Agnath

Evolution of vertebrates was accompanied by a gradual increase in oxygen availability, from the irrespirable atmosphere of the Precambrian to a PO₂ at 7 mmHg with the first vertebrates (agnaths), to the present sea level PO₂ value of 160 mmHg with the first reptiles.

The gnathostomes almost entirely replaced the agnaths, presumably because they were more effective fishes.

Thus, although tunicates are presumably intermediate between flatworms and agnaths, the larval tunicate more closely resembles the flatworm and adult agnath.