1. Experiments have been performed to determine some of the processes involved in urine formation in the octopus. The concentration of specific substances was followed in serial samples of the blood, pericardial fluid and urine for extended periods of time before and after the administration of metabolic poisons.

2. The results with inulin indicate that it is filtered since its concentration is approximately the same in the blood, pericardial fluid and urine. It is proposed that filtration of the blood occurs within the pericardial cavity.

3. The results with glucose indicate that it is reabsorbed from the filtrate in the pericardial cavity and reno-pericardial canal. Phlorizin administration increased the glucose concentration in the pericardial fluid and urine to the level of that in the blood.

4. The filtrate flows by way of the reno-pericardial canal into the renal sac. The results with phenolsulphonphthalein (PSP), para-amino hippuric acid (PAH) and urea indicate that active secretion into the filtrate takes place in the renal sacs. PSP, PAH and urea were in higher concentration in the urine than in the blood and pericardial fluid. The secretion of PAH and PSP was inhibited by DNP and benemid.

5. The theory of urine formation proposed is the following. Filtration of the blood occurs across the wall of the branchial heart appendage into the pericardial cavity. The filtrate formed passes by way of the reno-pericardial canal into the renal sacs. Reabsorption occurs en route to the renal sacs. Active secretion into the filtrate occurs in the renal sacs from where the filtrate is expelled to the outside as urine.

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