SUMMARY

1. The activity of swimmeret muscles of the lobster Nephrops norvegicus during beating in the upright and tilted animal has been examined. The responses to tilt are produced primarily by stimulation of the statocysts. 2. The anatomy of the swimmeret muscles is described. Although essentially similar to previous descriptions, important new aspects are presented. 3. The arrangement of the main powerstroke and returnstroke muscle groups in relation to the peg-and-hook articulation of the swimmeret produces a segregation of action, with different muscles contributing progressively to force production. 4. The auxiliary muscles of the basipodite, M9, MIO and M13 act to twist the swimmeret laterally. The auxiliary muscle Ml 1-12-14—15 acts to maintain a rearward powerstroke. 5. The innervation of swimmeret muscles and the location of their motoneurone cell bodies in the abdominal ganglia have been revealed by cobalt staining. There is a clear segregation of powerstroke and returnstroke motoneurones. Intraganglionic fibre tracts as well as four interganglionic fibres are identified