The rapid tail withdrawal of the tubificid worm, Branchiura sowerbyi, was studied using correlated electrophysiological and behavioural analyses. The minimal response latency (i.e. time from onset of mechanical stimulus to onset of withdrawal) was approximately 7 ms, faster than the escape responses of any previously studied invertebrate. Factors contributing to the speed and efficacy of this response include: (1) a sensitive mechanosensory system for detecting potential prey, (2) a short latency for excitation along afferent and efferent pathways, (3) a rapid intersegmental conduction of lateral giant fibre spikes, (4) a short coupling time from muscle excitation to the onset of shortening, and (5) the requirement of only a single lateral giant fibre spike for a complete (all-or-none) response. Videotaped sequences of predator—prey interactions showed that such reflex speed permits effective escape from the strike of the bluegill (Lepomis macrochirus).

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