1. Analyses have been made of the body fluids of Echinus esculentus, and two decapod Crustacea, Homarus vulgaris and Cancer pagurus, before and after dialysis with sea water in which they were living.
2. The composition of the perivisceral fluid of Echinus is identical with that of sea water, complete physico-chemical equilibrium existing between the two fluids.
3. The blood plasmas of Homarus and Cancer are maintained in dynamic equilibrium with sea water. They contain more Na, K and Ca and less Cl, Mg and SO4 than sea water.
4. The antennary gland fluid of Cancer contains less Na, K, Ca and Cl and more Mg and SO4 than the blood plasma.
5. The importance of the antennary glands and the surface membranes in regulating the inorganic composition of the blood is discussed.