Summary

1. 1. Body contraction in the sabellid worm, Pseudopotamilla occelata, during the rapid withdrawal reflex occurred only in the anterior half of the body. End-to-end shortening was never observed. The longitudinal muscles are well-developed in the anterior half, and poorly developed in the posterior half.

2. 2. Conduction of action potentials along the giant axons was blocked at the midbody, and was responsible for the anteriorly restricted body contraction.

3. 3. Electrophysiological and histological studies excluded the possibility that conduction block resulted from a safety factor attributable to the special geometry of the axons.

4. 4. Current injection across the giant axon membrane in the region of the conduction block indicated that changes in the properties of the membrane were responsible for the conduction block.