Bilateria

Bilateral animals
Scherengarnele ( Stenopus hispidus )
Systematics
without rank: Filozoa without rank: Choanozoa without rank: Multicellular animals (Metazoa) without rank: Epitheliozoa without rank: Tissue animals (Eumetazoa) without rank: Bilateral animals
Scientific name
Bilateria
Haeckel , 1874

The Bilateria are the bilaterally symmetrically built three-cotyledon tissue animals (Eumetazoa). The scientific name of the taxon is derived from the Latin bis and latus and means "two-sided" or "two-sided animals".

term

In 1841, the German geoscientist Heinrich Georg Bronn wrote that the body shape of many animals could be simplified to a kind of wedge. Twenty-one years later, the Canadian geoscientist John William Dawson introduced the word bilaterata to denote the group of animals with bilaterally symmetrical physique. In the following decade, the German zoologist Ernst Haeckel finally coined the term Bilateria for these animals . He also put the word Sphenota as a synonym for the term . The latter word coining should mean "wedge animals". It referred to the certain wedge-shape of the second side animals, which had been emphasized by Heinrich Georg Bronn. In more recent biological publications, Dawson's word bilaterata appears very rarely, although it is the older name . Ever since the term was first coined , Haeckel's alternative word Sphenota received hardly any attention. On the other hand, his second coinage, Bilateria, became generally accepted.

In addition to bilateral symmetry, the embryonic arrangement of three cotyledons is one of the fundamental characteristics of all bilateral animals. From this fact, separate group names were derived, which are used as synonyms for the expression bilateria . In 1873, the British zoologist Edwin Ray Lankester introduced the word triploblastica . It is still used today. However, the abraded expression Triploblasta is written with a similar frequency . It can be traced back to the 1970s and is a Latinized version of the English technical term "triploblasts" for the "triploblasts".

features

The Bilateria show bilateral symmetry at least in the larval stage , that is, there is a left and a right half of the body, each of which is mirror-inverted. There are no other planes of symmetry. In the adult stage, individual organs may not be symmetrical, for example the human liver or the front claws of fiddler crabs . The adult echinoderms are secondary to radial symmetry .

All bilateria originally have a front and a rear end and a mouth. In the course of the phylogenetic development, however, these characteristics may have been reduced, for example in the case of parasites . The main axis of the body goes through the front and rear ends. In some groups of the Bilateria, however, this original direction of movement is changed, for example cuttlefish swim morphologically up or down.

The adult Bilateria live mostly solitary and freely mobile. The main axis is usually in the direction of travel. They usually look actively for food, whereby the development of a head section with a higher-level part of the nervous system, i.e. a cerebral ganglion or brain, plays an essential role. This event, known as cephalization , has occurred in different lineages of Bilateria during evolution .

Usually a certain side of the body is directed towards the ground during movement. This is called the belly or ventral side. It has corresponding structures for moving the body, such as crawling soles or extremities . Opposite the ventral side is the back or dorsal side. The imaginary interface between the dorsal and ventral sides is called the dorsoventral plane . The plane of symmetry of the body of the bilateria, the sagittal plane, is at right angles to the dorsoventral plane . It separates the right from the left half of the body.

Sessile and colonized life forms are represented in a few groups of the Bilateria, for example Kamptozoa and Bryozoa . These also have a filtering food intake associated with this way of life .

Another characteristic characteristic of all bilateria is tricotyledonousness or triploblasty, i. H. the existence of the mesoderm , a third cotyledon that is created during gastrulation and slips between the two primary cotyledons, ectoderm and endoderm . It is mostly of mesentodermal origin, so it is formed by the endoderm. The musculature , the connective tissue and the epithelium around the secondary body cavity Coelom are formed from the mesoderm .

Systematics

The sister taxon of the bilateral animals (Bilateria) consists either of the hollow animals ( Coelenterata ) or of the rib jellyfish ( Ctenophora ) or of the cnidarians ( Cnidaria ). Together with comb jellyfish and cnidarians, the bilateral animals may form a common community of descent called tissue animals ( Eumetazoa ). However, there are still uncertainties and disagreements here. Different taxonomic working groups put forward arguments for three different concepts. The Coelenterata concept stands alongside the Acrosomata concept and the ParaHoxozoa / Planulozoa concept. Depending on the systematic used, the two other large groups of sponges ( Porifera ) and disc animals ( Placozoa ) are also located at different positions in the animal system ( Metazoa ).

