The Evolution of Vertebrates

Hemichordates ( "acorn worms" ) appear to be intermediates between echinoderms and chordates because of their gill-slitted pharynx and dorsal tubular nerve cord. 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. The nerve cord expanded into a brain.

More complex sense organs and nervous systems began to arise in fishes. Paired fins evolved, and gave rise to limbs. The fins of fishes were the starting point for the legs, arms, and wings seen among higher vertebrates.

Gills evolved, and then gave rise to lungs. Gradually, there was less reliance on gills and more on lungs and the circulatory system (heart and blood vessels).

REVIEW: A shift from reliance on _ to reliance on _ was pivotal in the evolution of all vertebrates.
a. the notochord; a backbone
b. filter feeding; jaws
c. gills; lungs
d. all of the above