The Protochordata or Prochorda are simpler forms of the chordate animals in which the notochord is wholly or partially formed and the vertebral column and skull are lacking (hence Acrania). This division includes a heterogeneous (polyphyletic) group of animals of the phylum Chordata, most of which are fairly related to the vertebrates but none possesses the full complement of vertebrate characters.
The Protochordata are divided into two classes: (i) the Tunicata (or Urochordata) or ascidians which in the adult forms are entirely different from the vertebrates, and (li) the Cephalochordata, a group containing various species of Amphioxus (-Branchiostomum). a small animal, which looks fish-like, but has no fish-like skeleton brain or eyes. A third group, the Hemichordata, was formerly included among the Protochordata. It comprises worm-like forms, whose vertebrate affinities are much more remote. Bateson (1885) included the Hemichordata in the phylum Chordata, a classification that has largely been followed in Zoology texts upto now. In many elementary books this practice still continues. But many French and German authors have unequivocally rejected this view. Hyman (1959) has separated this from chordates and placed it alongwith enterocoelous coelomates between Chaetognatha and Progonophora. In the present book the Hemichordata is being described as an appendix to the Protochordata.
The present system of the classes of the protochordates is different. As is evident from the above the hcmichordates, represented by the pterobranchs and enteropneusts occupy a controversial position and exhibit no more than a convergent resemblance to chordate organization. Amphioxus, on the other hand, is a degenerate form, and is not a satisfactory vertebrate prototype. Ascidians, through their larval forms, then constitute the actual stock from which the chordates have emerged.
Therefore, ascidians are being described first and Amphioxus following that. Although it is now, more or less, established that the hemichordates have no real relation with chordates.