The cell bodies of the position sensitive units form a row distal to the movement sensitive cells and their dendrites run in pairs in a narrow neck of tissue on the ventral side of the receptor strand. The scolopidia share the features of elongation and relaxation sensitive movement units. Thus the canal cell is absent, but there is some scolopale material in the enveloping cells. Also the scolopale is apposed by a mixture of strand cells and collagen. The more distal scolopidia are found in a region of large haemocoelic lacunae. The physiological differences between movement and position sensitive units could be explained in terms of how well the dendrites are anchored into the tube; with the position cells possibly being held at different levels with respect to their maximum sensitivity. On the other hand, the dendrites of both types of unit may behave identically and, if so, then the necessary physiological differences could occur in the transduction and/or impulse initiation sites. These alternative explanations are discussed.