1. Action potentials were observed in cerebrovisceral connectives of a marine bivalve, Mytilus edulis L., and two species of freshwater bivalve, Anodonta cataracta and Elliptio complanata.

2. In Mytilus nerve, responses to stimulation in both the intact and the de-sheathed nerve were blocked after two minutes in a sodium-free saline. Restoration of normal sodium levels reversed the block within 5 min.

3. In nerves of the freshwater species, Anodonta and Elliptio, the action potential of the sheathed nerve altered very slightly after one hour of stimulation in a sodium-free solution. The completely de-sheathed nerve, however, was blocked within one minute in sodium-free saline. The block was rapidly reversible.

4. It is concluded, contrary to conclusions of previous investigators, that the neural sheath in freshwater clams is just as vital to restriction of sodium loss as it is in amphibia and insects, and it would seem logical to seek out a common denominator of sheath function. Several possible mechanisms are put forth and discussed.

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