In vertebrates all inhibition of skeletal muscle has its seat of action in the central nervous system. Therefore the study of inhibition is complicated by the intricacy of central nervous organization. In certain invertebrates the conditions are simpler. Stimulation of the claw of the crayfish has been shown to cause opening or closing, depending on the strength of stimulus applied, the phenomenon involving a reciprocal inhibition. The authors studied this type of inhibition on the lobster (Homarus), the blue crab (Callinectis) and the spider crab (Libinia). It does not appear to have been previously recognized that the selective excitability and reciprocal inhibition is not limited to the claw muscles but is general in the appendages of arthropods. At least as far as the authors have examined, the various joints which are moved by antagonistic flexor and extensor muscles may each be flexed or extended depending on the character of the stimulus. Peripherally produced summation, rhythm, delay, arrest, reversal and alternation, rebound and after-contraction, are described. Superficially, at least, these resemble similar phenomena associated with central conduction in vertebrates.