1. The decalcification of the abdomen and the asymmetry of the central flexors is a ‘hydrostatic’ mechanism by which the abdomen is expanded to fit the shell tightly.
2. The single pair of giant fibres is homologous to the medial giants of the crayfish except that no contralateral activation of the flexor muscles was observed.
3. Two kinds of asymmetry in the motor system were observed; (a) loss of motor fibres in the first ganglionic roots of the right side; (b) greater frequency of ‘spontaneous activity’ in the slow flexor branch of the third roots on the left side.
4. Sensory stimulation evoked different sets of fibres in these third root branches; one set fired spontaneously even after total ganglionic isolation, the other, when reflexly activated, reciprocally inhibited the first group.
5. Antidromic and synaptic inputs failed to re-set the spontaneous activity; two kinds of reflex activation, phasic and tonic suggest two different sites for inputs, one to the motoneurons, the other to a pre-synaptic pacemaker.
6. Alternations of the two reflex groups occurred in the totally isolated ganglion; addition of picrotoxin to the saline abolished reciprocal inhibition but variations in the frequency of spontaneous activity persisted.
7. A possible mechanism for this system is proposed.