Postural control of shell position by the abdomen of the hermit crab, Pagurus pollicarus. I. Morphology of the superficial muscles and their nerves

As in other decapod crustacea, the muscles of the hermit crab abdomen are divided into a central mass of muscle, active during the escape response, and a superficial layer of muscles which is responsible for the normal postural movements of the abdomen. The ventral superficial muscles consist of three layers, a mesial layer of longitudinal fibers, and two distal layers of circular muscle. The longitudinal muscles are thicker on the right side and are divided into three longitudinal strips each innervated by a single excitor motoneuron. Six motor axons innervate the ventral superficial muscles on each side — those on the right are larger than those on the left. Also occurring in the third ganglionic root is a group of fine axons believed to be branches of the motoneurons. There does not appear to be any relationship between the size of the motoneuron and the amount of muscle innervated.