ABSTRACT
Several species of marine invertebrates, and an elasmobranch, have been kept in diluted media. The increase of body weight so caused was compared with the resulting dilution of the body fluids.
The bounding membrane of the invertebrates was permeable to salts when the animals were immersed in diluted sea water.
The bounding membrane of the elasmobranch was semipermeable, i.e. permeable to water but not to solute. There is a close quantitative agreement between the osmotic swelling observed and the diminution of the osmotic pressure of the blood.
All dilutions were of tank water with tap water: the animals had been for some time in tank water (which is slightly more concentrated than sea water) before use : Plymouth tap water may be taken for the present purpose as equivalent to distilled water. Osmotic pressures are expressed as a fraction of that of Plymouth sea water.
