Our goal is to understand how neuronal circuits coordinate behaviours extending to whole organ systems or to the entire body. Achieving this at cellular resolution in an entire nervous system is possible by studying small animals amenable to genetic and other manipulations. We are actively developing the marine annelid Platynereis dumerilii as a new system for circuit neuroscience. We use whole-body connectomics, neuronal activity imaging, and behavioural analysis to understand the circuit bases of behaviour in fully mapped, stereotypical circuits. Genome editing and transgenic access to single neurons allow us to link molecular function to network activity and behaviour. I will present recent results on the integration of phototactic and UV-avoidance responses and on a hydrodynamic startle behaviour.