Difficult to produce a detailed Classification.

http://en.wikipedia.org/wiki/Chordate

It has also proved difficult to produce a detailed classification within the living chordates. Attempts to produce evolutionary "family trees" give results that differ from traditional classes because several of those classes are not monophyletic. As a result, vertebrate classification is in a state of flux.

For a recent concise review of chordate relationships, see Holland, N. D. 2005, Chordates. Curr. Biol. 15: R911-R914.

Origin of name

Although the name Chordata is attributed to William Bateson (1885), it was already in prevalent use by 1880. Ernst Haeckel described a taxon comprising tunicates, hrotochordates, and vertebrates in 1866. Though he used the German vernacular form, it is allowed under the ICZN code because of its subsequent latinization.