Editing Ctenophora (section)

=== 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>