The Chordata Phylum is a phylum that includes mostly vertebrates. Most people are most familiar with the Chordata phylum because it includes most animals, including humans. However, contrary to popular belief, the Chordata phylum does not always include vertebrates. At some point in the lives of every single Chrodate, each organism has the following features, making it, by definition, a part of the Chrodata phylum:
"Pharyngeal slits - a series of openings that connect the inside of the throat to the outside of the "neck". These are often, but not always, used as gills.
Dorsal nerve cord - a bundle of nerve fibers which runs down the "back". It connects the brain with the lateral muscles and other organs.
Notochord - cartilaginous rod running underneath, and supporting, the nerve cord.
Post-anal tail - an extension of the body past the anal opening."
"Pharyngeal slits - a series of openings that connect the inside of the throat to the outside of the "neck". These are often, but not always, used as gills.
Dorsal nerve cord - a bundle of nerve fibers which runs down the "back". It connects the brain with the lateral muscles and other organs.
Notochord - cartilaginous rod running underneath, and supporting, the nerve cord.
Post-anal tail - an extension of the body past the anal opening."
- (Aquired from http://www.ucmp.berkeley.edu/chordata/chordata.html)