1. A technique of recording electrical activity from an intact, essentially normal specimen of Hydra is described.

2. The existence of regularly recurring co-ordinated longitudinal contractions of the ectodermal muscles is confirmed and re-emphasized.

3. Such contractions are found to consist of a patterned series of individual coordinated contractions, each preceded by a single large, compound potential. The overall contraction, consisting of a variable number between 5 and 12 or more contractions depending on species, is thus called a contraction burst.

4. These contraction-burst potentials originate endogenously; they are considered to be the most important effector activity in Hydra.

5. Contraction-burst potentials originate in the hypostome and are conducted throughout the column at approximately 15 cm./sec.

6. Contraction-burst patterns have been studied quantitatively in two species, showing interspecific differences between both regular contraction bursts and those associated with locomotion.

7. Certain extrinsic and intrinsic variables affect contraction-burst frequency. Daylight, and nutritional state, both modify this rate, with the former giving rise to a circadian activity cycle under natural conditions.

8. Single electric shocks usually cause a single co-ordinated muscle contraction. Such stimuli can markedly reduce endogenous contraction-burst activity.

9. Sudden illumination interrupts contraction bursts temporarily, even halting those in progress. Blue light is most effective. This stimulus has been used as a tool to investigate the properties of the pacemaker systems concerned with contraction bursts.

10. The nature and properties of these pacemakers is discussed.

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