We investigate the development and function of the chordate nervous system using two models: the amphibian Xenopus and the tunicate Ciona. While amphibians are familiar to everyone, tunicates are primitive chordates and the closest relatives of the vertebrates. While the Ciona larval CNS is very simple (177 neurons), it has homologs of the vertebrate forebrain, midbrain, hindbrain and spinal cord, and its development closely parallels that observed in vertebrates. In our work, we focus on neural tube closure (NTC). In NTC the embryonic CNS transforms from a sheet of cells to a closed tube. This research started with the isolation of a Ciona line carrying a null mutation in a T-type Ca2+ channel (TTCC). The defect in this mutant appears to disrupt one of the final steps in NTC – the sealing of the edges of the forming tube. We have now extended our findings to show that TTCCs have a conserved function in vertebrate NTC. In a second area of research we study the function of the larval Ciona CNS, guided by the recently published synaptic connectome. In our research we use behavioral studies of normal and mutant Ciona larvae to investigate neural circuits underlying behavior.