The motor innervation of the segmental musculature of nereid polychaetes appears to be both polyneuronal and multiterminal, which closely resembles the condition found in the Arthropoda. The cell bodies of the motor axons lie in the cord and terminate in endings of the 'en grappe' type on the surface of the muscle fibres. These observations provide an anatomical basis for the experiments that showed the muscles were capable of fast twitch-like contractions associated with the escape response and slower graded contractions normally seen during the ambulatory cycle of the parapodia. The sensory equipment of the parapodia includes several prominent neurons which function as rapidly adapting stretch receptors. Some of these are activated by the movements of the parapodia and body wall during locomotion and have inputs to the giant fibres and to an unpolarized network of sensory fibres within the cord. These receptors may serve to integrate the activity of individual segments and ensure co-ordinated flexibility of the movements of the worm as a whole.