Protochordate, any group of either of two invertebrate subphyla of the phylum. Like the remaining subphylum of the chordates, the
Vertebrata, the protochordates have a hollow dorsal nerve cord, gill
slits, and a stiff supporting rod, the notochord, the forerunner of the
backbone. The protochordates differ chiefly from the vertebrates in not
having a backbone. Recent protochordates are thought to have evolved
from the same ancestral stock as that which gave rise to the
vertebrates. More recent theory postulates that the chordates evolved from a small fossil group called the mitrates.