How to get from larval echinoderm to chordate?
a. Increase size of larvae: improve escape from predation & becoming established on substrate once metamorphosis begins. b. Surface ciliary propulsion would not keep up with increase in volume/mass, which would favor development of alternate locomotor system: segmental musculature, elongated body, stiffened support rod (notochord). c. Increase in volume would also necessitate feeding change from ciliary to pharynx w/slits. d. Larval echinoderm –> chordate tadpole: paedomorphosis (attainment of sexual maturity in the larva. 3. Problems: a. Existing chordates should not be viewed literally as examples of ancestral forms. b. Urochordate (ascidian) tadpoles were specialized for relatively short (safer) planktonic life; doubtful that reverse selection would prolong this stage. c. Structural problem: intestine of ascidian tadpole opens into atrium; not homologous to intestine of other chordates such as amphioxus. Also, ascidian tail musculature is not segmental. Ascidians seemed to be evolv
