Understanding the evolution of the diversity of animal forms is one of the major tasks in biology. The use of molecular biology led to a better understanding of how evolutionary changes in the development can result in changes in the body plan. Modifications of the wiring between genes caused the adoption and co-option of gene regulatory networks that shape the morphology of the adult during the development. To get a better picture of how these changes led to evolutionary novelties one has to investigate animals that are representatives of key-nodes in the animal tree of life. A fundamental innovation of complex animals is the third germ layer – the mesoderm, that is characteristic for all bilateral symmetric animals that include flies, worms and humans. The mesoderm gives rise to important organ systems such as skeletal elements, musculature, heart and kidneys. Recent insights gained from studies of the development of cnidarians – the most closely related group of the bilaterians – suggest, that the mesoderm evolved from the internal germ layer the endoderm. Comparative studies between animals can reveal how the gene regulatory network that specifies the endoderm of cnidarians has been changed so that it gives rise to a novel germ layer, the mesoderm. The ideal organisms to investigate for answering this question are the simple acoel worms, which form the earliest evolutionary branch of all bilaterian animals. The proposed research project has the aim to investigate the gene regulatory network that specifies the endoderm and mesoderm of the acoel species Convolutriloba longifissura using modern molecular tools. By describing the detailed temporal and spatial expression of key regulatory genes and a following functional analysis important information will be gained that delivers the basis for a comparisons to identify the changes in the gene regulatory network that promoted the evolution of three germ layers and thus the evolution of more complex animals.