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Bilateria

Bilaterians
Temporal range: EdiacaranPresent, 555–0 Ma[1]
Animal diversity October 2007.jpg
Diversity of bilaterians.
Scientific classification e
Kingdom: Animalia
Subkingdom: Eumetazoa
Clade: Bilateria
Hatschek, 1888
Synonyms

Triploblasts Lankester, 1973

The Bilateria /ˌbləˈtɪəriə/ or bilaterians, or triploblasts, are animals with bilateral symmetry, i.e., they have a head (anterior) and a tail (posterior) as well as a back (dorsal) and a belly (ventral); therefore they also have a left side and a right side.[2]

The bilateria are a major group of animals, including the majority of phyla but not sponges, cnidarians, placozoans and ctenophores. For the most part, bilateral embryos are triploblastic, having three germ layers: endoderm, mesoderm, and ectoderm. Nearly all are bilaterally symmetrical, or approximately so; the most notable exception is the echinoderms, which achieve near-radial symmetry as adults, but are bilaterally symmetrical as larvae.

Except for a few phyla (i.e. flatworms and gnathostomulids), bilaterians have complete digestive tracts with a separate mouth and anus. Some bilaterians lack body cavities (acoelomates, i.e. Platyhelminthes, Gastrotricha and Gnathostomulida), while others display primary body cavities (deriving from the blastocoel, as pseudocoeloms) or secondary cavities (that appear de novo, for example the coelom).[3][4]

Body plan

Idealised bilaterian body plan. With a cylindrical body and a direction of movement the animal has head and tail ends. Sense organs and mouth form the basis of the head. Opposed circular and longitudinal muscles enable peristaltic motion.

The typical bilaterian body can be imagined as a cylindrical form, with a gut running between two openings, the mouth and the anus. Around the gut it has an internal body cavity, a coelom or pseudocoelom.[a] Animals with this bilaterally symmetric body plan have a head ("anterior") and a tail ("posterior") as well as a back ("dorsal") and a belly ("ventral"); therefore they also have a left side and a right side.[6][7]

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.[8] The body stretches back from the head, and many bilaterians have a combination of circular muscles that constrict the body, making it longer, and an opposing set of longitudinal muscles, that shorten the body;[7] these enable soft-bodied animals with a hydrostatic skeleton to move by peristalsis.[9] They also have a gut that extends through the basically cylindrical body from mouth to anus. Many bilaterian phyla have primary larvae which swim with cilia and have an apical organ containing sensory cells. However, there are exceptions to each of these characteristics; for example, adult echinoderms are radially symmetric (unlike their larvae), and certain parasitic worms have extremely simplified body structures.[6][7]

Evolution

The hypothetical most recent common ancestor of all bilateria is termed the "Urbilaterian".[10][11] The nature of the 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)), while the other poses that the first bilaterian was a coelomate organism and the main acoelomate phyla (flatworms and gastrotrichs) have lost body cavities secondarily (the Archicoelomata hypothesis and its variations such as the Gastrea by Haeckel or Sedgwick, the Bilaterosgastrea by Gösta Jägersten (sv.), or the Trochaea by Nielsen).

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.[12] Earlier fossils are controversial; the fossil Vernanimalcula may be the earliest known bilaterian, but may also represent an infilled bubble.[13][14] Fossil embryos are known from around the time of Vernanimalcula (580 million years ago), but none of these have bilaterian affinities.[15] Burrows believed to have been created by bilaterian life forms have been found in the Tacuarí Formation of Uruguay, and are believed to be at least 585 million years old.[16]

Phylogeny

There are two main lineages, superphyla, of Bilateria. The deuterostomes include the echinoderms, hemichordates, chordates, and a few smaller phyla. The protostomes include most of the rest, such as arthropods, annelids, mollusks, flatworms, and so forth. There are a number of 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 recognize at least two more superphyla among the protostomes, Ecdysozoa[17] (molting animals) and Spiralia.[17][18][19][20] The arrow worms (Chaetognatha) have proven difficult to classify; recent studies place them in the gnathifera.[21][22][23]

