Bilateria

Bilateria (/ˌbaɪləˈtɪəriə/)[1] is a large clade of animals characterised by bilateral symmetry during embryonic development. This means their body plans are laid around a longitudinal axis with a front (or "head") and a rear (or "tail") end, as well as a left–right–symmetrical belly (ventral) and back (dorsal) surface. Nearly all bilaterians maintain a bilaterally symmetrical body as adults; the most notable exception is the echinoderms, which have pentaradial symmetry as adults, but bilateral symmetry as embryos. With few exceptions, bilaterian embryos are triploblastic, having three germ layers: endoderm, mesoderm and ectoderm, and have complete digestive tracts with a separate mouth and anus. Some bilaterians lack body cavities, while others have a primary body cavity derived from the blastocoel, or a secondary cavity, the coelom. Cephalization is a characteristic feature among most bilaterians, where the sense organs and central nerve ganglia become concentrated at the front end of the animal.

Bilaterians constitute one of the five main lineages of animals, the other four being Porifera (sponges), Cnidaria (jellyfish, hydrozoans, sea anemones and corals), Ctenophora (comb jellies) and Placozoa. They rapidly diversified in the late Ediacaran and the Cambrian, and are now by far the most successful animal lineage, with over 98% of known animal species.[2] Bilaterians are traditionally classified as either deuterostomes or protostomes, on the basis of whether the blastopore becomes the anus or mouth.[3] The phylum Xenacoelomorpha, once thought to be flatworms, was erected in 2011, and has provided an extra challenge to bilaterian taxonomy, as they likely do not belong to either group.[4]

## Contents

* 1 Body plan
* 2 Evolution
* 2.1 Common ancestor
* 2.2 Fossil record
* 2.3 Phylogeny
* 3 Taxonomic history
* 4 See also
* 5 References
* 6 External links

## Body plan

Animals with a bilaterally symmetric body plan that mainly move in one direction have a head end (anterior) and a tail (posterior) end as well as a back (dorsal) and a belly (ventral); therefore they also have a left side and a right side.[5][6] Having a front end means that this part of the body encounters stimuli, such as food, favouring cephalisation, the development of a head with sense organs and a mouth.[7] Most bilaterians (nephrozoans) have a gut that extends through the body from mouth to anus (sometimes called a "through gut"[8]), and sometimes a wormlike body plan with a hydrostatic skeleton. Xenacoelomorphs, on the other hand, have a bag gut with one opening. Many bilaterian phyla have primary larvae which swim with cilia and have an apical organ containing sensory cells.[5][6]

Some bilaterians have only weakly condensed nerve nets (similar to those in cnidarians), while others have either a ventral nerve cord, a dorsal nerve cord, or both (e.g. in Hemichordate).[9]

## Evolution

### Common ancestor

Main article: Biology:Urbilaterian

The hypothetical most recent common ancestor of all Bilateria is termed the 'urbilaterian'. The nature of this first bilaterian is a matter of debate. One side suggests that acoelomates gave rise to the other groups (planuloid–aceloid hypothesis by Ludwig von Graff, Elie Metchnikoff, Libbie Hyman, or Luitfried von Salvini-Plawen (nl)). This means that the urbilaterian had a solid body, and all body cavities therefore secondarily arose later in different groups. The other side poses that the urbilaterian had a coelom, meaning that the main acoelomate phyla (flatworms and gastrotrichs) have secondarily lost their body cavities.[10][11] This is the Archicoelomata hypothesis first proposed by A. T. Masterman in 1899.[12] Variations of the Archicoelomata hypothesis are the Gastraea by Ernst Haeckel in 1872[13] or Adam Sedgwick, and more recently the Bilaterogastrea by Gösta Jägersten [sv; sv],[14] and the Trochaea by Claus Nielsen.[15]

One view is that the original bilaterian was a marine worm somewhat like Xenoturbella.

