Abstract
Multi-cellular animals have evolved an impressive variety of forms or body plans. The evolutionary and developmental processes causing and determining this diversity remain largely unknown and present one of the great challenges in evolutionary biology. Based on recent molecular, morphological, developmental, and palaeontological data, the phylogenetic interrelationships of the major metazoan lineages are currently controversially discussed. In particular, the early evolution of the Bilateria, the largest and most diverse metazoan clade, is far from being settled. Seemingly well established taxa such as Platyhelminthes (flatworms) and Articulata (arthropods and annelids) are challenged and new clades such as Ecdysozoa (arthropods and nemathelminths) and Lophotrochozoa (spiralians and tentaculates) are suggested. These different views on bilaterian interrelationships greatly influence our interpretation of body plan evolution and lead to very different evolutionary scenarios.
Europe