amphioxus

Lancelets
	Lancelet (Branchiostoma lanceolatum)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Subphylum:	Cephalochordata
Class:	Leptocardii
Order:	Amphioxiformes
Families
	Asymmetronidae; Branchiostomidae

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2013/10/15 21:19:51」(JST)

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1. brain-like blister 2. notochord 3. dorsal nerve cord 4. post-anal tail 5. anus 6. food canal 7. blood system 8. abdominal porus 9. overpharynx lacuna 10. gill's slit 11. pharynx 12. mouth lacuna 13. mimosa 14. mouth gap 15. gonads (ovary/testicle) 16. light sensor 17. nerves 18. abdominal ply 19. liver-like sack

The lancelets (from "lancet"), also known as amphioxus, comprise some twenty-two species of fish-like marine chordates with a global distribution in shallow temperate (as far north as Scotland)^[2] and tropical seas, usually found half-buried in sand. They are the modern representatives of the subphylum Cephalochordata, formerly thought to be the sister group of the craniates.^[3] In Asia, they are harvested commercially as food for humans and domesticated animals. They are an important object of study in zoology as they provide indications about the origins of the vertebrates. Lancelets serve as an intriguing comparison point for tracing how vertebrates have evolved and adapted. Although lancelets split from vertebrates more than 520 million years ago, their genomes hold clues about evolution, particularly how vertebrates have employed old genes for new functions.^[4] They are regarded as similar to the archetypal vertebrate form.

The Amphioxiformes are often called "amphioxus" (from the Greek: "pointed on both sides "). Amphioxus is an obsolete synonym of the genus Branchiostoma. It is used as a common name alongside with "lancelet", especially in the English language.

The genome of the Florida lancelet (Branchiostoma floridae) has been sequenced.^[5]

Physical features[edit]

Lancelets grow up to about 5 centimetres (2.0 in) long, reaching 7 centimetres (2.8 in) at the longest. They have a translucent, somewhat fish-like body, but without any paired fins or other limbs. A relatively poorly developed tail fin is present, so they are not especially good swimmers. While they do possess some cartilage-like material stiffening the gill slits, mouth, and tail, they have no true skeleton.^[6]

In common with vertebrates, lancelets have a hollow nerve cord running along the back, pharyngeal slits and a tail that runs past the anus. Also like vertebrates, the muscles are arranged in blocks called myomeres.

Unlike vertebrates, the dorsal nerve cord is not protected by bone but by a simpler notochord made up of a cylinder of cells that are closely packed to form a toughened rod. The lancelet notochord, unlike the vertebrate spine, extends into the head. This gives the subphylum its name (cephalo- meaning 'relating to the head'). The nerve cord is only slightly larger in the head region than in the rest of the body, so that lancelets cannot be said to possess a true brain. Neither do they have any eyes, or other complex sense organs comparable to those of vertebrates.

Lancelets also have oral cirri, thin tentacle-like strands that hang in front of the mouth and act as sensory devices and as a filter for the water passing into the body. Water passes from the mouth into the large pharynx, which is lined by numerous gill-slits. The ventral surface of the pharynx contains a groove, called the endostyle, which, connected to a structure known as Hatschek's pit, produces a film of mucus. Ciliary action pushes the mucus in a film over the surface of the gill slits, trapping suspended food particles as it does so. The mucus is collected in a second, dorsal, groove, and passed back to the rest of the digestive tract. Having passed through the gill slits, the water enters an atrium surrounding the pharynx, then exits the body via the atriopore.^[6]

The remainder of the digestive system consists of a simple tube running from the pharynx to the anus. A single blind-ending caecum branches off from the underside of the gut, with a lining able to phagocytize the food particles; a feature never found in vertebrates. It used to be thought that the caecum might be homologous to the liver of vertebrates, but this is now thought to be less likely.^[6]

Lancelets have no respiratory system, breathing solely through their skin, which consists of a simple epithelium. Despite the name, little if any respiration occurs in the gill slits, which are solely devoted to feeding. The circulatory system does resemble that of primitive fish in its general layout, but is much simpler, and does not include a heart. There are no blood cells, and no haemoglobin.^[6]

The excretory system consists of segmented "kidneys" containing protonephridia instead of nephrons, and quite unlike those of vertebrates. Also unlike vertebrates, there are numerous, segmented gonads.^[6]

Taxonomy[edit]

The Cephalochordata is traditionally seen as a sister subphylum to the vertebrates, with which it is grouped together into a clade (sometimes called Notochordata) which in turn is the sister group to the simpler still Urochordata. Newer research suggests this may not be the case. The Cephalochordata may be the most basal subphylum of the chordates,^[7] while the sister group of the vertebrates may be the tunicates, previously known as the urochordates.^[5] However, other recent molecular studies (cit. in Benton 2005: 8) place cephalochordates nearer to vertebrates, and "[m]ost authors regard amphioxus as the closest relative of the Vertebrata on the basis of 10–15 [morphological] features that are not seen in tunicates".^[8]