A modern (2011) consensus phylogenetic tree for Bilateria is shown below, although the positions of certain clades are still controversial and the tree has changed considerably between 2000 and 2010.[24][23][25][26][27][28][29][26] It is indicated when approximately clades radiated into newer clades in millions of years ago (Mya).[30]

ParaHoxozoa


Cnidaria Cauliflour Jellyfish, Cephea cephea at Marsa Shouna, Red Sea, Egypt SCUBA.jpg



Placozoa



Bilateria

Proarticulata


Xenacoelomorpha

Xenoturbellida Xenoturbella japonica.jpg


Acoelomorpha

Nemertodermatida



Acoela Proporus sp.png




Nephrozoa
Deuterostomia
Chordata

Cephalochordata Branchiostoma lanceolatum (Pallas, 1774).jpg


Olfactores

Urochordata Tunicate komodo.jpg



Craniata (including Vertebrata) Cyprinus carpio3.jpg




Ambulacraria

Echinodermata Portugal 20140812-DSC01434 (21371237591).jpg



Hemichordata Balanoglossus by Spengel 1893.png



Cambroernida





Saccorhytus coronarius




†Vetulocystids



Vetulicolians Vetulicolia NT.jpg





Protostomia
Ecdysozoa

Nematoida

Nematoda CelegansGoldsteinLabUNC.jpg



Nematomorpha Paragordius tricuspidatus.jpeg





Loricifera Pliciloricus enigmatus.jpg


Panarthropoda

Onychophora Velvet worm.jpg


Tactopoda

Tardigrada Echiniscus L.png



Arthropoda Long nosed weevil edit.jpg






Scalidophora

Priapulida Priapulus caudatus 20150625.jpg



Kinorhyncha Pycnophyes zelinkaei.jpg



>529 mya
Spiralia
Gnathifera

Rotifera and allies Bdelloid Rotifer (cropped).jpg



Chaetognatha Chaetoblack.png



Platytrochozoa

Platyhelminthes and allies Sorocelis reticulosa.jpg


Lophotrochozoa

Mollusca Grapevinesnail 01.jpg



Annelida and allies Polychaeta (no).JPG


550 mya
580 mya


Kimberella Kimberella NT.jpg


610 mya
650 mya

680 mya

Evolutionary origin

The original bilateria are hypothesized to be a bottom dwelling worm with a single body opening.[5]

See also

Notes

  1. ^ The earliest Bilateria may have had only a single opening, and no coelom.[5])

