Sponges in time and space : biology, chemistry, paleontology : proceedings of the 4th International Porifera Congress, Amsterdam/Netherlands, 19-23 April 1993 / edited by Rob W.M. van Soest, Theo M.G. van Kempen, Jean-Claude Braekman ; assisted by Anita Brink, Frans R. Bianchi, Jan J. Vermeulen