WATER, SALTS AND MINERALS 



41 



vacuolar output remains at a high level. In the marine rhizopod Amoeba 

 mira there is no contractile vacuole, but hyaline vacuoles appear during 

 feeding, and the fluid in these vacuoles is eventually discharged. In dilute 

 sea water the rate of elimination of fluid during feeding is inversely pro- 

 portional to the osmotic concentration of the culture fluid. There is also 

 an initial increase in cell volume, which gradually returns to normal. 

 Amoeba lacerta, a freshwater species, possesses contractile vacuoles. 

 Vacuolar output also varies inversely as the concentration of the medium. 

 In concentrated media the Amoeba shrinks, then returns to normal size. 

 These protozoans display volume regulation and seem to be permeable to 



«n 140 

 5 120 

 5 WO 



10 





i Body volume 



^ 20 -100% 

 3 -s.w. |^ 



40% S.w. 



Rate of output 



100% s.w. 



] ''40%sw wo% s.w 



SO 



120 



300 



180 240 



lime (minutes) 



Fig. 2.9. Effect of Hypotonic Sea Water on the Body Volume 

 and Vacuolar Output of a Marine Peritrich, Zoothomnium marinum. 



(From Kitching, 1934) 



salts: in anisosmotic media they soon approach the concentration of the 

 external medium (50, 89). 



Metazoa. Forms with limited powers of osmoregulation are described 

 below. 



Turbellaria. Many lower metazoans inhabiting brackish and fresh waters 

 obviously possess some osmoregulatory capacity. Among flatworms the 

 nephridial system of Gyratrix hermaphroditus has been implicated in 

 osmoregulation on morphological evidence. In freshwater varieties of 

 Gyratrix there is a well-developed protonephridial system, with flame cells, 

 ampullae and bladders. The nephridial system is reduced in brackish- 

 water varieties, and absent as far as we can tell from salt-water forms. In 

 dilute sea water and fresh water the nephridial system appears to function 

 in a manner analogous to the contractile vacuoles of Protozoa, by rhyth- 

 mically pumping water out of the organism (78). 



In the triclad Procerodes (= Gunda) ulvae, the role of the nephridial 

 system is doubtful. This species lives in estuarine reaches of small streams 

 where salinity conditions fluctuate from salt to fresh water according to 

 the state of the tide. When placed in dilute sea water Procerodes under- 

 goes swelling but equilibrium is soon reached with some subsequent 

 decrease in weight. During this process there is loss of salt to the external 



M.A. — 2* 



