The Adaptation of Gunda Ulvae to Salinity

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Abstract
1. The effect of fresh waters and dilute solutions on the behaviour and water exchange of the estuarine flatworm, Gunda ulvae, has been studied. 2. In Plymouth tap water which contains little dissolved substances the majority of the worms die within 48 hours. While immersed in this water the worms swell rapidly during the first hour to about double their volume in sea water, the volume falling slightly after this. The effect is reversible. 3. In dilute sea water the worms swell to a greater extent the greater the dilution. At great dilutions the swelling is much less than would be expected if the worm behaved as though it were covered with a perfectly semi-permeable membrane. 4. In water from the stream which normally flows over the Gunda at low tide the swelling of the worms is much less than in Plymouth tap water or in distilled water. This stream-water is rich in CaCO3. 5. The effects of distilled water and of solutions of NaCl, NaHCO3, glycerol, CaCl2 and of Cambridge tap water are compared with the effects of Plymouth tap water and the stream water. It is found that the beneficial effects of the stream water can be imitated only by the solutions containing calcium. 6. The mode of action of calcium is discussed. It is suggested that it acts primarily by lowering the permeability of the worms to water.

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