WATER, SALTS AND MINERALS 



43 



In hypotonic media, water absorption and salt loss both take place 

 (Figs. 2.11 and 2.12). On transferring N. diversicolor from full strength to 

 20% sea water, the initial increase in weight is followed by a slow decline, 

 and on returning the animals to normal sea water they suffer a further loss 

 in weight. This is due to a reduction of internal salt concentration which 

 now leaves the animal hypotonic to its normal environment. Direct 

 measurements of ionic influx into the worm have been made with the aid 

 of 24 Na and 36 C1. In normal sea water the uptake of sodium by Nereis 

 diversicolor is about 260 /ig/g/hour wet weight. At high dilutions (9% ) per- 

 meability to water is reduced, and there is active uptake of NaCl against 

 the concentration gradient. Worms which have been exposed to dilute 

 media and then returned to normal sea water show increased uptake of 

 sodium, which compensates for the loss of salt at lower salinities. 



40 60 80 



Time (hours) 



120 



Fig. 2.11. Regulation of Volume in Nereis diversicolor when 



Transferred from Normal to Dilute (20%) Sea Water 



A, Roscoff worms ; B, Bangor worms. (From Ellis, 1937.) 



Calcium is also necessary for osmotic regulation, and in a dilute medium 

 lacking this ion the animal remains swollen. The maintenance of hyper- 

 tonicity in waters of lowered salt concentration must involve the expendi- 

 ture of energy. In N. diversicolor and in Neanthes virens, another eury- 

 haline species, oxygen consumption goes up in dilute sea water, and cyanide 

 prevents the operation of regulatory processes with the consequence that 

 the weight curve continues to rise (3, 27, 35, 55#, 130, 130a, 137). 



Nereis diversicolor thus adjusts itself to a large extent when subjected 

 to a hypotonic environment : first, by absorbing water and swelling ; 

 second, by regulating its volume through salt loss. Calcium aids in diminish- 

 ing the permeability of the integument to water but the ability of the 

 animal to keep its internal fluids hypertonic to the environment shows that 

 active regulatory processes come into operation. These involve the active 

 uptake of ions (Na + and Cl~) against a concentration gradient and possibly 



