Abstract
1. 1. Sodium balance was described quantitatively in the crab Pachygraspsus crassipes Randall. In order to obtain this description, a method was developed which enables Na movements between a crab and its environment to be measured. 2. 2. Crabs in an intermold condition exchanges their internal sodium with the sodium of the external sea water at a rate of 20 μequiv. Na/g/hr. About 15 per cent of the total internal sodium does not exchange during a 96 hr period. The 85 per cent portion which does exchange during this period exhibits the kinetics of a two-pool system. 3. 3. The effect of body size on sodium exchange = 40 μequiv.Na/g/hr (weight) 0.77. 4. 4. Crabs in a soft-shelled state exchange internal sodium with external sodium four times faster than intermolt animals. 5. 5. Sodium exchange rate was determined for crabs in a series of external concentrations ranging from 40% to 160% sea water. 6. 6. A small negative potential was found to exist between the haemlymoph and the external medium at all salinities tested. 7. 7. The partitioning of total sodium exchange, as found in P. crassipes exposed to a wide range of salinities, permits a summary description of sodium balance and regulation.
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