Water 



41 



in a hanging drop or in such a chamber that covcrglass pressure and other 

 experimental conditions do not influence the rate. Measurements must be 

 made on nonfeeding individuals. When these precautions are taken, there 

 is little doubt that freshwater Protozoa eliminate more fluid by their c. vacu- 

 oles than do marine or endoparasitic forms in natural media. A summary of 

 all available data on vacuolar output was made by Kitching. '''^ The time 

 required to eliminate a quantity of water equal in volume to the body of the 

 animal, for fifteen fresh-water species (except one), ranged between 4.1 and 

 53 minutes, whereas for four marine species it was 2^4 to 4^/4 hours, and for 



20 *0 6 80 



CONCENTRATION OF CULTURE FLUID IN X OF SEA WATER 



100 



Fig. 18. Rate of excretion in cubic micra of fluid per hour per cubic micron of proto- 

 plasm in Amoeba mira adapted to different dilutions of sea water: O actively feeding 

 specimens; (D nonf ceding specimens. From Mast and Hopkins.'" 



one endoparasitic ciliate (measured in fresh water) it was 4 hours. More recent 

 data on marine and fresh-water forms are in agreement. 



Further evidence for the osmoregulating function of c. vacuoles comes from 

 data on Protozoa transferred experimentally to solutions of various osmotic 

 concentrations. A few examples will be selected from the voluminous litera- 

 ture. 



Numerous marine and parasitic Protozoa can be cultured in media of 

 different tonicities. Amoeba vi'ira can live in distilled water or in sea water 

 concentrated to one-tenth volume by boiling. The average size of the amoebae 