One proposal, by Johanna Taylor Cannon and colleagues, is that the original bilaterian was a bottom dwelling worm with a single body opening, similar to Xenoturbella.[16] An alternative proposal, by Jaume Baguñà and colleagues, is that it may have resembled the planula larvae of some cnidarians, which unlike the radially-symmetric adults have some bilateral symmetry.[17] However, Lewis I. Held presents evidence that it was segmented, as the mechanism for creating segments is shared between vertebrates (deuterostomes) and arthropods (protostomes).[18]

Bilaterians, presumably including the urbilaterian, share many more Hox genes controlling the development of their more complex bodies, including of their heads, than do the Cnidaria and the Acoelomorpha.[19]

### Fossil record

Ikaria wariootia, living 571–539 million years ago, is one of the oldest bilaterians identified.[20]

The first evidence of Bilateria in the fossil record comes from trace fossils in Ediacaran sediments, and the first bona fide bilaterian fossil is Kimberella, dating to 555 million years ago.[21] Earlier fossils are controversial; the fossil Vernanimalcula may be the earliest known bilaterian, but may also represent an infilled bubble.[22][23] Fossil embryos are known from around the time of Vernanimalcula (580 million years ago), but none of these have bilaterian affinities.[24] Burrows believed to have been created by bilaterian life forms have been found in the Tacuarí Formation of Uruguay, and were believed to be at least 585 million years old.[25] However, more recent evidence shows these fossils are actually late Paleozoic, not Ediacaran.[26]

### Phylogeny

The Bilateria are now by far the most successful animal lineage, with over 98% of known animal species.[2] The group has traditionally been divided into two main lineages or superphyla.[27] The deuterostomes traditionally include the echinoderms, hemichordates, chordates, and the extinct Vetulicolia. The protostomes include most of the rest, such as arthropods, annelids, molluscs, and flatworms. There are several differences, most notably in how the embryo develops. In particular, the first opening of the embryo becomes the mouth in protostomes, and the anus in deuterostomes. Many taxonomists now recognise at least two more superphyla among the protostomes, Ecdysozoa[28] and Spiralia.[28][29][30] The arrow worms (Chaetognatha) have proven difficult to classify. Studies published in 2004 and 2017 place them in the Gnathifera.[31][32][33]

The traditional division of Bilateria into Deuterostomia and Protostomia was challenged when new morphological and molecular evidence supported a sister relationship between the acoelomate taxa, Acoela and Nemertodermatida (together called Acoelomorpha), and the remaining bilaterians.[27][3][34] The latter clade was called Nephrozoa by Jondelius et al. (2002) and Eubilateria by Baguña and Riutort (2004).[27] The acoelomorph taxa had previously been considered flatworms with secondarily lost characteristics, but the new relationship suggested that the simple acoelomate worm form was the original bilaterian body plan and that the coelom, the digestive tract, excretory organs, and nerve cords developed in the Nephrozoa.[27][35] Subsequently, the acoelomorphs were placed in phylum Xenacoelomorpha, together with the xenoturbellids, and the sister relationship between Xenacoelomorpha and Nephrozoa supported in phylogenomic analyses.[35]

A cladogram for Bilateria under the Nephrozoa hypothesis from a 2014 review by Casey Dunn and colleagues, is shown below.[36] The cladogram indicates approximately when some clades radiated into newer clades, in millions of years ago (Mya).[37]

Bilateria |  | Xenacoelomorpha |  |  |

Xenoturbellida 70 px

|

Acoelomorpha 60 px

Nephrozoa |  | Deuterostomia  |  |  |

Ambulacraria 60 px

|

Chordata 60 px

anus before mouth
Protostomia  |  |  |

Ecdysozoa 60 px

|

Spiralia 60 px

610 mya
650 Mya

A different hypothesis is that Ambulacraria is sister to Xenacoelomorpha, together forming Xenambulacraria. Xenambulacraria may be sister to Chordata or to Centroneuralia (corresponding to Nephrozoa without Ambulacraria, or, as shown here, to Chordata + Protostomia).[38] A 2019 study by Hervé Philippe and colleagues presents the tree, cautioning that "the support values are very low, meaning there is no solid evidence to refute the traditional protostome and deuterostome dichotomy".[39] As of 2024, the issue of which hypothesis is correct remains unresolved.[40][41]

Cladogram showing Xenambulacraria hypothesis with a paraphyletic Deuterostomia:[42]

