Excitable mechanics embodied in a walking cilium*

Rapid transduction of sensory stimulation to action is essential for an animal to survive. To this end, most animals use the sub-second excitable and multistable dynamics of a neuromuscular system. Here, studying an animal without neurons or muscles, we report analogous excitable and multistable dynamics embedded in the physics of a 'walking' cilium.  We begin by showing that cilia can walk without specialized gait control and identify the characteristic scales of spatio-temporal height fluctuations of the tissue. With the addition of surface interaction, we construct a low-order dynamical model of this single-cilium sub-unit.  En route to an emergent model for ciliary walking, we demonstrate the limits of substrate mediated synchronization between cilia. In the desynchronized limit, our model shows evidence of a multi-stability mediated by the crosstalk between locomotive forcing and height. The out-of-equilibrium mechanics directly control the locomotive forcing of a walking cilia bypassing the role of the synaptic junctions between neurons and muscles. We show a minimal mechanism -- trigger waves -- by which these walking cells may work together to achieve organism-scale collaboration, such as coordination of hunting strikes across 10⁵ cells without central control.