Abstract
1. 1. Themiste dyscritum, apparently stenohaline in nature, tolerates a wide range of salinities in the laboratory (168–639 mM Cl −, or 30–114% sea water). Long-term survival probably does not occur below about 50% sea water. 2. 2. T. dyscritum is a passive osmoconformer over this entire salinity range, but has an apparent ion deficit in the coelomic fluid, as indicated by low internal chloride concentrations relative to external levels. This apparent ion deficit is most marked in worms in low salinities, and may be made up by small molecular weight organic molecules. 3. 3. Water content is strongly dependent upon salinity, and the worms absorb large amounts of water osmotically after a transfer to a lower salinity. Volume regulation is restricted to worms adapted to high salinities and to transfers which are not excessive. 4. 4. Nephridial contents are little different in concentration from the coelomic fluid. 5. 5. Limited data on Phascolosoma agassizii in 84% sea water indicate the coelomic fluid is slightly hyperosmotic to the medium, but iso-ionic with respect to chloride.
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