Hydra is representative of the most primitive nervous systems. Yet, it turns out that it does not function like its structure (a nerve net) would suggest and there might be more to learn about neurons in hydra than previously thought. This cnidarian offers a convenient preparation for calcium imaging, especially because it is possible to image the entire animal simultaneously. Our experiments using such technique revealed multiple groups of neurons (or conduction systems) that are anatomically distinct and that fire simultaneously, together with a series of neurons that seem to fire independently. Accordingly, we are interested in dissecting the nervous system of hydra and answering the following questions: How many different circuits are there in hydra? What type of computation does each of these circuits do, and to what extent are these circuits dependent on each other? Answering these questions will help understand the properties of the most primitive nervous systems and how they produce the appropriate behavioral response to a given situation.