The medicinal leech, Hirudo medicinalis, responds to surface waves by first orientating and then swimming towards the source of the disturbance. Leeches deduce the source of water motion from both the movement of the waves over the surface of the body and from the movement of light bars produced by differential reflection of light from the surface of the waves. Water movement is detected by motion sensitive cilia distributed over the entire body surface. We describe here a bilaterally distributed population of cells (CBW cells) located in each segmental ganglion, which are sensitive to water movement. These cells have a characteristic morphology within the ganglion in which their cell bodies lie but their axons may project to the adjacent ganglion through the anterior contralateral connective, through the posterior connective or in some cases in both directions. These cells are depolarized by water movement on both sides of the body. The CBW cells are also depolarized by ipsilateral touch cells through electrical synapses, by ipsilateral pressure cells through chemical synapses and are inhibited by contralateral touch cells. The CBW cells thus do not distinguish between ipsilateral touch or bilateral water movement. The role of the CBW cells is discussed.