The nervous system of the nematode worm Ascaris contains about 250 nerve cells; of these, the motoneurons consist of five segmental sets, each containing 11 cells. Morphologically, the motoneurons can be divided into seven different types. Their geometry is simple: some are unbranched, others have one branch point, and the most complex have two. There is no neuropil in the nerve cords; synapses are made by axo-axonal contact or onto short spines. These features enable us to study the anatomy and physiology of the system with a degree of completeness that would be difficult in other systems. The physiological activity of five of the motoneurons has been investigated, three being excitatory and two inhibitory. The excitatory motoneurons receive input from intersegmental interneurons. The inhibitory motoneurons do not receive input from the interneurons; instead they receive their input from the excitatory motoneurons in a circuit that can mediate reciprocal inhibition between the dorsal and the ventral musculature.