Solute and Water Balance in the Muscle Fibres of the East African Fresh-Water Crab, Potamon Niloticus (M. Edw.)

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Abstract
1. An account is given of the normal composition of the muscle fibres of the fresh-water crab, Potamon niloticus. 2. The effect of various dilutions of sea water is considered in relation to the concentrations of a number of the muscle constituents. 3. The water content of the muscle decreases as the blood concentration is increased, but at the higher concentrations the muscle dehydration is not so extensive as expected. There is evidence of a small addition of osmotically active substances to the muscle. 4. Potassium and muscle phosphate compounds behave passively and their concentrations are determined by the muscle water content. 5. The concentration of free amino acids and related compounds may be increased beyond that due to water loss. The addition of these substances to the muscle is not greater than 50-90 mM./kg. water. 6. Sodium enters the muscle when the blood sodium concentration is increased. At blood concentrations not exceeding 450 mM./l. sodium probably only penetrates into a freely exchanging region of the fibre: at higher concentrations sodium may penetrate into the fibre interior. 7. These results are discussed in relation to the adaptability of the cell to increased blood concentrations.