Postural control of shell position by the abdomen of the hermit crab, Pagurus pollicarus. III. Analysis of movements and calculations of forces exerted by the muscles

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Abstract
Experiments with a transparent plastic snail shell indicated that the hermit crab carries the columella in the crook of its bent abdomen. There are three components to the normal curvature of the abdomen in the region of the third segment, a ventroflexion of 30°, a pitch to the right of 10°, and a clockwise rotation of 20° of the whole abdomen about the thoracic‐abdominal junction. The forces and bending moments produced by the ventral superficial muscles were estimated from the sizes of the muscles, their position on the circumference of the abdomen, the amplitudes of the EJPs of the motoneurons, and their firing frequency. These calculations are in reasonable agreement with the behavioral measurements.