1. Several discrete postures have been generated by electrical stimulation at frequencies of 20-75 Hz of command interneurons within the circumoesophageal connectives of the crayfish. The behaviour patterns were recorded by cine photography. The similarity of the patterns in different individuals generated by stimulation of interneurons found in the same positions within the cord, strongly suggested that certain of the interneurons examined in this study were identical.

2. A number of tonic command fibres were found, each of which evoked a specific body geometry. Tonic command units were classified on the basis of evoked cheliped, walking leg, and abdominal positions. The study concentrated on those command fibres which generated spatially distributed behaviour patterns involving both abdomen and appendages, and neglected command elements with more restricted outputs.

3. The types of tonic command fibres characterized include the following: bilateral appendage promotion, ipsilateral appendage promotion, bilateral cheliped lift, ipsilateral appendage lift and flex, general flexion, abdominal extension, and both ipsilateral and contralateral turning.

4. A movement-suppressing command fibre, ultimately termed the ‘statue fibre’ was found twice. Stimulation of this element terminated spontaneous activity by ‘freezing’ the animal, irrespective of position. An additional consequence of stimulation of the statue fibre was the overriding of the excitatory effect of mechanical stimulation which in the absence of the command drive would provoke the animal into activity.

5. It is suggested that a restricted number of tonic command fibres could be functionally operative in the generation of their corresponding behaviour patterns. For the majority of tonic command fibres, however, it is felt that the unit is merely one of a group of elements each of which is activated simultaneously or sequentially for discrete behavioural performances.

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