How many and what are the body cavities? How often do they appear through phylogeny and how? How many and what are their functions? How valuable are the different theories so far elaborated by the scientists? Therefore, we decided to call a meeting of some Italian and foreign zoologists, all experts in some way in phylogeny and investigators of the body cavities, to hear about their investigations and ideas and to discuss with them this fascinating problem. The problem is certainly wast and heterogeneous and must be compared with different types of thinking and different methodologies, from classical embryology to molecular analysis, from structural and ultrastructural anatomy to studies of the functions, from critical reviews of older and new theories to the modern and futuristic views that are possible today because of the rapid development of biological knowledge, from a knowledge that might be obtained from studies of animals that are living today to interpretations of ancient Cambrian and Pre-Cambrian fauna.