Ascaris suum has a nervous system that is very simple both numerically and morphologically. It comprises only 298 neurons almost all of which are extremely simple in shape. Extensive anatomical descriptions of the morphology of neurons and of their synaptic connections, together with the study, by using intracellular recording techniques, of their physiological properties, have led to a prediction of how the motor nervous system controls behavior. Subsequent discovery of endogenous neuropeptides that have potent activity on subsets of the motor neurons suggests that the description of the motor circuitry is more complex than is apparent from its anatomy.