Abstract 1. 1. The superficial flexor muscles of Callianassa californiensis are divided into two layers of muscle fibers which differ in contraction speed. 2. 2. The flexor motor system consists of five excitatory neurons and one inhibitory neuron per half segment. 3. 3. The two layers of muscle are served by different subsets of the excitatory neurons whose physiological properties are matched to the contractile properties of the muscle fibers they innervate. 4. 4. Serially homologous neurons in Callianassa differ in size and physiological properties. 5. 5. Flexor neurons in Upogebia pugettensis and Procambarus clarkii differ from their homologs in Callianassa in physiological properties, in central connections and in peripheral connections.