1. Oxygen affinity of Limulus hemocyanin is lowered by the addition of Cl-and raised by the addition of H+. Therefore the oxygen transport system should be influenced by ionic changes in the blood, such as those which accompany the spring migrations into estuaries.2. When Limulus enters dilute waters, blood NaCl is reduced by almost half, even though active hyperosmotic regulation occurs. At very low salinities, the excretory organ plays a role in the imperfect regulation of blood NaCl by forming a dilute urine.3. Unlike crustaceans, H+ concentration of blood changes very little at low salinity, because little additional NH3 is produced and no appreciable increase in H+ binding to molecular NH3 occurs. This response is believed to be related to the small role of free amino acids in intracellular osmotic adjustment.4. Blood PO2 decreases in low salinity as a result of bradycardia. While the oxygenation of hemocyanin at the gill does not change appreciably, deoxygenation at the tissues is enhanced and thus,...