We seek to answer how animal behavior is set up by the collective behaviors of individual cells, over the entire course of brain and spinal cord development. We want to understand how gene activity can instruct developing neurons to move around, change shape, and connect to other cells. To do this, we study the simple larval nervous system of our closest invertebrate relatives, the tunicates. Tunicates, like us, belong to the Chordate phylum, but have very simple embryos and compact genomes. The laboratory model tunicate Ciona has only 177 neurons and is the only chordate with a fully mapped "connectome". We take advantage of this simplicity to understand molecular mechanisms that may underlie human neurodevelopment.