The asymmetric nature of juveniles is unique to the cephalochordates and demonstrates (as do certain other features, including the seriated gonads) that lancelets are more derived than would be expected. This is a reminder that the "living fossil" representative of a basal clade has as long an evolutionary history as any other living thing, and thus is more derived than the actual primitive ancestor it otherwise so closely resembles. The following are the species recognised by ITIS. Other sources, for instance Tudge,^[9] show that there might be up to thirty species.^{[verification needed]}

Family Asymmetronidae
- Genus Asymmetron
  - Asymmetron lucayanum
  - Asymmetron maldivense
- Genus Epigonichthys
  - Epigonichthys australis (Raff, 1912)
  - Epigonichthys bassanus (Günther, 1884)
  - Epigonichthys cingalense (Kirkaldy, 1894)
  - Epigonichthys cultellus Peters, 1877
  - Epigonichthys hectori (Benham, 1901)
  - Epigonichthys lucayanum (Andrews, 1893)
  - Epigonichthys maldivensis (Foster Cooper, 1903)
Family Branchiostomidae
- Genus Branchiostoma
  - Branchiostoma belcheri
  - Branchiostoma californiense
  - Branchiostoma capense
  - Branchiostoma caribaeum
  - Branchiostoma clonaseum
  - Branchiostoma floridae
  - Branchiostoma lanceolatum
  - Branchiostoma minucauda
  - Branchiostoma moretonensis
  - Branchiostoma valdiviae
  - Branchiostoma virginiae

References[edit]

^ Classification of Class: Leptocardii - Ocean Biogeographic Information System: Canadian Museum of Nature (OBIS Canada)
^ "Year of discovery". Scotland.gov.uk. 2011-12-29. Retrieved 2012-08-13.
^ Webb, J. E. (1958). "The Ecology of Lagos Lagoon. III. The Life History of Branchiostoma nigeriense Webb". Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences (The Royal Society) 241 (683): 335–353. doi:10.2307/92531. JSTOR 92531. edit
^ Worm-like Marine Animal Providing Fresh Clues About Human Evolution Newswise, Retrieved on July 8, 2008.
^ ^a ^b Putnam, N. H.; Butts, T.; Ferrier, D. E. K.; Furlong, R. F.; Hellsten, U.; Kawashima, T.; Robinson-Rechavi, M.; Shoguchi, E.; Terry, A.; Yu, J. K.; Benito-Gutiérrez, E. L.; Dubchak, I.; Garcia-Fernàndez, J.; Gibson-Brown, J. J.; Grigoriev, I. V.; Horton, A. C.; De Jong, P. J.; Jurka, J.; Kapitonov, V. V.; Kohara, Y.; Kuroki, Y.; Lindquist, E.; Lucas, S.; Osoegawa, K.; Pennacchio, L. A.; Salamov, A. A.; Satou, Y.; Sauka-Spengler, T.; Schmutz, J.; Shin-i, T. (Jun 2008). "The amphioxus genome and the evolution of the chordate karyotype". Nature 453 (7198): 1064–1071. Bibcode:2008Natur.453.1064P. doi:10.1038/nature06967. ISSN 0028-0836. PMID 18563158. edit
^ ^a ^b ^c ^d ^e Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 18–21. ISBN 0-03-910284-X.
^ Henry Gee (2008). "Evolutionary biology: The amphioxus unleashed". Nature 453: 999-1000.
^ Michael J. Benton (2005). Vertebrate Palaeontology, Third Edition 8. Oxford: Blackwell Publishing. ISBN 0-632-05637-1.
^ Tudge, Colin (2000). The Variety of Life. Oxford University Press. ISBN 0198604262.

Cephalochordata from Berkeley

External links[edit]

Wikispecies has information related to: Cephalochordata

Error in the Genealogy of Human
A special issue of Amphioxus Research (I)
A special issue of Amphioxus Research (II)
Amphioxus and the T-box gene
A movie of the amphioxus embryonic development on YouTube
Scripps Scientists Discover Fluorescence in Key Marine Creature
Amphioxus at GeoChemBio: taxonomy, brief facts, life cycle and embryology

English Journal

Zoology: A New Mouth for Amphioxus.

Soukup V1, Kozmik Z2.
Current biology : CB.Curr Biol.2016 May 9;26(9):R367-R368. doi: 10.1016/j.cub.2016.03.016.
Deuterostomes - a key subdivision of animals - are characterized by the mouth developing anteriorly as a rupture between the outer epithelium and the foregut wall. A new study of amphioxus challenges this view and proposes separate evolutionary origins of deuterostome oral openings.Copyright © 2016
PMID 27166696

Normal development and experimental embryology: Edmund Beecher Wilson and Amphioxus.