Bilateria |  | Xenambulacraria  |  |  |

Xenacoelomorpha 70 px

|

Ambulacraria 60 px

Centroneuralia  |  |  |

Chordata 60 px

|

Protostomia  |  |  |

Ecdysozoa 60 px

|

Spiralia 60 px

Cladogram showing hypothesis of Xenambulacraria within a monophyletic Deuterostomia:[42]

Bilateria |  | Deuterostomia  |  | Xenambulacraria  |  |  |

Xenacoelomorpha 70 px

|

Ambulacraria 60 px

Chordata 60 px

Protostomia  |  |  |

Ecdysozoa 60 px

|

Spiralia 60 px

## Taxonomic history

The Bilateria were named by the Austrian embryologist Berthold Hatschek in 1888. In his classification, the group included the Zygoneura, Ambulacraria, and Chordonii (the Chordata).[43][44] In 1910, the Austrian zoologist Karl Grobben renamed the Zygoneura to Protostomia, and created the Deuterostomia to encompass the Ambulacraria and Chordonii.[43][45]

## See also

* Embryological origins of the mouth and anus

## References

1. ↑ "Bilateria". Merriam-Webster Dictionary. https://www.merriam-webster.com/dictionary/bilateria. Retrieved 2024-05-12.
2. ↑ 2.0 2.1 "Animalia". https://www.catalogueoflife.org/.
3. ↑ 3.0 3.1 Hejnol, A.; Martindale, M. Q. (2009). "The mouth, the anus, and the blastopore - open questions about questionable openings". in M. J. Telford. Animal Evolution — Genomes, Fossils, and Trees. pp. 33–40. https://www.researchgate.net/publication/230766195.
4. ↑ Philippe, H.; Brinkmann, H.; Copley, R. R.; Moroz, L. L.; Nakano, H.; Poustka, A. J.; Wallberg, A.; Peterson, K. J. et al. (10 February 2011). "Acoelomorph flatworms are deuterostomes related to Xenoturbella". Nature 470 (7333): 255–258. doi:10.1038/nature09676. PMID 21307940. Bibcode: 2011Natur.470..255P.
5. ↑ 5.0 5.1 Minelli, Alessandro (2009). Perspectives in Animal Phylogeny and Evolution. Oxford University Press. p. 53. ISBN 978-0-19-856620-5. https://books.google.com/books?id=jIASDAAAQBAJ&pg=PA53.
6. ↑ 6.0 6.1 Brusca, Richard C. (2016). "Introduction to the Bilateria and the Phylum Xenacoelomorpha: Triploblasty and Bilateral Symmetry Provide New Avenues for Animal Radiation". Invertebrates. Sinauer Associates. pp. 345–372. ISBN 978-1-60535-375-3. http://www.sinauer.com/media/wysiwyg/samples/Brusca3e_Chapter_9.pdf.
7. ↑ Finnerty, John R. (November 2005). "Did internal transport, rather than directed locomotion, favor the evolution of bilateral symmetry in animals?". BioEssays 27 (11): 1174–1180. doi:10.1002/bies.20299. PMID 16237677. http://faculty.weber.edu/rmeyers/PDFs/Finnerty - symmetry evol.pdf. Retrieved 2018-03-07.
8. ↑ Nielsen, Claus; Brunet, Thibaut; Arendt, Detlev (2018-08-22). "Evolution of the bilaterian mouth and anus" (in en). Nature Ecology & Evolution 2 (9): 1358–1376. doi:10.1038/s41559-018-0641-0. ISSN 2397-334X. PMID 30135501. Bibcode: 2018NatEE...2.1358N. https://www.nature.com/articles/s41559-018-0641-0.
9. ↑ Martín-Durán, J. M.; Pang, K.; Børve, A.; Semmler Lê, H.; Furu, A.; Cannon, J. T.; Jondelius, U.; Hejnol, A. (2017). "Convergent evolution of bilaterian nerve cords". Nature 553 (7686): 45–50. doi:10.1038/nature25030. PMID 29236686.
10. ↑ Knoll, Andrew H.; Carroll, Sean B. (25 June 1999). "Early Animal Evolution: Emerging Views from Comparative Biology and Geology". Science 284 (5423): 2129–2137. doi:10.1126/science.284.5423.2129. PMID 10381872.