Concepts for the systematics of the Metazoa including the Bilateria
Coelenterata concept	Acrosomata concept	ParaHoxozoa / Planulozoa concept
Metazoa Porifera Epitheliozoa Placozoa Eumetazoa Coelenterata Ctenophora Cnidaria Bilateria	Metazoa Porifera Epitheliozoa Placozoa Eumetazoa Cnidaria Acrosomata Ctenophora Bilateria	Metazoa Ctenophora unnamed Porifera ParaHoxozoa Placozoa Planulozoa Cnidaria Bilateria

The bilateral animals (Bilateria) split into the lines of the Xenacoelomorpha and the kidney animals ( Nephrozoa ). While the former are divided into Xenoturbellida and Acoelomorpha , a distinction is made in the latter between primordial mouth animals ( Protostomia ) and new mouth animals ( Deuterostomia ). The primordial mouth animals combine the taxa of the molting animals ( Ecdysozoa ) and the spiral furrows ( Spiralia ). The new mouth animals include the groups of Ambulacraria and the back-stringed animals ( Chordata ).

Internal systematics of the bilateria
Bilateria   Xenacoelomorpha  Xenoturbellida     Acoelomorpha  Nephrozoa   Protostomia  Ecdysozoa     Spiralia  Deuterostomia  Ambulacraria     Chordata
See also : Systematics of multicellular animals

evolution

The population of the last common ancestors of all bilateral animals is called the Urbilateria . The animals had evolved one main direction of movement forward . In the course of this, a head region and a bilaterally symmetrical body shape had developed. There is still no evolutionary consensus on whether the Urbilateria already had a central nervous system . Regardless, they were able to actively move their tiny worm-shaped bodies with the help of muscles. The muscle tissue was derived from cells of an embryonic mesoderm .

Any further reconstruction of the Urbilateria depends on which ancestry hypothesis is followed. According to the Nephrozoa hypothesis , the group of kidney animals (Nephrozoa) finally split off from morphologically more original Bilateria. Descendants of those more original bilateral animals also survived to this day. They are summarized as the group of Xenacoelomorpha and compared to the Nephrozoa. Recent representatives of the Xenacoelomorpha can still display many of the more original Bilateria traits. Accordingly, it is derived from morphological studies of the Xenacoelomorpha that the Urbilateria did not yet form a secondary body cavity . They had a digestive tract, which was lined with epithelial tissue , but as a gastric space only had an opening to the outside. If the Nephrozoa hypothesis is true, the kidney animals evolved from the acoelomorpha-like animals just described. Only the Nephrozoa then developed a secondary body cavity as well as a continuous digestive tract with mouth and anus. However, the Nephrozoa hypothesis is not entirely undisputed.

About 800 million years ago the bilateral animals split off from the rest of the animals. This is suggested by phylogenomic studies with a molecular clock . The split occurred in the neoproterozoic period of the Tonian . After that, the bilateria continued to evolve. According to the Nephrozoa hypothesis, Xenacoelomorpha and Nephrozoa separated. The latter were finally divided into primitive mouth animals (Protostomia) and new mouth animals (Deuterostomia) about 680 million years ago during the Sturtic Ice Age .

The oldest fossil evidence for the presence of bilateral animals, however, is at least 250 million years younger than their calculated first occurrence. They do not come from the Tonium, but rather from the last phase of the Ediacarium . Accordingly, fossils of the genus Kimberella , for example , are dated 555 million years ago. The remains of these animals were first made known in 1959 and their first careful scientific description took place in 1966. At that time, Kimberella was still considered a coelenterate . From 1997, however, new fossil discoveries suggested that the genus belonged to the bilateral animals. In March 2020, the discovery of a similarly ancient fossil, a worm that was named Ikaria wariootia , was published. The fossil, about the size of a grain of rice, was discovered in South Australia and dated to an age of 571 to 539 million years.

literature

• Rüdiger Wehner, Walter Jakob Gehring: Zoology . Georg Thieme Verlag, New York 2007, ISBN 978-3-13-367424-9 .
• Wilfried Westheide, Reinhard Rieger: Special Zoology. Part 1: Protozoa and invertebrates . Spektrum Akademischer Verlag, Heidelberg 2007, ISBN 978-3-8274-1575-2 .

