A study of the time course of penetration of 3H2O into whole worms suggests that worms immersed in a hypo-osmotic environment (15% artificial sea water) reach full exchange equilibrium more slowly than worms in an iso-osmotic environment (40% artificial sea water). The apparent water content, determined by dry mass, matches that determined by 3H2O exchange when worms are immersed for 24 h in 40% artificial sea water (ASW), but the water content measured by 3H2O exchange is lower when worms are kept in a hypo-osmotic environment for 24 h. These differences disappear after 48 h. No such differences are apparent when sacs, consisting of cylinders of body wall lacking their intestines and pseudocoelomic fluid and closed at both ends by ligatures, are immersed in either 40% or 15% ASW for 24 h. The placing of ligatures at the head, but not at the tail, results in a failure of worms immersed in 40% ASW or 15% ASW containing 3H2O to achieve full exchange equilibrium within 24 h. These results suggest that although worms immersed in an iso-osmotic environment drink, those immersed in a hypo-osmotic environment do not, a conclusion supported by studies involving the addition of [14C]inulin to the medium. The application of ligatures to the head and tail of worms immersed in 40% ASW results in a slower penetration of 3H2O into the pseudocoelomic fluid, whereas similar ligatures do not further retard the penetration in worms exposed to 15% ASW. The results are consistent with a model which sees the pseudocoelomic fluid as consisting of two compartments containing water, one of which exchanges more slowly than the other.

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