Chordates are named for the notochord, present in all chordate embryos as well as in
some adult chordates. The notochord is a longitudinal, flexible rod located between the
digestive tube and the nerve cord. It is composed of large, fluid–filled cells encased in
fairly stiff, fibrous tissue. The notochord provides skeletal support throughout most of
the length of a chordate, and in larvae or adults that retain it, it also provides a firm but
flexible structure against which muscles can work during swimming. In most
vertebrates, a more complex, jointed skeleton develops, and the adult retains only
remnants of the embryonic notochord. In humans, the notochord is reduced to
gelatinous disks sandwiched between the vertebrae.