Web links

Commons : Bilateria  - collection of images, videos and audio files

• Tree of Life Web Project - Bilateria

Individual evidence

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2. ^ John William Dawson: Zoological Classification; or Cœlenterata and Protozoa, versus Radiata. In: The Canadian Naturalist and Geologist . Volume 07, 1862, p. 440.
3. ↑ Ernst Haeckel: The Gastraea theory, the phylogenetic classification of the animal kingdom and the homology of the germ layers . In: Jenaische Zeitschrift für Naturwissenschaft . Volume 08, 1874, p. 35.
4. ↑ for example Wolfgang Hennig: Genetics . Springer-Verlag, Berlin / Heidelberg, 2002, p. 17.
5. ↑ for example Joanna Fietz, Jutta Schmid: Successfully passing exams in the subject of zoology . Verlag Eugen Ulmer, Stuttgart, 2019, pp. 64, 262, 265, 266.
6. ↑ Edwin Ray Lankester: On the primitive cell-layers of the embryo as the basis of genealogical classification of animals, and on the origin of vascular and lymph systems . In: The Annals and magazine of natural history; zoology, botany, and geology . Volume 11, 1873, p. 330.
7. ↑ Ernst Haeckel: The Gastraea theory, the phylogenetic classification of the animal kingdom and the homology of the germ layers . In: Jenaische Zeitschrift für Naturwissenschaft . Volume 08, 1874, p. 32.
8. ↑ for example Vitaly V. Kozin, Roman P. Kostyuchenko: Evolutionary Conservation and Variability of the Mesoderm Development in Spiralia: A Peculiar Pattern of Nereid Polychaetes . In: Biology Bulletin . Volume 43, 2016, p. 216.
9. ↑ for example Roberto Ligrone: Biological Innovations that Built the World . Springer International Publishing, Cham, 2019, p. 322.
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14. ↑ Martin Dohrmann, Gert Wörheide: Novel Scenarios of Early Animal Evolution — Is It Time to Rewrite Textbooks? . In: Integrative and Comparative Biology . Volume 53, 2013, pp. 504, 507-508.
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18. ↑ Gert Wörheide, Martin Dohrmann, Dirk Erpenbeck, Claire Larroux, Manuel Maldonado, Oliver Voigt, Carole Borchiellini, Dennis V. Lavrov: Deep Phylogeny and Evolution of Sponges (Phylum Porifera) . In: Advances in Marine Biology . Volume 61, 2012, p. 1.
19. ^ Johanna Taylor Cannon, Bruno Cossermelli Vellutini, Julian Smith, Fredrik Ronquist, Ulf Jondelius, Andreas Hejnol: Xenacoelomorpha is the sister group to Nephrozoa . In: Nature . Volume 530, 2016, p. 89.
20. ↑ John Buckland-Nicks, Kennet Lundin, Andreas Wallberg: The sperm of Xenacoelomorpha revisited: implications for the evolution of early bilaterians . In: Zoomorphology . Volume 138, 2019, p. 13.
21. ↑ Andreas C. Fröbius, Peter Funch: Rotiferan Hox genes give new insights into the evolution of metazoan bodyplans . In: Nature Communications . Volume 8, 2017, p. 3.
22. ^ Alfred Goldschmid: Deuterostomia . In: Wilfried Westheide, Gunde Rieger (ed.): Special Zoology • Part 1 . Springer-Verlag, Berlin / Heidelberg 2013, p. 716.
23. ^ Edward M. de Robertis, Yoshiki Sasai: A common plan for dorsoventral patterning in Bilateria . In: Nature . Volume 380, 1996, p. 37.
24. ↑ François Bailly, Emmanuelle Pouydebat, Bruno Watier, Vincent Bels, Philippe Souères: Should Mobile Robots Have a Head? . In: Vasiliki Vouloutsi, José Halloy, Anna Mura, Michael Mangan, Nathan Lepora, Tony J. Prescott, Paul FMJ Verschure (eds.): Biomimetic and Biohybrid Systems . Springer International Publishing, Cham, 2018, p. 30.
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26. ↑ Linda Z. Holland, João E. Carvalho, Hector Escriva, Vincent Laudet, Michael Schubert, Sebastian M. Shimeld, Jr-Kai Yu: Evolution of bilaterian central nervous systems: a single origin? . In: EvoDevo . Volume 04, 2013, Article 27, p. 15.
27. ↑ José M. Martín-Durán, Kevin Pang, Aina Børve, Henrike Semmler Lê, Anlaug Furu, Johanna Taylor Cannon, Ulf Jondelius, Andreas Hejnol: Convergent evolution of bilaterian nerve cords . In: Nature . Volume 553, 2018, p. 45.
