ABSTRACT
The freshwater isopod Asellus aquaticus can be adapted to media ranging in concentration from 0 · 088 mM./l. NaCl to 200 mM./l. NaCl.
In fresh water O.P.1 has a mean value of 150 · 5 mM./l. NaCl. In more concentrated media O.P.I approaches isotonicity with the medium.
The mean ion concentrations of the haemolymph of animals in fresh water are Na1 = 137 m-equiv./l., Ch = 125 m-equiv./l. and K1 = 7·4 m-equiv./l.
Na and Cl account for most of O.P.I throughout the viable range of haemolymph concentration.
Evidence is presented which suggests that the animal is fairly permeable to salts and water.
During 8 days’ starvation there is no change in O.P.I, Na1 or Cl1. Sodium and chloride lost from the body can therefore be replaced by active uptake in the absence of food.