References

  1. ^ Martin, M. W.; Grazhdankin, D. V; Bowring, S. A; Evans, D. A; Fedonkin, M. A; Kirschvink, J. L. (5 May 2000). "Age of Neoproterozoic bilatarian [sic] body and trace fossils, White Sea, Russia: implications for metazoan evolution". Science. 288: 841–5. doi:10.1126/science.288.5467.841. PMID 10797002. 
  2. ^ Brusca, Richard C. (2016). Introduction to the Bilateria and the Phylum Xenacoelomorpha | Triploblasty and Bilateral Symmetry Provide New Avenues for Animal Radiation (PDF). Invertebrates. Sinauer Associates. pp. 345–372. ISBN 978-1605353753. 
  3. ^ "Probable ancestor of cephalochordates" (PDF). PENICHEFOSSIL. Retrieved 9 January 2017. 
  4. ^ "Ediacaran fauna worms". Walter Jahn. Retrieved 9 January 2017. Suny Orange
  5. ^ a b 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. 
  6. ^ a b Minelli, Alessandro (2009). Perspectives in Animal Phylogeny and Evolution. Oxford University Press. p. 53. ISBN 978-0-19-856620-5. 
  7. ^ a b c Brusca, Richard C. (2016). Introduction to the Bilateria and the Phylum Xenacoelomorpha | Triploblasty and Bilateral Symmetry Provide New Avenues for Animal Radiation (PDF). Invertebrates. Sinauer Associates. pp. 345–372. ISBN 978-1605353753. 
  8. ^ Finnerty, John R. (2005). "Did internal transport, rather than directed locomotion, favor the evolution of bilateral symmetry in animals?" (PDF). BioEssays. 27: 1174–1180. doi:10.1002/bies.20299. PMID 16237677. 
  9. ^ Quillin, K. J. (May 1998). "Ontogenetic scaling of hydrostatic skeletons: geometric, static stress and dynamic stress scaling of the earthworm lumbricus terrestris". The Journal of Experimental Biology. 201 (12): 1871–83. PMID 9600869. 
  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. 
  12. ^ Fedonkin, M. A.; Waggoner, B. M. (Nov 1997). "The Late Precambrian fossil Kimberella is a mollusc-like bilaterian organism". Nature. 388 (6645): 868–871. Bibcode:1997Natur.388..868F. doi:10.1038/42242. Retrieved 2007-03-08. 
  13. ^ Bengtson, S.; Budd, G. (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. 
  14. ^ Bengtson, S.; Donoghue, P. C. J.; Cunningham, J. A.; Yin, C. (2012). "A merciful death for the 'earliest bilaterian,' Vernanimalcula,". Evolution & Development. 14: 421–427. doi:10.1111/j.1525-142X.2012.00562.x. PMID 22947315. 
  15. ^ Hagadorn, J. W.; Xiao, S.; Donoghue, P. C. J.; Bengtson, S.; Gostling, N. J.; Pawlowska, M.; Raff, E. C.; Raff, R. A.; Turner, F. R.; Chongyu, Y.; Zhou, C.; Yuan, X.; McFeely, M. B.; Stampanoni, M.; Nealson, K. H. (2006). "Cellular and Subcellular Structure of Neoproterozoic Animal Embryos". Science. 314 (5797): 291–294. Bibcode:2006Sci...314..291H. doi:10.1126/science.1133129. PMID 17038620. 
  16. ^ 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. 
  17. ^ a b 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. 
  18. ^ 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. 
  19. ^ 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. 
  20. ^ The Invertebrate Animals
  21. ^ 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. PMC 489987Freely accessible. PMID 15249679. 
  22. ^ 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. 
  23. ^ a b Fröbius, Andreas C.; Funch, Peter (2017-04-04). "Rotiferan Hox genes give new insights into the evolution of metazoan bodyplans". Nature Communications. 8 (1). doi:10.1038/s41467-017-00020-w. 
  24. ^ Edgecombe, Gregory D.; Giribet, Gonzalo; Dunn, Casey W.; Hejnol, Andreas; Kristensen, Reinhardt M.; Neves, Ricardo C.; Rouse, Greg W.; Worsaae, Katrine; Sørensen, Martin V. (June 2011). "Higher-level metazoan relationships: recent progress and remaining questions". Organisms, Diversity & Evolution. 11 (2): 151–172. doi:10.1007/s13127-011-0044-4. 
  25. ^ Smith, Martin R.; Ortega-Hernández, Javier (2014). "Hallucigenia's onychophoran-like claws and the case for Tactopoda". Nature. 514 (7522): 363–366. doi:10.1038/nature13576. 
  26. ^ a b "Palaeos Metazoa: Ecdysozoa". palaeos.com. Retrieved 2017-09-02. 
  27. ^ Yamasaki, Hiroshi; Fujimoto, Shinta; Miyazaki, Katsumi (June 2015). "Phylogenetic position of Loricifera inferred from nearly complete 18S and 28S rRNA gene sequences". Zoological Letters. 1: 18. doi:10.1186/s40851-015-0017-0. 
  28. ^ Nielsen, C. (2002). Animal Evolution: Interrelationships of the Living Phyla (2nd ed.). Oxford University Press. ISBN 0-19-850682-1. 
  29. ^ "Bilateria". Tree of Life Web Project. 2001. Retrieved August 11, 2014. 
  30. ^ 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. PMC 2614224Freely accessible. PMID 18192191. 

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