MUSCULAR ACTIVITY UNDERLYING VENTILATION AND SWIMMING IN THE HORSESHOE CRAB, LIMULUS POLYPHEMUS (LINNAEUS)

¹ Department of Biological Sciences, University of California, Santa Barbara, California 93106 and the Marine Biological Laboratory, Woods Hole, Massachusetts

1. Ventilatory and swimming movements in Limulus polyphemus are described and the underlying muscular activity is analyzed.

2. Ventilatory and swimming muscle activity in gill plates consists of repetitive, metachronal bursts in both extensor and flexor muscles beginning in the caudal gill plate muscles and proceeding sequentially forward.

3. Ventilatory muscle bursts are approximately symmetrical, being of relatively long duration, small amplitude and long intersegmental delay, affecting exclusively the gill plate musculature.

4. Hyperventilation involves the active participation of the genital operculum in the rhythmic cycle. Rapidly decreasing intersegmental delay, increasing muscle burst amplitude and transition in the muscle burst shapes accompany decreasing interburst intervals.

5. Swimming movements consist of comparatively high frequency, large amplitude excursions of the gill plates, genital operculum and, to various degrees, the walking legs. Metachronous, high amplitude, square shaped muscle bursts, which proceed rostrally with a short intersegmental delay, are responsible for these propulsive movements.

6. Swimming movements of the gill plates require descending information from the brain, whereas the neural circuitry required for ventilation and hyperventilation is endogenous to the opisthosomal ganglia.

7. The pacemaker function for swimming and ventilatory movements may be served by any of the five involved opisthosomal ganglia if its caudal connectives are severed.

8. The ventilatory coordinating mechanism is not polarized; whichever ganglion functions as the pacemaker, the wave of excitation propagates both rostrally and caudally, sequentially exciting neighboring ganglia.