1. Two further co-ordinating systems, the rhythmic potential (or RP) and the pre-locomotor burst (PLB) systems, are described from several species of Hydra.

2. RP impulses may be recorded from anywhere in the column. They are small, distinctive in shape, through-conducted at 4 cm. per sec. and without direct behavioural correlates.

3. RP pacemakers occur throughout the column, but those near the base are the most active. Impulses are initiated endogenously in a more or less regular, rhythmic manner, with a frequency of 1-10 per min.

4. Mechanical or electrical stimulation of the column enhance pacemaker activity and often shift the active loci towards the points stimulated. Starvation depresses activity after several days.

5. Moderate illumination has a dual effect on RP pacemakers: it causes almost immediate inhibition of the currently active locus, and then an extended period of rhythmic enhancement triggered by basal disk pacemakers. Both effects show the same spectral sensitivity with the greatest sensitivity to blue light and total insensitivity to the red. RP conduction is unaffected by light.

6. Chloretone affects first the contraction-burst system and the epidermal muscles, then endogenous RP pacemaker activity, while RP responsiveness and conduction are still unaffected. These effects are reversible.

7. RP impulses can first be detected in still-attached buds, but their rhythms are slower, less regular and not light-sensitive.

8. The PLB system resembles the RP system in the character of its impulses, and the contraction-burst system in its occasional bursts of activity. PLB pacemakers are in the subhypostomal region. These impulses precede (and presumably cause) the distinctive locomotor contraction bursts.

9. It is concluded that the RP system consists of two distinct components, the pacemakers and the conducting network; it is suggested that the pacemakers are protoneurons and that both RP and PLB conduction is myoid.

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