Historical introduction, overview, and reproductive biology of the protochordates

This issue of the Canadian Journal of Zoology exhaustively reviews most major aspects of protochordate biology by specialists in their fields. Protochordates are members of two deuterostome phyla that are exclusively marine. The Hemichordata, with solitary enteropneusts and colonial pterobranchs, share a ciliated larva with echinoderms and appear to be closely related, but they also have many chordate-like features. The invertebrate chordates are composed of the exclusively solitary cephalochordates and the tunicates with both solitary and colonial forms. The cephalochordates are all free-swimming, but the tunicates include both sessile and free-swimming forms. Here I explore the history of research on protochordates, show how views on their relationships have changed with time, and review some of their reproductive and structural traits not included in other contributions to this special issue.