The phylum Chordata contains all animals that have, at some point during their lives, a dorsal nerve cord - a bundle of nerve fibers which runs down the "back", a notochord, a post-anal tail and pharyngeal slits - a series of openings that connect the inside of the throat to the outside of the "neck". The phylum has about 43,700 species, most of them in the subphylum Vertebrata, making it the third-largest phylum in the animal kingdom. This animal phylum is the one with which everyone is most intimately familiar, since it includes humans and other vertebrates.