1. The heart of Palinurus lalandii and the ventricle of Octopus horridus maintain their normal rhythm for long periods in solutions containing only sodium and calcium chlorides.

2. The removal of calcium from the perfusion fluids causes systolic arrest in both hearts.

3. Both hearts are sensitive to very low concentration of potassium when perfused with potassium-free fluid.

4. Excess of magnesium and strontium produce the same effect as excess of calcium, but magnesium cannot be successfully substituted for calcium in the perfusing fluid.

5. The physiological range of H ion concentration is determined for the two preparations.

6. Both hearts have low temperature optima, and the high limit of their range is in the neighbourhood of 20° C.

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