1. Measurements have been made of the inorganic ion and free amino acid content of a number of lamellibranch muscles. The volumes of extracellular fluid in the muscles have also been determined so that the intracellular concentrations can be calculated.

2. The fast portion of the adductor muscle of Pecten contains about 160 mM. K/kg. fibre water and only 43 mM. Na/kg. fibre water and 34 mM. Cl/kg. fibre water. The potassium and chloride are approximately in a Donnan equilibrium with the potassium and chloride in the blood.

3. In the slow portion of the adductor muscle of Pecten and in the two parts of the adductor and in the byssus retractor of Mytilus, the concentrations of potassium in the fibres are from 150 to 160 mM./kg fibre water, of sodium 73 to 95 mM./kg. fibre water and chloride 94 to 152 mM./kg. fibre water. The potassium and chloride in the fibres are not in a Donnan equilibrium with the potassium and chloride in the blood.

4. The fast and slow fibres of the adductor muscles of Anodonta contain 21 and 17 mM./kg. fibre water of potassium respectively, 5·3 and 7·2 mM./kg. fibre water of sodium, and 2·4 and 5·4 mM./kg. fibre water of chloride. The potassium and chloride in the fibres is not in a Donnan equilibrium with the potassium and chloride in the blood.

5. The fast fibres of Mytilus adductor contain 295 mM./kg. fibre water of free amino acids and 39 mM./kg. fibre water of acid-soluble phosphate compounds. The fast fibres of Anodonta adductor contain only 11 mM./kg. fibre water of amino acids and 19·8 mM./kg. fibre water of phosphate compounds.

6. Mytilus muscles fibres adapt to a reduced blood concentration, partly by an increase in water content and partly by a reduction in the sodium, chloride and free amino acid content.

7. Anodonta muscle fibres adapt to an increased blood concentration, partly by a reduction in the water content and partly by an increased sodium and chloride content.

8. The significance of these results is discussed. It is concluded that the ionic contents of the lamellibranch smooth muscles are consistent with equilibria systems in which the permeability to sodium is significant compared with the permeability to potassium and in which both a sodium and a potassium pump operate.

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