Abstract
1. 1. Blood concentrations of H. ulvae were determined in normal, dilute and concentrated sea-waters, and on exposure and reimmersion. 2. 2. The species proved to be poikilosmotic and very euryhaline, and had a very effective means of volume control. 3. 3. Equilibration to new media took about an hour in active animals, but retraction delayed the process at extreme salinities so that it took several days at least. 4. 4. Increases in blood concentration of up to 100 per cent were permissible before death by desiccation on exposure. 5. 5. These results are similar to those previously found for littoral species although there seems to be a greater tolerance of blood concentration changes in H. ulvae. 6. 6. Small size, salt and water permeability and volume control are discussed in relation to euryhalinity.
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