ABSTRACT
It has been shown by McCutcheon & Lucke (1928) that in glucose solutions isotonic with 40 per cent sea water, the eggs of the sea-urchin Arbacia absorb twice as much water as they do from 40 per cent sea water, and that this increased imbibition is prevented by a small concentration of calcium ion in the glucose solution, even in the absence of the other cations of sea water. Pantin (1931 a) showed that the Platy-helminth, Procerodes (Gunda) ulvae, which in its estuarine habitat withstands tidal changes in osmotic pressure, is only able to survive in very dilute salt solutions if these contain calcium.
These quantities, which were found empirically, are such that, while the change in concentration of the sea water, due to salt loss from the worm, is sufficiently large to be readily measurable, it is not so large as seriously to change the osmotic gradient between body fluid and external medium.
Although, of course, like every apparent status quo in the organism, it must represent the resultant of dynamic equilibria: there is no reason to believe that increased metabolism should be necessary for the maintenance of the weight equilibrium in dilute sea water.
