Figure 1.

The many forms of cellular excitability. (a) Stochastic or non-oriented navigation strategies (e.g. prokaryotic chemotaxis), (b) sensing by spatial comparison (e.g. amoeboid chemotaxis), (c) sensing by helical klinotaxis (e.g. flagellate phototaxis), (d) cell–cell recognition as a prelude to fusion (e.g. ciliate conjugation and gametic fusion in Chlamydomonas), (e) active feeding by selective engulfment of prey organisms, (f) mechanosensitivity and flow interactions, and (g) ultrafast escape responses and reversal of ciliary beating by action potentials. (Online version in colour.)