11. ↑ Balavoine, G.; Adoutte, Andre (2003). "The segmented Urbilateria: A testable scenario". Integrative and Comparative Biology 43 (1): 137–147. doi:10.1093/icb/43.1.137. PMID 21680418.
12. ↑ Masterman, A. T. (1899). "On the Theory of Archimeric Segmentation and its bearing upon the Phyletic Classification of the Cœlomata". Proceedings of the Royal Society of Edinburgh 22: 270–310. doi:10.1017/S0370164600051245.
13. ↑ Levit, Georgy S.; Hoßfeld, Uwe; Naumann, Benjamin; Lukas, Paul; Olsson, Lennart (2022). "The biogenetic law and the Gastraea theory: From Ernst Haeckel's discoveries to contemporary views". Journal of Experimental Zoology Part B: Molecular and Developmental Evolution 338 (1–2): 13–27. doi:10.1002/jez.b.23039. PMID 33724681. Bibcode: 2022JEZB..338...13L.
14. ↑ Olsson, Lennart (2007). "A clash of traditions: the history of comparative and experimental embryology in Sweden as exemplified by the research of Gösta Jägersten and Sven Hörstadius". Theory in Biosciences 126 (4): 117–129. doi:10.1007/s12064-007-0008-6. PMID 18008099.
15. ↑ Nielsen, Claus (1985). "Animal phylogeny in the light of the trochaea theory". Biological Journal of the Linnean Society 25 (3): 243–299. doi:10.1111/j.1095-8312.1985.tb00396.x.
16. ↑ Cannon, Johanna Taylor; Vellutini, Bruno Cossermelli; Smith, Julian; Ronquist, Fredrik; Jondelius, Ulf; Hejnol, Andreas (2016). "Xenacoelomorpha is the sister group to Nephrozoa". Nature 530 (7588): 89–93. doi:10.1038/nature16520. PMID 26842059. Bibcode: 2016Natur.530...89C. http://urn.kb.se/resolve?urn=urn:nbn:se:nrm:diva-1844.
17. ↑ Baguñà, Jaume; Martinez, Pere; Paps, Jordi; Riutort, Marta (April 2008). "Back in time: a new systematic proposal for the Bilateria". Philosophical Transactions of the Royal Society B: Biological Sciences 363 (1496): 1481–1491. doi:10.1098/rstb.2007.2238. PMID 18192186.
18. ↑ Held, Lewis I. (2014). How the Snake Lost its Legs. Curious Tales from the Frontier of Evo-Devo. Cambridge University Press. p. 11. ISBN 978-1-107-62139-8.
19. ↑ Hombría, James C.-G.; García-Ferrés, Mar; Sánchez-Higueras, Carlos (5 August 2021). "Anterior Hox Genes and the Process of Cephalization". Frontiers in Cell and Developmental Biology 9. doi:10.3389/fcell.2021.718175. PMID 34422836.
20. ↑ Evans, Scott D.; Hughes, Ian V.; Gehling, James G.; Droser, Mary L. (7 April 2020). "Discovery of the oldest bilaterian from the Ediacaran of South Australia". Proceedings of the National Academy of Sciences 117 (14): 7845–7850. doi:10.1073/pnas.2001045117. PMID 32205432. Bibcode: 2020PNAS..117.7845E.
21. ↑ Fedonkin, M. A.; Waggoner, B. M. (November 1997). "The Late Precambrian fossil Kimberella is a mollusc-like bilaterian organism". Nature 388 (6645): 868–871. doi:10.1038/42242. Bibcode: 1997Natur.388..868F.
22. ↑ Bengtson, S.; Budd, G. (19 November 2004). "Comment on 'small bilaterian fossils from 40 to 55 million years before the Cambrian'.". Science 306 (5700): 1291a. doi:10.1126/science.1101338. PMID 15550644.
23. ↑ Bengtson, S.; Donoghue, P. C. J.; Cunningham, J. A.; Yin, C. (2012). "A merciful death for the 'earliest bilaterian,' Vernanimalcula". Evolution & Development 14 (5): 421–427. doi:10.1111/j.1525-142X.2012.00562.x. PMID 22947315. http://urn.kb.se/resolve?urn=urn:nbn:se:nrm:diva-597.
