The emergence of the endomesoderm in metazoan evolution was fundamental for the morphological diversification of animals. It promoted interactions between different tissues that led to novel structures and organs, such as the heart, bones, and kidneys. Drosophila and C. elegans are the most well studied models within the Ecdysozoa, the largest group of invertebrate animals, but show evolutionary divergent mechanisms of endomesoderm formation. Deciphering the ancestral ecdysozoan condition requires models that form endomesoderm in a manner more representative of the clade. The marine priapulid worms are distantly related to arthropods and nematodes and thus occupy a phylogenetic position that is key to reconstructing the ancestral ecdysozoan body plan. This proposal aims to generate a detailed description of the development of the priapulid Priapulus caudatus, characterize endomesoderm specification, and understand its relationship with the establishment of the priapulid body plan. I will apply advanced cell-lineage tracing methodologies (4D-microscopy and blastomere injections) and analyze the function of core molecular components of the conserved endomesodermal gene regulatory network. This project will produce an in-depth morphological and molecular characterization of priapulid embryogenesis, thus offering new insights into the origin of the Ecdysozoa and the global evolution of animals. The project involves genuine mobility and will promote and enhance the scientific cooperation between Member States and Associate Countries at all levels, thereby increasing the Europe’s reputation as a center of excellence for the analysis of evolution and development. The proposed research will have a significant impact on my scientific career, fostering my development as a visible, independent leading researcher in the European Research Area.