Hydra belongs to the phylum of cnidarians which are representative of the earliest nervous systems in evolution. There is renewed interest in the use of Hydra as an animal model since it is now possible to create stable transgenic lines in this species. Accordingly, I created a line that expresses a calcium indicator in neurons and developed tools to study this new preparation. I was surprised by the advantages offered by this animal model when using calcium imaging, as it allows to monitor neuronal activity over entire animals simultaneously, at high temporal resolution and over hours. For the upcoming years, I want to take advantage of this preparation to study how the various circuits that I discovered in the nervous system of Hydra coordinate their activity to control the behavior of the animal. I believe that this approach will help us understand some of the most fundamental principles by which the nervous system is organized.