1. The survival of Ligia in artificial sea water is doubled if the Mg content is reduced by one half. 2. In spite of the fact that Ligia cannot survive long periods in natural sea water, no artificial sea water has been found to be a more favorable medium. 3. Exposure of about one hour to sea water at 38° C. or to moist air at 42° C. is fatal to Ligia. 4. In natural sea water the beating of the pleopods of Ligia baudiniana yields temperature characteristics of 16,000 calories between 10 and 16° C. and 8,400 calories between 16 and 28° C. These are typical of respiratory processes. 5. The reduction in the frequency of the gill-beat in sea water containing 96 per cent heavy water is equivalent to that produced by a drop in temperature of about 3–4° C. Similarly the critical temperature in heavy water is shifted three degrees higher. 6. In sea water containing 96 per cent heavy water the gill-beat yields temperature characteristics of 15,000 calories between 10 and 19 and 8,400 calories between 19 and 28° C. 7. It is concluded that the lower energy content of heavy water slows down the beating of the gills but does not change the fundamental regulatory mechanism.