Uptake of radioactive sodium (24Na) by Nereis diversicolor Mueller and Perinereis cultrifera (Grube)

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Abstract
Nereis diversicolor is typically euryhaline: it is in equilibrium with normal sea water (Schlieper, 1929; Beadle, 1937), and develops hypertonicity in more dilute media which it is able to maintain indefinitely. The maintenance of this steady state is preceded by a transition period. During this there is a rapid uptake of water accompanied by a loss of salts from the worm, resulting in a fall in the osmotic pressure of the body fluids, and then a subsequent water loss which, according to Ellis (1937), is not accompanied by an uptake of salts. Worms accommodated to 25% sea water weigh about 140% of their original weight (Beadle, 1931). Beadle (1937) found that when the steady state is attained the body fluid concentration in 50% sea water approaches a value which is about 5% higher than that of the external medium, whereas in 25% sea water the concentration of the body fluid is equivalent to about 44% sea water. His worms were collected from the Northumberland coast, and concentration measurements of the body fluid were determined by Baldes's modification of the Hill vapour-pressure method. The results of Schlieper (1929) on worms from Heligoland, where they may live in a salinity as low as 4%, are based on freezing-point determinations and give a higher internal concentration in the dilute media: after 3 days in 50% sea water the concentration of the body fluid is equivalent to 68% sea water, and after 2 days in 25% sea water equivalent to 49% sea water. These varying results suggest the occurrence of local physiological races.