Although not the largest phylum, Chordata contains the most familiar species, including humans. All chordates have several things in common that occur at some stage of development. They have pharyngeal slits, which are openings that connect the inside of the throat to the outside of the neck. These are often used as gills. Their main feature, what they are named after, is the notochord, which is a rod that supports the nerve cord. The nerve cord is also present in all species. This is a bundle of nerve fibers which connect the brain with the muscles and organs, and is through which messages from the brain are sent. A tail is also present, which extends past the anal opening. In most species these features disappear with age. For example, the pharyngeal slits are only present in the human fetus. There are approx 44 000 species.