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Chordate

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Chordates (phylum Chordata) include the vertebrates, together with several closely related invertebrates. They are united by having, at some stage in their life, a notochord, a hollow dorsal nerve cord, pharyngeal slits, a tail extending past the anus, and bands of muscles that go around the body.

The phylum chordata is broken down into three subphyla: Urochordata, Cephalochordata, and Vertebrata. 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 In vertebrates, the notochord has been replaced by a bony vertebral column.

The phylum chordata consists of twelve extant classes. Three of these are in the subphylum Urochordata, one in the subphylum Cephalochordata, one is the hagfish, and the other seven (or eight or nine, depending on how the fish are split) in the subphylum Vertebrata.

The traditional classification of vertebrates contains a wide variety of paraphyletic groups, which in newer systems may either be abandoned or greatly extended. No particular standard system has developed yet, and the groups given at right should be considered tentative.

Other groups that have been used (in alphabetical order):

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