Cephalochordata (Lancelets)

Abstract

The Cephalochordata constitute one of the three traditionally recognized subphyla of the Chordata, the other two being the Tunicata (Urochordata) and the Craniata (including the Vertebrata). They are also known as Acraniata or lancelets. There are only two extant genera, Branchiostoma (amphioxus) and Epigonichthys (Asymmetron).

Keywords: chordata; left–right asymmetry; calcichordates; echinodermata

Figure 1.

Externally visible features of an adult amphioxus (Branchiostoma) seen from the left.

Figure 2.

Internal structure of Branchiostoma (right view).

Figure 3.

Swimming larva of Branchiostoma seen from the left. Note the left‐opening mouth and the single row of primary gill slits. In the adult these will become left gill slits.

Figure 4.

The mitrate Lagynocystis (Ordovician, Czech Republic, 470 million years old): (a) right view; (b) gill bars and slits preserved inside the posterior part of the head, seen from in front. Most of these gill slits would have opened into a median ventral atrium, debouching by a median ventral atrial opening, as in extant amphioxus.

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Further Reading

Briggs DEJ, Erwin DH and Collier FJ (1994) The Fossils of the Burgess Shale. Washington DC: Smithsonian Institution Press.

Gans C, Kemp N and Poss S (1996) The lancelets (Cephalochordata): a new look at some old beasts. Israel Journal of Zoology 42 (supplement): S1–S446.

Gee H (1996) Before the Backbone. Views on the Origin of the Vertebrates. London: Chapman and Hall.

Harrison FW and Ruppert EE (eds) (1997) Hemichordata, Chaetognatha and the Invertebrate Chordates, vol. 15. In: Harrison FW (ed.) Microscopic Anatomy of the Invertebrates. New York: Wiley‐Liss.

Holland LZ, Kene M, Williams NA and Holland ND (1997) Sequence and embryonic expression of the amphioxus engrailed gene (AmphiEn): the metameric pattern of transcription resembles that of its segment‐polarity homolog in Drosophila. Development 124: 1723–1732.

Holland PWH and Garcia‐Fernàndez J (1996) Hox genes and chordate evolution. Developmental Biology 173: 382–395.

Jefferies RPS (1973) The Ordovician fossil Lagynocystis pyramidalis and the ancestry of amphioxus. Philosophical Transactions of the Royal Society of London B 265: 409–469.

Jefferies RPS (1986) The Ancestry of the Vertebrates. London and Cambridge: British Museum (Natural History) and Cambridge University Press.

Jefferies RPS, Brown NA and Daley PEJ (1996) The early phylogeny of chordates and echinoderms and the origin of chordate left‐right asymmetry and bilateral symmetry. Acta Zoologica (Stockholm) 77: 101–122.

Shu D‐G, Conway Morris S and Zhang X‐L (1996) A Pikaia‐like chordate from the Lower Cambrian of China. Nature 384: 157–158.

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How to Cite close
Jefferies, Richard PS(Apr 2001) Cephalochordata (Lancelets). In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0001530]