What are the ‘Calcichordata’? and the larger question of the origin of Chordates

Argument around the question of the validity of the ‘Calcichordata’ emphasizes the need for a more functional and utilitarian view of the origin of the chordates. The calcichordate question is reviewed and ‘tested’ in terms of our usual ideas of biological reality. The concept is rejected and the Stylophora are viewed as aberrant and still largely unknown echinoderms. Rejection of the calcichordate concept turns us once more to the question of the origin of the chordates. A number of competing hypotheses are compared: the tunicate, dipleuruloid and archicoelomate models. The conclusion is that the dipleuruloid, a simple metazoan, offers the least explanatory difficulties. Also it is concluded that discussion of such a subject as this refines and improves our thinking concerning it.