 The law states that at equilibrium, the partial pressure of oxygen in water will be equal to that in air; but the concentrations will differ due to the differing solubility.This law explains why "O. vulgaris" has to alter the amount of water cycled through its mantle cavity as the oxygen concentration in water changes.The gills are in direct contact with water – carrying more oxygen than the blood – that has been brought into the mantle cavity of the octopus.