24. ↑ Hagadorn, J. W.; Xiao, S.; Donoghue, P. C. J.; Bengtson, S.; Gostling, N. J.; Pawlowska, M.; Raff, E. C.; Raff, R. A. et al. (13 October 2006). "Cellular and Subcellular Structure of Neoproterozoic Animal Embryos". Science 314 (5797): 291–294. doi:10.1126/science.1133129. PMID 17038620. Bibcode: 2006Sci...314..291H.
25. ↑ Pecoits, E.; Konhauser, K. O.; Aubet, N. R.; Heaman, L. M.; Veroslavsky, G.; Stern, R. A.; Gingras, M. K. (June 29, 2012). "Bilaterian burrows and grazing behavior at >585 million years ago". Science 336 (6089): 1693–1696. doi:10.1126/science.1216295. PMID 22745427. Bibcode: 2012Sci...336.1693P.
26. ↑ Verde, Mariano (15 September 2022). "Revisiting the supposed oldest bilaterian trace fossils from Uruguay: Late Paleozoic, not Ediacaran". Palaeogeography, Palaeoclimatology, Palaeoecology 602. doi:10.1016/j.palaeo.2022.111158. Bibcode: 2022PPP...60211158V. https://www.sciencedirect.com/science/article/abs/pii/S0031018222003285.
27. ↑ 27.0 27.1 27.2 27.3 Nielsen, Claus (2008). "Six major steps in animal evolution: are we derived sponge larvae?". Evolution and Development 10 (2): 241–257. doi:10.1111/j.1525-142X.2008.00231.x. PMID 18315817.
28. ↑ 28.0 28.1 Halanych, K.; Bacheller, J.; Aguinaldo, A.; Liva, S.; Hillis, D.; Lake, J. (17 March 1995). "Evidence from 18S ribosomal DNA that the lophophorates are protostome animals". Science 267 (5204): 1641–1643. doi:10.1126/science.7886451. PMID 7886451. Bibcode: 1995Sci...267.1641H.
29. ↑ Paps, J.; Baguna, J.; Riutort, M. (14 July 2009). "Bilaterian phylogeny: a broad sampling of 13 nuclear genes provides a new Lophotrochozoa phylogeny and supports a paraphyletic basal Acoelomorpha". Molecular Biology and Evolution 26 (10): 2397–2406. doi:10.1093/molbev/msp150. PMID 19602542.
30. ↑ Telford, Maximilian J. (15 April 2008). "Resolving animal phylogeny: A sledgehammer for a tough nut?". Developmental Cell 14 (4): 457–459. doi:10.1016/j.devcel.2008.03.016. PMID 18410719.
31. ↑ Helfenbein, Kevin G.; Fourcade, H. Matthew; Vanjani, Rohit G.; Boore, Jeffrey L. (20 July 2004). "The mitochondrial genome of Paraspadella gotoi is highly reduced and reveals that chaetognaths are a sister group to protostomes". Proceedings of the National Academy of Sciences of the United States of America 101 (29): 10639–10643. doi:10.1073/pnas.0400941101. PMID 15249679. Bibcode: 2004PNAS..10110639H.
32. ↑ Papillon, Daniel; Perez, Yvan; Caubit, Xavier; Yannick Le, Parco (November 2004). "Identification of chaetognaths as protostomes is supported by the analysis of their mitochondrial genome". Molecular Biology and Evolution 21 (11): 2122–2129. doi:10.1093/molbev/msh229. PMID 15306659.
33. ↑ Fröbius, Andreas C.; Funch, Peter (2017-04-04). "Rotiferan Hox genes give new insights into the evolution of metazoan bodyplans". Nature Communications 8 (1): 9. doi:10.1038/s41467-017-00020-w. PMID 28377584. Bibcode: 2017NatCo...8....9F.
34. ↑ Martín-Durán, José M.; Passamaneck, Yale J.; Martindale, Mark Q.; Hejnol, Andreas (2016). "The developmental basis for the recurrent evolution of deuterostomy and protostomy". Nature Ecology & Evolution 1 (1): 0005. doi:10.1038/s41559-016-0005. PMID 28812551. https://qmro.qmul.ac.uk/xmlui/handle/123456789/54816.
