1. The chordotonal organ, CB, spanning the coxo-basal joint in walking legs of the crab, not being attached to a muscle tendon like the receptors of the more distal joints, is stretched during extension of the joint (i.e. limb depression) and relaxes during flexion (elevation).
2. Afferent impulses have been recorded extracellularly from whole sensory nerves of CB, and from fibres isolated from it: (a) during passive movement of the coxo-basal joint, with CB in situ in a preparation containing no other joints, and (b) during stretching and releasing of the excised receptor strand.
3. Unidirectional movement fibres and unidirectional position fibres for both directions were recorded. Thus, of the afferents responding primarily during movement, with a constant frequency for steady movement over the whole joint arc, some responded only during extension (or stretch of receptor), others only during flexion (release). Similarly, distinct fibres responded tonically in the extended positions of the joint (stretched), and others in the flexed (relaxed) positions.
4. Fibres intermediate between true movement and true position fibres, and phasic movement--or ‘acceleration’-- fibres, were also observed.
5. Since the scolopidia of CB are isodynal, with only ciliary-type distal processes (Whitear, 1962), the simple hypothesis that the ciliary cell responds solely to relaxation of the receptor strand--and the paraciliary cell to stretch--is now untenable. Alternative hypotheses on the mechanism of activation of crab chordotonal organs, and on the function of CB in the intact animal, are discussed.