Tunicates, or sea squirts, are marine organisms covered with a gelatinous tunic. The larval stage resembles a tadpole and has a notochord in the tail. Metamorphosis to the adult results in a loss of the notochord and tail, a regression of the nerve cord, and an expansion of the pharynx for filter feeding. Lancelets are small fishlike animals with tapered bodies. They lie buried in the sand filtering food from the stream of water passing through the pharynx.
Lancelets display all four of the vertebrate characteristics throughout their lives.
Among the Vertebrates A. Puzzling Origins, Portentous Trends 1. Perhaps vertebrates evolved from a tunicate-like ancestor whose larva became sexually functional. The single, continuous notochord was replaced by a column of separate, hardened vertebrae, parts of which became modified near the head to form jaws. More complex sense organs and nervous systems began to arise in fishes. The fins of fishes were the starting point for the legs, arms, and wings seen among higher vertebrates.