35. ↑ 35.0 35.1 Cannon, Johanna Taylor; Vellutini, Bruno Cossermelli; Smith, Julian; Ronquist, Fredrik; Jondelius, Ulf; Hejnol, Andreas (2016). "Xenacoelomorpha is the sister group to Nephrozoa". Nature 530 (7588): 89–93. doi:10.1038/nature16520. PMID 26842059. Bibcode: 2016Natur.530...89C. http://urn.kb.se/resolve?urn=urn:nbn:se:nrm:diva-1844.
36. ↑ Dunn, Casey W.; Giribet, Gonzalo; Edgecombe, Gregory D.; Hejnol, Andreas (23 November 2014). "Animal Phylogeny and Its Evolutionary Implications". Annual Review of Ecology, Evolution, and Systematics 45 (1): 371–395. doi:10.1146/annurev-ecolsys-120213-091627.
37. ↑ Peterson, Kevin J.; Cotton, James A.; Gehling, James G.; Pisani, Davide (2008-04-27). "The Ediacaran emergence of bilaterians: congruence between the genetic and the geological fossil records". Philosophical Transactions of the Royal Society of London B: Biological Sciences 363 (1496): 1435–1443. doi:10.1098/rstb.2007.2233. PMID 18192191.
38. ↑ Kapli, Paschalia; Natsidis, Paschalis; Leite, Daniel J.; Fursman, Maximilian; Jeffrie, Nadia et al. (2021-03-19). "Lack of support for Deuterostomia prompts reinterpretation of the first Bilateria". Science Advances 7 (12). doi:10.1126/sciadv.abe2741. PMID 33741592. Bibcode: 2021SciA....7.2741K.
39. ↑ Philippe, Hervé; Poustka, Albert J.; Chiodin, Marta; Hoff, Katharina J.; Dessimoz, Christophe et al. (2019). "Mitigating Anticipated Effects of Systematic Errors Supports Sister-Group Relationship between Xenacoelomorpha and Ambulacraria". Current Biology 29 (11): 1818–1826.e6. doi:10.1016/j.cub.2019.04.009. PMID 31104936. Bibcode: 2019CBio...29E1818P.
40. ↑ Abalde, Samuel; Jondelius, Ulf (2025-02-10). Whelan, Nathan. ed. "A Phylogenomic Backbone for Acoelomorpha Inferred From Transcriptomic Data". Systematic Biology 74 (1): 70–85. doi:10.1093/sysbio/syae057. ISSN 1063-5157. PMID 39451056. PMC 11809588. https://academic.oup.com/sysbio/article/74/1/70/7841810.
41. ↑ Robertson, Helen E.; Sebé-Pedrós, Arnau; Saudemont, Baptiste; Loe-Mie, Yann; Zakrzewski, Anne-C.; Grau-Bové, Xavier; Mailhe, Marie-Pierre; Schiffer, Philipp et al. (2024-03-19). "Single cell atlas of Xenoturbella bocki highlights limited cell-type complexity". Nature Communications 15 (1): 2469. doi:10.1038/s41467-024-45956-y. ISSN 2041-1723. PMID 38503762. Bibcode: 2024NatCo..15.2469R.
42. ↑ 42.0 42.1 Mulhair, Peter O.; McCarthy, Charley G.P.; Siu-Ting, Karen; Creevey, Christopher J.; O'Connell, Mary J. (December 2022). "Filtering artifactual signal increases support for Xenacoelomorpha and Ambulacraria sister relationship in the animal tree of life". Current Biology 32 (23): 5180–5188.e3. doi:10.1016/j.cub.2022.10.036. PMID 36356574. Bibcode: 2022CBio...32E5180M. https://linkinghub.elsevier.com/retrieve/pii/S0960982222016840.
43. ↑ 43.0 43.1 Nielsen, Claus (2009). "The 'new phylogeny'. What is new about it?". Annual Review of Ecology, Evolution, and Systematics 35: 229–256. https://www.palaeodiversity.org/pdf/03Suppl/Supplement_Nielsen.pdf.
44. ↑ Hatschek, Berthold (1888) (in de). Lehrbuch der Zoologie (1st ed.). Jena: Gustav Fischer. pp. 1–144.
45. ↑ Grobben, Karl; Claus, Carl Friedrich Wilhelm (1910). Lehrbuch der Zoologie (2nd ed.). Marburg: Elvert'sche Verlagsbuchhandlung.

