Machemer H, Bräucker R, Machemer-Röhnisch S, Nagel U, Neugebauer DC, Weskamp M (1998): The linking of extrinsic stimuli to behaviour: roles of cilia in ciliates.

Behaviour in unicellular organisms occurs under unique conditions. The structural organization of receptors and effectors is subject to spatial restrictions. Limited energy resources would not allow for wasteful mechanisms. On the other hand, the distances to be covered by intracellular signals are small as compared to multicellular organisms. Why did a stimulus-coupled behaviour evolve at the level of a single cell, and how can it work? Part of the answer was given by eukaryotic evolution: there was obviously no survival of a species without adequate cellular responses to meaningful, often dangerous, stimuli such as heat, pH, light or gravity. Thus, interactions between stimulus and behavioural response have presumably been invented in conjunction with the eukaryotic organism. Looking today at modern descendants of those unicellular predecessors, the ciliates, we see some essential features of behavioural organization: simplicity, parsimony, quickness, plasticity, all of which are likely to be ancestral properties. This paper summarizes some answers and continuing questions to sensorimotor coupling in ciliates, and casts some light on the transduction and behavioural processing of one of the oldest stimuli, gravity.