This volume comprises a selection of the papers read at the Second International Workshop of the Hydrozoan Society held at the Centre d´Estudis Avançats de Blanes (CSIC), Spain (September 1991). This volume represents an up-to-date overiew of research in hydrozoan biology, especially in its ecological and evolutionary aspects. Through a series of original research papers and selected reviews, the authors provide accounts of the most exiting and challenging recent development. The volume cover a wide range of topics in the area of ecology, evolution, systematics, behaviour, reproduction, nematocysts and zoogeography of hydroids, hydromedusae and siphonophores