Literature
- Alexander, R. M. 1975. The chordates. Cambridge University Press, London.
- Barington, E. J. W. 1965. The biology of Hemichordata and Protochordata. W.H. Freeman, NY.
- Margulis, L. & Schwartz K. V. 1988. Five Kingdoms. W. H. Freeman and Company, New York. 376 pp.
- Romer, A. S. 1971. The vertebrate story, rev. ed. University of Chicago Press, Chicago.