1. Electrical activity recorded in the first roots of the abdominal nerve cord show bursts of activity which accompany rhythmic movements of the swimmerets. These bursts persist when a root is cut distal to the electrodes.
2. Proprioceptive impulses have been recorded from the distal portion of these nerves during passive movements of the swimmerets. They have also been recorded in the connectives during both passive and active movements.
3. Cutting all second and third roots of the abdominal ganglia does not interfere with the rhythmic swimmeret movements of the isolated abdomen. Rhythmic efferent bursts persist in first roots even when only a single first root remains intact in such a preparation.
4. Intermittent bursts are also found in the first roots of a completely isolated cord but their pattern, frequency and phase relationships differ from those recorded in the same roots before isolation was completed. Such bursts are very rare in second and third roots.
5. Stimulation of small bundles of fibres in the circumoesophageal commissure produces well-defined inhibitory and excitatory effects on the discharge of single units in the first roots.
6. One specific bundle found in many preparations, when stimulated at 50/sec., caused a tonic retraction of all the swimmerets which then began to beat rhythmically and continued to do so when stimulation was discontinued.
7. Intermittent bursts in the first roots took place with correct phasing during this stimulation even when the abdominal ganglia were isolated except for their connexion with the last thoracic ganglion. Again, such bursts were absent in the second and third roots.
8. It is concluded that both the inflow from peripheral proprioceptors and intrinsic properties of the central ganglia play essential parts in the co-ordination of the metachronal movements of the swimmerets.
Supported by Grant G-5461 of the National Science Foundation.