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Deuterostomia topics: | | | | | |
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Chordates (phylum Chordata) are a group of animals that includes the , together with several closely related . They are united by having, at some time in their life, a , a hollow dorsal nerve cord, , an , and a muscular extending past the anus. Some scientists argue, however, that the true qualifier should be rather than slits.[citation needed]
The phylum Chordata is broken down into three subphyla: , , and . Urochordate larvae have a notochord and a nerve cord but they are lost in adulthood. Cephalochordates have a notochord and a nerve cord but no vertebrae. In the vertebrates the dorsal hollow nerve cord has been surrounded with cartilaginous or bony and the notochord generally reduced. (This classification leaves out the , which constitute the sister group to the vertebrates, and together with them forms the clade . Hagfish have notochords with no vertebrae, resembling in this respect cephalochordates, but other characters show them to be vertebrate allies.)
The chordates and two sister phyla, the hemichordates and the echinoderms, make up the , a superphylum.
The extant groups of chordates are related as shown in the , below. They do not match up very well with the traditional groups, and as a result vertebrate classification is in a state of flux, although their relationships are not very well understood
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Chordata [Acraniata]
| i. s.: Cathaymyrus diadexus
|--Branchiostoma [Acrania, Cephalochordata]
| |--B. belcheri
| |--B. floridae
| `--B. lanceolatum [=Limax lanceolatus]
`--Craniata [Agnatha, Craniota]
| i. s.: Pipiscius zangerli
| Myllokunmingia
|--+--Myxinikela siroka
| `--
`--+--Haikouichthys ercaicunensis
`--
* Type species of genus indicated
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Boorman, C. J., & S. M. Shimeld. 2002. The evolution of left-right asymmetry in chordates. BioEssays 24 (11): 1004-1011.
Conway Morris, S. 1998. The Crucible of Creation. Oxford University Press: Oxford.
Gee, H. 2003. You aren’t what you eat. Nature 424: 885-886.
Giribet, G., D. L. Distel, M. Polz, W. Sterrer & W. C. Wheeler. 2000. Triploblastic relationships with emphasis on the acoelomates and the position of Gnathostomulida, Cycliophora, Plathelminthes, and Chaetognatha: A combined approach of 18S rDNA sequences and morphology. Systematic Biology 49: 539-562.
Martini, F. H., & D. Flescher. 2002. Hagfishes. Family Myxinidae. In Bigelow and Schroeder’s Fishes of the Gulf of Maine (B. B. Collette & G. Klein-MacPhee, eds.) 3rd ed. pp. 9-16. Smithsonian Institute Press: Washington.
Shu, D.-G., L. Chen, J. Han & X.-L. Zhang. 2001. An Early Cambrian tunicate from China. Nature 411: 472-473.
Shu, D., S. Conway Morris, X.-L. Zhang, L. Chen, Y. Li & J. Han. 1999. A pipiscid-like fossil from the Lower Cambrian of south China. Nature 400: 746-749.
Shu, D.-G., S. Conway Morris, J. Han, Z.-F. Zhang, K. Yasul, P. Janvier, L. Chen, X.-L. Zhang, J.-N. Liu, Y. Li & H.-Q. Liu. 2003. Head and backbone of the Early Cambrian vertebrate Haikouichthys. Nature 421: 526-529.
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