Lowe JW1.
Studies in history and philosophy of biological and biomedical sciences.Stud Hist Philos Biol Biomed Sci.2016 Apr 4;57:44-59. doi: 10.1016/j.shpsc.2016.03.004. [Epub ahead of print]
This paper concerns the concept of normal development, and how it is enacted in experimental procedures. To that end, I use an historical case study to assess the three ways in which normal development is and has been produced, used, and interpreted in the practice of experimental biology. I argue t
PMID 27054569

Transcriptome analysis of different developmental stages of amphioxus reveals dynamic changes of distinct classes of genes during development.

Yang KY1,2, Chen Y3, Zhang Z4, Ng PK1, Zhou WJ1, Zhang Y1, Liu M1,2, Chen J5, Mao B4, Tsui SK1,2.
Scientific reports.Sci Rep.2016 Mar 16;6:23195. doi: 10.1038/srep23195.
Vertebrates diverged from other chordates approximately 500 million years ago and have adopted several modifications of developmental processes. Amphioxus is widely used in evolutionary developmental biology research, such as on the basic patterning mechanisms involved in the chordate body plan and
PMID 26979494

Japanese Journal

A comparative examination of neural circuit and brain patterning between the lamprey and amphioxus reveals the evolutionary origin of the vertebrate visual center

Suzuki Daichi G.,Murakami Yasunori,Escriva Hector,Wada Hiroshi
The Journal of comparative neurology 523(2), 251-261, 2015-02
… To bridge this gap, we present the detailed development of retinofugal projections in the lamprey, the neuroarchitecture in amphioxus, and the brain patterning in both animals. … Both the lateral eye in larval lamprey and the frontal eye in amphioxus project to a light-detecting visual center in the caudal prosencephalic region marked by Pax6, which possibly represents the ancestral state of the chordate visual system. …
NAID 120005526737

Complete Mitochondrial Genome of Epigonichthys cultellus (Cephalochordata: Branchiostomatidae)

Li Wei-Ye,Fang Shao-Hua,Wang Yi-Quan
Zoological science 31(11), 766-772, 2014-11
NAID 40020278746

A novel N-terminal motif is responsible for the evolution of neural crest-specific gene-regulatory activity in vertebrate FoxD3

Ono Hiroki,Kozmik Zbynek,Yu Jr-Kai,Wada Hiroshi
Developmental biology 385(2), 396-404, 2014-01
… Amphioxus and vertebrate (Xenopus) FoxD proteins both exhibited transcriptional repressor activity in Gal4 transactivation assays and bound to similar DNA sequences in vitro. … However, whereas vertebrate FoxD3 genes induced the differentiation of ectopic NCCs when overexpressed in chick neural tube, neither amphioxus FoxD nor any other vertebrate FoxD paralogs exhibited this activity. …
NAID 120005396072

Related Pictures

[1] Classification of Class: Leptocardii - Ocean Biogeographic Information System: Canadian Museum of Nature (OBIS Canada)

[2] "Year of discovery". Scotland.gov.uk. 2011-12-29. Retrieved 2012-08-13.

[3] Webb, J. E. (1958). "The Ecology of Lagos Lagoon. III. The Life History of Branchiostoma nigeriense Webb". Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences (The Royal Society) 241 (683): 335–353. doi:10.2307/92531. JSTOR 92531. edit

[4] Worm-like Marine Animal Providing Fresh Clues About Human Evolution Newswise, Retrieved on July 8, 2008.

[putnam-5] Putnam, N. H.; Butts, T.; Ferrier, D. E. K.; Furlong, R. F.; Hellsten, U.; Kawashima, T.; Robinson-Rechavi, M.; Shoguchi, E.; Terry, A.; Yu, J. K.; Benito-Gutiérrez, E. L.; Dubchak, I.; Garcia-Fernàndez, J.; Gibson-Brown, J. J.; Grigoriev, I. V.; Horton, A. C.; De Jong, P. J.; Jurka, J.; Kapitonov, V. V.; Kohara, Y.; Kuroki, Y.; Lindquist, E.; Lucas, S.; Osoegawa, K.; Pennacchio, L. A.; Salamov, A. A.; Satou, Y.; Sauka-Spengler, T.; Schmutz, J.; Shin-i, T. (Jun 2008). "The amphioxus genome and the evolution of the chordate karyotype". Nature 453 (7198): 1064–1071. Bibcode:2008Natur.453.1064P. doi:10.1038/nature06967. ISSN 0028-0836. PMID 18563158. edit

[VB-6] Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 18–21. ISBN 0-03-910284-X.

[7] Henry Gee (2008). "Evolutionary biology: The amphioxus unleashed". Nature 453: 999-1000.

[8] Michael J. Benton (2005). Vertebrate Palaeontology, Third Edition 8. Oxford: Blackwell Publishing. ISBN 0-632-05637-1.

[9] Tudge, Colin (2000). The Variety of Life. Oxford University Press. ISBN 0198604262.

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