The Water and Electrolyte Exchange ofNereis Diversicolor(Müller)

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Abstract
1. The rate of weight regulation in diluted sea water of Nereis diversicolor from Brittany (Roscoff) is much greater than that of the same species from the Menai Straits (Wales) or Plymouth. This difference is believed to be racial and not due to environmental factors. 2. Weight regulation in diluted sea water is inhibited by the absence of any one of the chief cations found in normal sea water. 3. Water is reabsorbed by N. diversicolor if, after weight regulation, the worms are transferred to water of the same osmotic pressure but containing no calcium. No reabsorption of water occurs in solutions lacking sodium, potassium, or magnesium. 4. The uptake of water from diluted sea water is accompanied by a loss of salts from the worm : this loss is not affected by the presence or absence of calcium. The subsequent water loss in diluted sea water is not accompanied by an uptake of salts from the water. Only a very small loss of salts occurs when weight-regulated worms absorb water from calcium-free solutions. 5. If a "regulated" worm be transferred from dilute sea water back to 100 per cent sea water, there is a water loss and a salt intake. As a result of the water loss the weight of the worm is now only 80 per cent of the initial weight in 100 per cent sea water. By repeated transferences from 100 to 20 per cent sea water, and back again, the weight in 100 per cent sea water can be reduced to half the initial weight. 6. It is concluded that weight regulation is not accompanied by osmotic regulation.