1. The sea water at Woods Hole, Mass., freezes at—1.82°C., at Pacific Grove, Cal., Δ is—1.90° C. 2. The blood or body fluid of a marine invertebrate has the ame freezing point as the sea water from which it is taken, and therefore has the same osmotic pressure. This is also true of selachian blood, although the salt content is lower than that of the sea water, the deficit of salts in the blood of this latter group is compensated by the osmotic pressure of the urea in the blood. 3. The osmotic pressure of teleost blood is about half that of sea water (Δ = 0.8° to – 0.96° C.). 4. A dilution or concentration of the aquarium water always causes an equivalent change in the blood of invertebrates, and osmotic equilibrium between "internal and external media" is established. Their membranes are completely permeable. This permeability is proven for the integument of worms and the gills of Limulus. 5. Dilution or concentration of the aquarium water causes a change in the same sense in the blood of selachians, but death ensues before osmotic equilibrium is established. The membranes of selachians are semi-permeable. 6. Only slight if any change takes place when teleosts (Anguilla) are transferred from salt to fresh water and vice versa. Normal Fundulus heteroclitus will live in water varying in osmotic sure from that of the tap to sea water concentrated to double its strength. The membranes of teleosts are impermeable, or the fish possess some regulative mechanism which keeps the osmotic pressure of the blood nearly constant. Extensive abrasion of the skin of Fundulus results in death in aquarium water of less or greater osmotic pressure than that of their blood, for example they die in fresh water and in normal sea water but not in sea water diluted with an equal volume of distilled water.