1. Command interneurons from the circumoesophageal connectives of crayfish which are capable of evoking various cyclical, phasic behavioural outputs are described. By pooling evoked-behaviour data and information on command-fibre location for a number of preparations it has been possible to repeatedly identify certain individual interneurons.
2. Five escape or swimming-command identities have been established. Electrical stimulation of any of these elements activates the fast abdominal musculature, generating single or multiple tail-flips.
3. Three forward-walking identities have been established within the experimental connective. Stimulation of a forward-walking command fibre results in extension of the abdomen and chelipeds in conjunction with locomotory movements of the walking legs. Output has been characterized as walking by virtue of the fact that the walking wheel is actually rotated by the stepping appendages.
4. Four backward-walking identities have been established. For these command elements a variety of cheliped and abdominal geometries are superimposed on the basic backward-stepping patterns of the walking legs.
5. Cyclical command fibres capable of evoking branchiostegite scratching, feeding-like activity, cleaning of the dorsal aspect of the abdomen, and slow rocking of appendages have also been noted.