Adaptation to Changes of Salinity in the Polychaetes

Abstract

1. Nereis diversicolor collected from the same locality at different times showed smaller weight increases in dilute sea water (25 per cent) during the winter than during the summer months. 2. In spite of great variations in the weight curve, the body fluid concentration curve was very constant. 3. The maintenance of hypertonic body fluids and the regulation of body volume are largely unconnected. 4. The lowering of the weight curve below that theoretically expected from the concentration curve cannot be attributed to passive salt loss through the body surface. It is suggested that this is due to the removal of fluid through the nephridia under the hydrostatic pressure produced by the contraction of the body wall muscles. 5. Animals previously subjected to dilute sea water, when placed in water isotonic with the body fluids, will increase the concentration of the latter. This result is more marked when the internal hydrostatic pressure is high. 6. The results suggest that the osmotic regulatory mechanism involves the removal by the nephridia of fluid hypotonic to the body fluids. But no direct evidence for this is available. 7. Calcium deficiency and cyanide in dilute sea water cause an increase of weight and ultimately inhibit the maintenance of hypertonic body fluids. Both these effects are reversible. 8. The mechanism by which body fluids are maintained hypertonic to the external medium is not sufficiently developed to be of survival value in the locality in which the animals were found. 9. The control of body volume is probably of greater importance. 10. The majority of the extra oxygen consumption in dilute sea water is not the result of osmotic work. It is suggested that it may be due to work done by the body wall muscles in resisting swelling.