1. The urine of Viviparus is hypotonic to the blood by about 30 mM./l. NaCl in tap water, and remains hypotonic in concentrations of up to 10% sea water.

2. The rate of production of urine is between 0·25 and 0·91 µl./g./min. in tap water at 19° C. The rate decreases in proportion to the decrease in osmotic difference between blood and external medium. Viviparus may be able to detect changes in salt concentration of the external medium and alter its rate of urine production accordingly.

3. Pericardial fluid is similar to blood in composition; the rate of flow of pericardial fluid through the reno-pericardial canal is proportional to the blood pressure; and when inulin is injected into the blood, concentrations in blood and pericardial fluid are approximately the same. For these reasons it is supposed that blood is filtered through the heart into the pericardium.

4. About 20 mM./l. NaCl, and probably some water, are reabsorbed in the kidney. Liquid is passed through the kidney by rhythmic contractions of the kidney musculature. Pericardial pressure does not influence the overall rate of urine production but blood pressure does have an effect.

5. About 5 mM./ NaCl, and probably a little water, are reabsorbed in the ureter.

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