1. Examination of escape behaviour in freely moving animals with chronically implanted nerve cord electrodes has clarified the normal function of the giant fibres and the general organization of escape behaviour in the crayfish.

2. The giant fibres react almost exclusively to sudden stimuli; they do so at very short latency; and they produce rather stereotyped actions. Certain visual stimuli such as very rapidly approaching objects and rostral tactile stimuli such as taps to the anterior carapace or antennae fire the medial giants; caudal stimuli such as taps to the abdomen fire the lateral giants. In free animals even very weak stimuli often excite the giants.

3. Spontaneous tail flips and responses to more gradual stimuli are mediated by non-giant systems. When latencies can be meaningfully measured, they are at least an order of magnitude greater than those of giant-fibre reactions. Visual stimuli such as reaching for an animal or walking past its aquarium and somatic stimuli such as pinching or even cutting typically excite these systems.

4. Animals commonly escape with a sequence of tail flips (swimming). When they do this, the first flip of the sequence is determined as above, but subsequent flips are almost always mediated by non-giant systems. Both swimming and delayed (i.e. non-giant) escape involve a variety of phasic abdominal movements.

5. Experiments with animals whose nerve cords were cut at various levels indicate that non-giant escape either originates in or is strongly facilitated by the suboesophageal ganglion complex.

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