coelenterata
{{Short description|Term encompassing animal phyla Cnidaria and Ctenophora}}
{{automatic_Taxobox
| name = Coelenterates
| image = Comb_jellies-mba.jpg
| image_caption = Comb jellies (Beroe spp.)
| taxon = Coelenterata
| authority = Hatschek, 1888
| subdivision_ranks = Phyla
| subdivision = *Ctenophora
- Cnidaria
- Placozoa
- {{extinct}}Trilobozoa(?)Ivantsov, A. Yu. (2021). "Proarticulates—an extinct phylum of soft-bodied metazoans, or a group of vendobionts par excellence?" Symmetry. 13(2): 160. [https://www.mdpi.com/2073-8994/13/2/160 Full text]. doi:10.3390/sym13020160.
- {{extinct}}Dipleurozoa
}}
Coelenterata is a term encompassing the animal phyla Cnidaria (corals, true jellies, sea anemones, sea pens, and their relatives) and Ctenophora (comb jellies). The name comes {{etymology|grc|{{wikt-lang|grc|κοῖλος}} ({{grc-transl|κοῖλος}})|hollow||{{wikt-lang|grc|ἔντερον}} ({{grc-transl|ἔντερον}})|intestine}}, referring to the hollow body cavity common to these two phyla.{{Cite OED|coelenterate}}{{cite Merriam-Webster|coelenterate|access-date=2017-02-25}} They have very simple tissue organization, with only two layers of cells (ectoderm and endoderm), along with a middle undifferentiated layer called the mesoglea, and radial symmetry. Coelenterata lack a specialized circulatory system, relying instead on diffusion across the tissue layers.
Characteristics
All coelenterates are aquatic, mostly marine, animals. The body form is radially symmetrical, diploblastic and does not have a coelom. The body has a single opening, the {{wikt-lang|en|hypostome}}, surrounded by sensory tentacles equipped with either nematocysts or colloblasts to capture mostly planktonic prey. These tentacles are surrounded by a spacious cavity called the gastrovascular cavity, or coelenteron. Digestion is both intracellular and extracellular. Respiration and excretion are accomplished by simple diffusion. A nerve net is spread throughout the body. Many Cnidaria exhibit polymorphism, wherein different types of individuals are present in a colony for different functions. These individuals are called zooids. These animals generally reproduce asexually by budding, though sexual reproduction does occur in some groups.
History of classification
The scientific validity of the term coelenterate is currently rejected, as the Cnidaria and Ctenophora have less in common than previously assumed.{{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 |doi-access=free |pmid=25840473 |bibcode=2015TEcoE..30..282D }} Coelentera may only be monophyletic if both Placozoa and Bilateria are included.{{Cite journal |last1=Pisani |first1=Davide |last2=Pett |first2=Walker |last3=Dohrmann |first3=Martin |last4=Feuda |first4=Roberto |last5=Rota-Stabelli |first5=Omar |last6=Philippe |first6=Hervé |last7=Lartillot |first7=Nicolas |last8=Wörheide |first8=Gert |date=2015-12-15 |title=Genomic data do not support comb jellies as the sister group to all other animals |journal=Proceedings of the National Academy of Sciences |language=en |volume=112 |issue=50 |pages=15402–15407 |doi=10.1073/pnas.1518127112 |issn=0027-8424 |pmc=4687580 |pmid=26621703 |bibcode=2015PNAS..11215402P |doi-access=free}} In particular, the phylogenetic position of Ctenophora is controversial; it was first considered a sub-group of coelenterata but Hyman regarded it as a separate phylum.{{Cite journal |title=Error, signal, and the placement of Ctenophora sister to all other animals |journal=Proceedings of the National Academy of Sciences |date=2015-05-05 |issn=0027-8424 |pmc=4426464 |pmid=25902535 |pages=5773–5778 |volume=112 |issue=18 |doi=10.1073/pnas.1503453112 |first1=Nathan V. |last1=Whelan |first2=Kevin M. |last2=Kocot |first3=Leonid L. |last3=Moroz |first4=Kenneth M. |last4=Halanych |bibcode=2015PNAS..112.5773W |doi-access=free}} Most researchers think that Coelenterata is not monophyletic, and therefore any group containing Cnidaria and Ctenophora but excluding other phyla would be paraphyletic.
Previously, some genomic studies have found support for monophyletic coelenterates.{{Cite journal |title=Phylogenomics Revives Traditional Views on Deep Animal Relationships |journal=Current Biology |issn=0960-9822 |pmid=19345102 |pages=706–712 |volume=19 |issue=8 |doi=10.1016/j.cub.2009.02.052 |first1=Hervé |last1=Philippe |first2=Romain |last2=Derelle |first3=Philippe |last3=Lopez |first4=Kerstin |last4=Pick |first5=Carole |last5=Borchiellini |first6=Nicole |last6=Boury-Esnault |first7=Jean |last7=Vacelet |first8=Emmanuelle |last8=Renard |first9=Evelyn |last9=Houliston |date=April 2009 |doi-access=free | bibcode=2009CBio...19..706P }}{{Cite journal |title=Deep metazoan phylogeny: When different genes tell different stories |journal=Molecular Phylogenetics and Evolution |date=2013-04-01 |pages=223–233 |volume=67 |issue=1 |doi=10.1016/j.ympev.2013.01.010 |first1=Tetyana |last1=Nosenko |first2=Fabian |last2=Schreiber |first3=Maja |last3=Adamska |first4=Marcin |last4=Adamski |first5=Michael |last5=Eitel |first6=Jörg |last6=Hammel |first7=Manuel |last7=Maldonado |first8=Werner E. G. |last8=Müller |first9=Michael |last9=Nickel |pmid=23353073 | bibcode=2013MolPE..67..223N |hdl=10261/93095 |hdl-access=free }} Despite this uncertainty, the term coelenterate is still used in informal settings to refer to the Cnidaria and Ctenophora.
Complicating the issue is the 1997 work of Lynn Margulis (revising an earlier model by Thomas Cavalier-Smith) that placed the Cnidaria and Ctenophora alone in the branch Radiata within Eumetazoa.Margulis, Lynn and Karlene V. Schwartz, 1997, Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth, W.H. Freeman & Company, {{ISBN |0-613-92338-3}} (The latter refers to all the animals except the sponges and Placozoa). Neither grouping is accepted universally;{{Cite web |url=https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?mode=Tree&id=6072&lvl=3&p=mapview&p=has_linkout&p=blast_url&p=genome_blast&lin=f&keep=1&srchmode=1&unlock |title=Taxonomy browser (Eumetazoa) |website=www.ncbi.nlm.nih.gov}} however, both are commonly encountered in taxonomic literature.
References
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