The major aspect of this study was to evaluate the effect of changes in external salinity on the concentrations of Na⁺, C1ˉ , K⁺, Ca⁺⁺, and Mg⁺⁺ in the blood, urine, and foot muscle cells of Acmaea scutum. To estimate intracellular ion values, measurements of muscle tissue ion values and the extracellular volume (as represented by the inulin space) were made. As well, aspects of the water balance of A. scutum have been studied; the effect of changes in external salinity on water content of whole animal, and muscle cells has been documented; and the water content of animals maintained at a constant salinity has been studied. The results showed that ion values of the blood, except for K⁺, were the same as respective ion values of external salinities. At all salinities the concentration of K⁺ in the blood was greater than sea water K⁺ values. Dialysis experiments showed the gradient was not due to a Donnan equilibrium. All urine ion values were the same as ion values of external salinities. The K⁺ gradient observed between blood and sea water did not exist between urine and sea water. Extracellular volume of foot muscle changed linearly with change in external salinity (16.7% at 50% sea water and 3l.0% at 125% sea water). This change in extracellular space indicated that cellular volume changed with changes in external salinity. Intracellular ion values were different than blood ion values. Intracellular estimates of Na⁺ and C1ˉ were, at all salinities tested, close to or not significantly different from zero. Intracellular K⁺ estimates were, at all salinities, much higher than blood K⁺ values. Intracellular K⁺ values also changed significantly with changes in external salinity. Over a range of salinities from 50 to 125% sea water intracellular K⁺ values appeared to increase linearly. Intracellular values of.Ca⁺⁺ and Mg⁺⁺ were lower than corresponding values of experimental salinities. At a given salinity the sum of intracellular ion values was much lower than the sum of blood ion values. Results on seasonal distribution of ions and water of foot muscle for animals from a constant salinity showed that over an 18 month period muscle ion and water values varied significantly. As well, water content values of whole animal for the same field samples varied significantly. Water content of whole animal also changed significantly with changes in external salinity. For A. scutum from a marine environment changes in water content caused by changes in experimental salinities were maintained for up to one week immersion. Whole animal water content for A. scutum from an estuarine environment however, returned to starting water content values after 48 hr immersion in experimental salinities. Whole animal water content data for field samples from an estuarine environment indicated little volume regulation. Over a range of environmental salinities from 18% to 82% sea water, water content ranged from 89.0% to 77.0%. As well as showing changes in total body water with changes in external salinity, A. scutum demonstrated large changes in water content at a given constant salinity. Changes in water content at a constant salinity resulted from sea water entering the blood space from the external environment. When the molecules inulin and amaranth were dissolved in experimental salinities and the water uptake response tested, these molecules also entered the blood space from the external salinities. The ion values of blood, urine, and muscle cells were similar to values of these parameters recorded for other gastropod molluscs. Water content values from experiments on the effect of change in external salinity also agreed with data from studies on other molluscs. The water uptake response of A. scutum however, was not in accordance with measurements on change in blood volume at constant salinities for other gastropod molluscs. A definite example of a gastropod that can take sea water from the external environment into the blood space must now be added to the literature. / Science, Faculty of / Zoology, Department of / Graduate