## External links

* Tree of Life web project — Bilateria
* University of California Museum of Paleontology — Systematics of the Metazoa

* v
* t
* e

Extant Animal phyla

Domain
Archaea
Bacteria
Eukaryota
(Supergroup
Plant
Hacrobia
Heterokont
Alveolata
Rhizaria
Excavata
Amoebozoa
Opisthokonta

Animal
Fungi)

| Animalia|

* Porifera (sponges)

Diploblasts
(Eumetazoa)|

* Ctenophora (comb jellies)

ParaHoxozoa
(Planulozoa)|

| P+C|

* Placozoa (Trichoplax)
* Cnidaria (jellyfish and relatives)

Bilateria
(Triploblasts)|

* Xenacoelomorpha
* Xenoturbellida (Xenoturbella)
* Acoelomorpha
* acoels
* nemertodermatids

Nephrozoa|

* (see below↓)

Nephrozoa

| Deuterostomia|

* Chordata
* lancelets
* tunicates
* Vertebrata/Craniata

Ambulacraria|

* Echinodermata (starfish and relatives)
* Hemichordata
* acorn worms
* pterobranchs

Protostomia|

| Ecdysozoa|

| Scalidophora|

* Kinorhyncha (mud dragons)
* Priapulida (penis worms)

|

N+L+P|

| Nematoida|

* Nematoda (roundworms)
* Nematomorpha (horsehair worms)

|

L+P|

* Loricifera (corset animals)

Panarthropoda|

* Onychophora (velvet worms)

Tactopoda|

* Arthropoda (arthropods)
* Tardigrada (waterbears)

Spiralia|

| Gnathifera|

* Chaetognatha (arrow worms)
* Gnathostomulida (jaw worms)

M+S|

* Micrognathozoa (Limnognathia)
* Syndermata
* Rotifera (wheel animals)
* Acanthocephala (thorny-headed worms)

Platytrochozoa|

| R+M|

| Mesozoa|

* Orthonectida
* Dicyemida or Rhombozoa
* Monoblastozoa (Salinella)†

|

Rouphozoa|

* Platyhelminthes (flatworms)
* Gastrotricha (hairybacks)

Lophotrochozoa|

* Cycliophora (Symbion)
* Annelida (ringed worms)
* Sipuncula ‡

M+K|

* Mollusca (molluscs)

Kryptotrochozoa|

* Nemertea (ribbon worms)

Lophophorata|

| Bryozoa s.l.|

* Entoprocta or Kamptozoa
* Ectoprocta (moss animals)

|

Brachiozoa|

* Brachiopoda (lamp shells)
* Phoronida (horseshoe worms)

* * *

| Major groups
within phyla|

* Sponges
* Hexactinellid
* Demosponge
* Calcareous
* Homoscleromorpha
* Cnidarians
* Anthozoa inc. corals
* Hexacorallia
* Octocorallia
* Medusozoa inc. jellyfish
* Myxozoa
* Vertebrates
* Jawless fish
* Cartilaginous fish
* Bony fish
* Amphibians
* Reptiles
* Birds
* Mammals
* Echinoderms
* Sea lilies
* Asterozoa inc. starfish
* Echinozoa
* Nematodes
* Chromadorea
* Enoplea
* Secernentea
* Arthropods
* Chelicerates/Arachnids
* Myriapods
* Crustaceans
* Hexapods/Insects
* Platyhelminths
* Turbellaria
* Trematoda
* Monogenea
* Cestoda
* Bryozoans
* Phylactolaemata
* Stenolaemata
* Gymnolaemata
* Annelids
* Polychaetes
* Clitellata
* Echiura
* Sipuncula
* Molluscs
* Gastropods
* Cephalopods
* Bivalves
* Chitons
* Tusk shells

|

Phyla with ≥1000 extant species bolded
Potentially dubious phyla †
Disputed phyla ‡
See also
Diploblasts

Wikidata ☰ Q5173 entry

0.00

(0 votes)

Original source: https://en.wikipedia.org/wiki/Bilateria. Read more

|

Retrieved from "https://handwiki.org/wiki/index.php?title=Biology:Bilateria&oldid=4255716"
*[v]: View this template
*[t]: Discuss this template
*[e]: Edit this template