28. ↑ Linda Z. Holland, João E. Carvalho, Hector Escriva, Vincent Laudet, Michael Schubert, Sebastian M. Shimeld, Jr-Kai Yu: Evolution of bilaterian central nervous systems: a single origin? . In: EvoDevo . Volume 04, 2013, Article 27, p. 3.
29. ↑ Herve Philippe, Albert J. Poustka, Marta Chiodin, Katharina J. Hoff, Christophe Dessimoz, Bartlomiej Tomiczek, Philipp H. Schiffer, Steven Müller, Daryl Domman, Matthias Horn, Heiner Kuhl, Bernd Timmermann, Noriyuki Satoh, Tomoe Hikosaka-Katayama , Hiroaki Nakano, Matthew L. Rowe, Maurice R. Elphick, Morgane Thomas-Chollier, Thomas Hankeln, Florian Mertes, Andreas Wallberg, Jonathan P. Rast, Richard R. Copley, Pedro Martinez, and Maximilian J. Telford: Mitigating Anticipated Effects of Systematic Errors Supports Sister-Group Relationship between Xenacoelomorpha and Ambulacraria . In: Current Biology . Volume 29, 2019, p. 1818.
30. ↑ Gonzalo Giribet, Gregory D. Edgecombe: “Perspectives in Animal Phylogeny and Evolution”: A decade later . In: Giuseppe Fusco (ed.): Perspectives on Evolutionary and Developmental Biology . Padova University Press, 2019, p. 170.
31. ^ Johanna Taylor Cannon, Bruno Cossermelli Vellutini, Julian Smith, Fredrik Ronquist, Ulf Jondelius, Andreas Hejnol: Xenacoelomorpha is the sister group to Nephrozoa . In: Nature . Volume 530, 2016, p. 89.
32. ↑ Ulf Jondelius, Olga I. Raikova, Pedro Martinez: Xenacoelomorpha, a Key Group to Understand Bilaterian Evolution: Morphological and Molecular Perspectives . In: Pierre Pontarotti (ed.): Evolution, Origin of Life, Concepts and Methods . Springer International Publishing, Cham, 2019, p. 287.
33. ↑ Herve Philippe, Albert J. Poustka, Marta Chiodin, Katharina J. Hoff, Christophe Dessimoz, Bartlomiej Tomiczek, Philipp H. Schiffer, Steven Müller, Daryl Domman, Matthias Horn, Heiner Kuhl, Bernd Timmermann, Noriyuki Satoh, Tomoe Hikosaka-Katayama , Hiroaki Nakano, Matthew L. Rowe, Maurice R. Elphick, Morgane Thomas-Chollier, Thomas Hankeln, Florian Mertes, Andreas Wallberg, Jonathan P. Rast, Richard R. Copley, Pedro Martinez, and Maximilian J. Telford: Mitigating Anticipated Effects of Systematic Errors Supports Sister-Group Relationship between Xenacoelomorpha and Ambulacraria . In: Current Biology . Volume 29, 2019, p. 1824.
34. ↑ Kim Mikkel Cohen, David AT Harper, Philip Leonard Gibbard, Jun-Xuan Fan: International Chronostratigraphic Chart v 2018/08 . International Commission on Stratigraphy, 2018, ( Link ).
35. ↑ Martin Dohrmann, Gert Wörheide: Dating early animal evolution using phylogenomic data . In: Scientific Reports . Volume 07, 2017, p. 4.
36. ↑ John A. Cunningham, Alexander G. Liu, Stefan Bengtson, Philip CJ Donoghue: The origin of animals: Can molecular clocks and the fossil record be reconciled? . In: BioEssays . Volume 39, 2017, p. 1.
37. ^ Ilya Bobrovskiy, Janet M. Hope, Andrey Ivantsov, Benjamin J. Nettersheim, Christian Hallmann, Jochen J. Brocks: Ancient steroids establish the Ediacaran fossil Dickinsonia as one of the earliest animals . In: Science . tape 361 , no. 6408 , September 21, 2018, p. 1246–1249 , doi : 10.1126 / science.aat7228 (English).
38. ^ Martin F. Glaessner, Brian Daily: The geology and Late Precambrian fauna of the Ediacara fossil reserve . In: Records of the South Australian Museum . Volume 13, 1959, p. 391.
39. ^ Martin F. Glaessner, Mary Wade: The late Precambrian fossils from Ediacara, South Australia . In: Palaeontology . Volume 09, 1966, pp. 611-612.
40. ↑ Mikhail A. Fedonkin, Benjamin M Wagoner: The Late Precambrian fossil Kimberella is a mollusc-like bilaterian organism . In: Nature . Volume 388, 1997, p. 869.
41. ^ Scott D. Evans, Ian V. Hughes, James G. Gehling, Mary L. Droser: Discovery of the oldest bilaterian from the Ediacaran of South Australia . In: Proc. Natl. Acad. Sci. USA . doi : 10.1073 / pnas.2001045117 (English).