1. The ionic composition of the haemolymph, and the concentrations of uric acid and protein, have been determined for the amphibious prosobranchs Pomacea lineata and P. depressa. Ionic composition of the haemolymph is similar to that of freshwater gastropods.

2. The urine is decidedly hypo-osmotic to the haemolymph, reabsorption of ions occurring in the anterior chamber of the kidney. The rate of production of urine is approximately 1 µl./g./min. at 25° C.

3. The onset of aestivation appears to be related to a loss of 20% of the normal wet weight of the tissues. The loss of weight during aestivation averages 0.009% of the initial wet weight/hr., and aestivation may continue for over 400 days.

4. During aestivation the osmotic pressure of the haemolymph may rise to twice its normal value; but this is 30 mM/l. NaCl less than that predicted from weight losses. The relative composition of the haemolymph alters little, except that the percentages of sodium and chloride increase.

5. Uric acid accumulates round the blood vessels during aestivation, especially in the lung. Relatively little accumulates in the two chambers of the kidney, and only that in the posterior chamber is excreted later. Since the volume of fluid in the posterior chamber falls to about 10% of its normal value, while the total loss of weight of the snail is about 50%, the fluid in the posterior chamber acts as a water reserve.

6. Recovery from aestivation occurs in about 24 hr., when the snails are placed in water. The posterior chamber of the kidney excretes high concentrations of uric acid during this time.

7. The characteristics of aestivation are discussed, and compared with those shown by the Pulmonata.

Contribution No. 898 from the Marine Laboratory, Institute of Marine Sciences, University of Miami.