Urochordata (Tunicates)


The Urochordata are a subphylum of the Chordata comprising marine organisms such as the benthic ascidians and the pelagic appendicularians, salps, doliolids and pyrosomes.

Keywords: ciona; chordata; tadpole; tunicate; notocord; zooid

Figure 1.

The ascidian Ciona. (a) Cutaway diagram of adult. (b) Transverse section through adult to show the pharynx surrounded at left, right and dorsally by the atrium, the endostyle in the mid‐ventral line of the pharynx and the dorsal languets hooked ventralwards and to the right (in the diagram the right side of the animal is on the right). (c) Tadpole larva in right anterodorsal view. Note the clear distinction between head and tail, and the adhesive papillae for attachment to a substrate.

Figure 2.

Pelagic urochordates. (a) The appendicularian Oikopleura inside its house. The animal is tadpole‐like and beating of the tail pulls water through the house, filtering out plankton. (b) The doliolid Doliolum seen from the left.

Figure 3.

The Ordovician mitrate Anatifopsis. (a) Dorsal view to show the atrial openings on the dorsal surface of the head as in a tunicate tadpole. (b) Left view to show tail, head and the dorsal atrial openings. The animal would have locomoted by pushing the tail forwards in the mud, so pulling the head rearward.


Further Reading

Berrill NJ (1950) The Tunicata with an Account of the British Species. London: Ray Society.

Burighel P and Cloney RA (1997) Urochordata: Ascidiacea. In: Harrison FW and Ruppert EE (eds) Microscopic Anatomy of the Invertebrates, pp. 221–347. New York: Wiley‐Liss.

Garstang W (1928) The morphology of the Tunicata and its bearing on the phylogeny of the Chordata. Quarterly Journal of Microscopical Sciences 72: 51–87.

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

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

Jefferies RPS (1997) A defence of the calcichordates. Lethaia 30: 1–10.

Jefferies RPS and Jacobson AG (1998) An episode in the ancestry of the vertebrates: from mitrate to crown‐group craniate. Integrative Biology 1: 115–132.

Millar RH (1953) Ciona. Liverpool Marine Biology Committee Memoirs on Typical British Marine Plants and Animals 35: 1–84.

Wada H (1998) Evolutionary history of free‐swimming and sessile lifestyles in urochordates as deduced from 18S rDNA molecular phylogeny. Molecular Biology and Evolution 15: 1189–1194.

Wada H, Saiga H, Satoh N and Holland PWH (1998) Tripartite organization of the ancestral chordate brain and the antiquity of placodes: insights from ascidian Pax‐2/5/8, Hox and Otx genes. Development 125: 1113–1122.

Werner E and Werner B (1954) Über den Mechanismus des Nahrungerwerbs der Tunicaten, speziell der Ascidien. Helgoländer wissenschaftliche Meeresuntersuchungen 5: 57–92.

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How to Cite close
Jefferies, Richard(Apr 2001) Urochordata (Tunicates). In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0001529]