When one taps on the side of the substrate (ie, dirt) that a nematode is moving across, it stops, reverses direction, then resumes moving forward in a different direction. This is known as the nematode tap withdrawal reflex.
This is of interest because nematodes in general, and Caenorhabditis elegans in particular are a favorite subject of biological study.
The tap withdrawal reflex in C. elegans has been studied extensively. The connectivity of the neural circuitry responsible for the reflex has been mapped, and the polarity of the connections in the network has been investigated somewhat successfully via lesion studies and simulations.
References:
Wicks, S.R., Roehrig, C.J. and Rankin, C.H. (1996). A dynamic network simulation of the nematode tap withdrawal circuit: Predictions concerning synaptic function using behavioral criteria. Journal of Neuroscience 16:4017-4031