarnes2004Ctenophora/>
* [[Cestida]], ribbon-shaped and the largest ctenophores<ref name=RuppertBarnes2004Ctenophora/>