1. The reflex role of the crayfish abdominal muscle receptor organ (MRO) was studied in intact preparations under relatively physiological conditions by electro-physiological and photographic methods.

2. By separately stretching the receptor muscles of the two receptors in each half-segment it was found that discharge of the tonic MRO is sufficient to evoke activity in the tonic superficial extensors but has no effect on the deep twitch extensors. No effect on either muscle group was produced by activity in the phasic MRO.

3. Discharge of the tonic MRO was found to activate only one of the five motoneurones supplying the superficial extensors. This motoneurone is generally tonically active and has a low threshold to tactile input. Evidence is given that this motoneuronedoes not innervate the receptor muscle of the tonic MRO.

4. The excitatory effect of tonic MRO output on ipsilateral superficial extensor motoneurones is only one-tenth as great in adjacent segments as in the same segment. The effect contralaterally in the same segment is negligible.

5. Simultaneous electrical recording of tonic MRO output with photographic recording of abdominal position proves that the response of the tonic MRO is a function of the difference between actual segment length and some centrally deter-mined ‘desired’ length. Evidence is presented supporting the concept that the tonic MRO functions as an error detector in a follow-up length servo control system which governs the posture of the crayfish abdomen. A model of this system is presented anddiscussed.

Supported by grants (to Dr Donald Kennedy) from the U.S. Public Health Service (B-2944) and the U.S. Air Force Office of Scientific Research (AF-AFOSR).