The term 'differential permeability' is frequently used to designate the property due to which a substance passes less rapidly in one direction than in the other, though identical gradients induce the passage. This property of membranes may be of great consequence in regulating the exchanges of materials by organisms. Relatively few instances of it have been described, since its identification depends upon the evaluation of all the ordinary forces that are concerned in diffusion or osmosis.
Differential permeability manifested itself wyile a survey was being made of osmotic exchanges in Phascolosoma. The original objectives of the measurements to be described were : To ascertain the rate of passage of water through the body wall, to define the permeability to water and to various other molecules and ions, to find whether the permeability could be modified, and to discover any regulation of water or salt content over long periods of time.
The gephyrean Phascolosoma gouldi (Pourtales) was chosen for the study because of several suitable qualities. Its body surface is smooth and uniform, the body wall is tough and not injured by handling, the worm survives exposure to a considerable variety of solutions, and the measurements of body weight are highly reproducible and controllable.