The origin of the body plan of our own phylum, Chordata, is one of the most fascinating ... the homologous structure should occupy the ventral side of the hemichordate endomesoderm.

All chordates, such as vertebrates, eel-like lancelets, and tunicates, or sea squirts, at some point in their lives have a flexible, rod-shaped nerve structure called a notochord in their backs.

Chordates include all animals with a notochord ... This form of locomotion was aided by the ridged structure notochords and early backbones, which is something all vertebrates share.