The pilidium is a fascinating planktonic larva found in one group of marine nemertean worms. In this group of worms the development is maximally-indirect. This means that the two life history stages, the planktonic larva and the benthic adult worm, are built and function entirely differently. The young worm forms inside the pilidium larva over the course of many weeks, then erupts from the larva during catastrophic metamorphosis, and promptly devours the larval body. Many marine invertebrate animals have similarly indirect development. In many of those the development has been abbreviated or simplified, giving rise to direct development, in which the larval stage, if present, resembles the adult. In nemerteans, however, it is clear, that indirect developmental mode evolved from ancestral direct development. This project aims to investigate what changes in development contributed to the evolution of the pilidium larva. The general approach is to compare fates of homologous embryonic cells and expression of developmental regulatory genes in nemertean species with and without the pilidium. The results will clarify the evolutionary origin of the different domains of the larval body and the adult rudiments inside the pilidium, and shed light on how the antero-posterior axes of the body plan is established in nemerteans with direct and indirect development. This project is expected to contribute significantly to central issues in evolutionary developmental biology, such as the origin of novel animal body plans. This project provides training and research opportunities for diverse undergraduate and graduate students, including community college students, many of which are from underrepresented groups. Finally, this project, by maintaining a molecular lab and advanced microscopy facility at a marine biological field station, expands its research and teaching capacity and serves visiting scientists from other institutions.