Chordates (phylumChordata) 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. In the subphylum urochordata the larvae have a notochord and nerve cord but the adults have neither. Cephalochordates have a notochord and a nerve cord but no vertebra and in vertebrates the notochord has been replaced by a bony vertebral column.
The phylum chordata consists of ten extant classes. One in the subphylum urochordata, one in the subphylum cephalochordata, and eight 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):
