Protochordata (group)

Group of marine animals characterized by having at some point in their life cycle a hollow dorsal nerve cord, a notochord, and gill slits, including hemichordates, urochordates, cephalochordates, or any hypothetical ancestral form of any of these. The protochordates lack a skull and vertebrae and are believed to resemble the ancestors of the vertebrates. They include the subphylums of Tunicata and Cephalochordata.