This book embraces general problems of animal classification of animals and new information on their molecular sequences.

1. Origin of metazoans: a phylogeny deducted from sequences of the 28S ribosomal RNA R. Christen, A. Ratto, A. Baroin, R. Persasso, K. Grell and A. Adoutte; 2. Major factors in the rapidity and extent of the metazoan radiation during the Proterozoic-Phanerozoic transition J. W. Valentine; 3. Divergence and persistence of highly ranked taxa R. R. Strathmann; 4. Problematical taxa: a problem for biology or biologists? S. Conway Morris; 5. Metazoan evolution around the Precambrian-Cambrian transition J. Bergström; 6. Cladistic analysis of metazoan phyla and the placement of fossil problematica F. R. Schram; 7. Is fossil evidence consistent with traditional views of the early metazoan phylogeny? J. Dzik; 8. Lower Cambrian lagerstatte from Chengjiang, Yunnan, China: insights for reconstructing early Metazoan life Chen Jun-Yuan and D. D. Erdtmann; 9. Middle Cambrian biotic diversity: examples from four Utah Lagerstatten L. A. Robison; 10. Extant 'problematica' within or near the Metazoa G. Haszprunar, R. M. Riefer and P. Schuchert; 11. On the origin of the Bilateria: traditional views and recent altneratuve concepts R. M. Rieger, G. Hazprunar and P. Schuchert; 12. Problematic reef-building spongers R. A. Wood; 13. Beach and laboratory experiments with the jellyfish Aurelia, and remarks on some fossil 'medusoid' traces D. L. Bruton; 14. Early Cambrian medusiform fossils from Chengjiang, Yannan, China Sun Weiguo; 15. The enigma of conulariid affinities L. E. Babcock; 16. Evolutionary affinities of conulariids H. Van Iten; 17. Functional morphology of the Class Helcionelloida nov., and the early evolution of the Molusca J. S. Peel; 18. The arthropods of the Lower Cambrian Chengjiang fauna, with relationships and evolutionary significance Hou Xian-Guang and J. Bergström; 16. Early Palaeozoic arthropods and problems of arthropod phylogeny; with some notes on taxa of doubtful affinities A. M. Simonetta and L. Delle Cave; 17. New observations on the thylacocephala (Arthropoda, Crustacea) A. Alessandrello, P. Arduini, G. Pinna and G. Teruzzi; 18. Fossil Acarida: contribution of palaeontolgical data to acarid evolutionary history F. Bernini; 19. Are Protura really insects R. Dallai; 20. The Tully Monster and a new approach to analyzing problematica J. S. Beall; 21. Problematica-incertae sedis E. L. Yochelson.