Student Resources

Chordate Origins - Workbook Questions

What are the other stages, or steps, in Garstang’s theory on the origins of the Chordata?  (p. 100)

Remember that chordates, protochordates (Hemichordata and Urochordata), and echinoderms are deuterostomes, and they share the characteristics that define the taxon: a tripartate coelom, mesoderm formed from enterocoelic pouches, and an oral opening that doesn’t form at the position of the blastopore. Garstang’s theory on the origins of the Chordata is based, in part, on the apparent similarity between larval stages of echinoderms and hemichordates. In both, the larvae have a similar arrangement of surface ciliature, organization of the gut, and are the depressive stage for a sedentary adult. Presumably there were selective pressures that brought about changes in the larva stage increasing its size, longevity, and how it swam. We get a glimpse of these changes in the larval urochordates, which still have a sedentary adult stage in their life cycle. The larval stage dominates the life cycle of the chordate ancestor, and Garstang proposed that pedomorphosis, a form of neoteny, resulted in an animal that resembles today’s cephalochordates. The larval urochordate and the cephalochordates share the chordate characters of pharyngeal gill slits, a notochord, a dorsal tubular nerve cord, and a tail.

Protozoa || Porifera || Cnidaria || Platyhelminthes || Nematoda || Annelida || Mollusca || Arthropoda
Echinodermata || Chordate Origins || Jawed Fishes || Amphibia || Mammalia