Osmotic and Ionic Regulation in the Isopod Crustacean Ligia Oceanica

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Abstract
1. Osmotic pressure of the blood of Ligia oceanica, measured by the freezing-point depression, has a mean value of δ 2.15 ± 0.04° C. (≡3.58% NaCl on weight/ volume basis). 2. Osmotic pressure of Ligia blood is much higher than that of other terrestrial isopods: Oniscus sp. δ1.04° C.; Armadillidium sp. δ1.18° C.;Porcellio sp. δ1.30° C. or of the fresh-water Asellus sp. δ0.50° C. 3. The osmotic pressure of the blood increases during the process of moulting, but no subsequent decrease is observed in the 4 days following. 4. Animals kept at low humidities lose water. They may be desiccated without permanent adverse effects until δblood is 3.48° C. (≡ 5.8% NaCl). Recovery to a normal level takes about 24 hr. in moist conditions. 5. In well-aerated sea water between 50 and 100% concentration, animals survive without much alteration in δblood. Above and below this range δblood rises and falls. 6. In animals kept on filter-paper moistened with distilled water δblood may fall to 1.44° C. (≡2.4% NaCl) without permanent adverse effects. 7. Analyses of inorganic ions in the blood show that sodium, potassium and chloride are all higher in concentration than in sea water; calcium is much more concentrated; and magnesium and sulphate much reduced.