Asymmetry of the motor system in the hermit crab Pagurus granosimanus Stimpson.

Although much is known about the physiology of the neuromuscular junction and its role in initiating muscular contraction in crustaceans, less attention has been devoted to the central events which select and control contraction. The mechanisms which shape the discharge patterns of motoneurons are best known for the escape reflex of the crayfish abdomen, mediated by giant fibres, and for the neurogenic heart beat in decapods. The latter has provided a model (Maynard, 1955) which has proved useful in the analysis of rhythmic reflexes, and has been applied to explain the formation of repetitive-reflex patterns in a number of preparations. These systems appear to use pacemaker cells which are the sources of excitation linking together various motoneurons. Interconnexions of several pacemakers with one another and with motoneurons (which may also be interconnected one with another) are complex versions of the same general model. The tonic slow flexor system of the hermit crab is similar in many respects to that of its symmetrical relative, the crayfish, which has recently been studied by Kennedy & Takeda (1965 a, b) and Kennedy Evoy & Fields (1966). The functional asymmetry of the hermit crab abdomen might be expected to have imposed changes upon the nervous system, and in particular upon the central mechanisms which maintain the tonic control of the slow flexor system; these changes might lead us to a greater knowledge of these central mechanisms. The hermit crab should provide us with insight into the ways in which natural selection alters a nervous system to a different function from that for which it was originally evolved. The conservative nature of the decapod nervous system was demonstrated in a study of comparative innervation patterns in the limbs of decapods by Wiersma & Ripley (1952). Within the Reptantia there was no variation in the distribution of excitor motoneurons to the various muscles of the pereiopods but some changes had occurred in the patterns of inhibitory innervation. Bush (1963) showed that the reflex resistance to passive movement of the limbs was similar in spite of different patterns of inhibitory innervation. He suggested, however, that the more stereotyped reflex response of brachyurans and anomurans, in comparison with macrurans, might be due to a shift from polysynaptic to monosynaptic reflex control of passive movement. These two studies suggested that an investigation of the reflex control of the abdominal muscles of the hermit crab