Water 



17 



regulation is less well developed than that of \olume regulation. Morphologi- 

 cal evidence tor a mechanism ol salt reabsorption in the nephridia is sum- 

 marized by Jiirgens, ^-"' who shows that the nephridial canal oF N. diversi- 

 color is long and coiled, pro\'iding ample epithelium for reabsorption, whereas 

 the nephridium of N. ciiltrifera is a simple sac (Fig. 8). 



It is likely that Neanthes virens behaves much like Nereis diversicolor. Data 

 on blood concentration are not available, but studies have shown that on 

 transfer to 20 per cent sea water weight initially increased and then slowly 

 declined; when the worms were replaced in 100 per cent sea water the weight 

 went below the original level. -^^ When transferred through gradual stages 

 of salinity, Neanthes virens could eventually live for 14 days in 12.5 per 

 cent sea water. A significant increase in oxygen consumption occurred in N 

 virens transferred to dilute solutions; ^^^ the weight remained constant with 



I 

 O 



LJ 

 < 



Z 



O 



0^ 



I 00 



HOURS 



Fig. 7. Weight changes as per cent of initial weight in Nereis diversicolor when trans- 

 ferred from 100 per cent sea water to normal 20 per cent sea water (curve B); or to 20 

 per cent sea water lacking calcium (curve A), calcium added to 20 per cent sea water 

 at arrow. From Ellis."* 



dilution until the animals were in about 16 per cent sea water, when weight 

 increased and oxvgen consumption dechned. 



Among the marine worms there are several levels of osmotic function. 

 Phascolosoma, Sipuncidus, and probably Nereis cuUrifera adapt in body con- 

 centration bv swelling and do not shrink during at least a day in dilute sea 

 water. Nereis pelagica also changes in concentration but loses a little salt so 

 that there is slight volume regulation. Nereis diversicolor and probably 

 Neanthes virens show a limited regulation of both body concentration and 



volume. 



Parasitic Worms. Parasitic worms are usually assumed to be at osmotic 

 equilibrium with host tissues. Intestinal parasites may be subjected to con- 

 siderable variations in medium from time to time. The coelomic Huid ot 

 Ascaris megalocephala and the tissues of A. hnnhricoides are isotonic or slight- 

 ly hvpotonic to the intestinal fluid in which the worms live. "' Cestodes 



