16 



Comparative Animal Physiology 



adjusted to 20 per cent sea water was put back into 100 per cent sea water 

 there was water loss and salt intake so that the worm soon had only 80 per 

 cent of the weight it initially had in 100 per cent sea water. By repeated 

 transfer from 100 per cent to 20 per cent sea water and back, the weight in 

 100 per cent sea water could be reduced to half. This indicates that water is 

 transferred more rapidly than salt. The ratio of chloride output to water intake 

 in 20 per cent sea water is nearly double the ratio of the exchanges in 100 per 

 cent sea water, hence swelling is much less than in N. cultrifera, where the 



Fig. 6. Freezing point of body fluids, Ai^Oi Nereis diversicolor, •, Arenicola marina, 

 and +, N. pelagica; animals adapted to dilutions of sea water indicated by freezing 

 points, Ao. Data from Schlieper."" 



chloride loss is very slow. Whether the avenue of chloride loss is through the 

 nephridia or not is unknown. 



Several mechanisms favor osmoregulation in N. diversicolor. The permea- 

 bility of the skin is important in volume regulation. Calcium decreases the 

 skin permeability to water and thus prevents a continued rise in volume. Body 

 wall contraction opposes swelling. After an initial increase the volume de- 

 creases as chloride is lost and hypertonicity is maintained; this must, as Krogh 

 points out, necessitate the elimination of hypotonic fluid, possibly by way of 

 nephridia, and is an active process requiring energy. There may, in addition, 

 be active salt absorption from the dilute medium. The mechanism of osmo- 



