Water 39 



sions.^'^'*' ^-'^ Of the four rhizopods, A. mira and A. lacerta adjust rapidly 

 (1-2 hours) after transfer to a changed medium, so that their volume is similar 

 at all concentrations. Pelomyxa is less tolerant of extreme changes and 

 requires 6 to 12 hours to adjust its volume after transfer from freshwater 

 culture to distilled water or to a solution of nonelectrolyte. Amucha proteus 

 transferred to a solution of sugar (lactose) above 0.005 M showed a steady 

 decline in volume tor 6 to 7 days, after which there was sometimes a slight 

 rise, but survival was rare after that time in the nonelectrolyte. Starvation 

 complicated the late reactions, but it seems certain that Aviochn proteus has 

 very little volume regulation. The fresh-water ciliate Hhabdostyla hrevipes 

 and the marine Zoothmnnmm niarinwn also failed to show any tendency to 

 return to their original volume, at least for 3 hours of measurement in a solu- 

 tion of changed tonicity, whereas the brackish-water Cothtirnia curvula when 

 transferred to a dilute medium swelled initially but after 2 3 hours showed a 

 slight decline in volume. Three species of Euplotes which occi: in l:jsh 

 water, in 2.5 per cent, and in 4 per cent salinity are of diminishing size, in 

 that order. ^- If the fresh-water species becomes adapted over a period of a 

 month or more to 1 per cent salinity it decreases in size. 



Membrane Permeability. The preceding observations indicate a consider- 

 able permeability to water but an extremely low permeability to salts in 

 Pelomyxa, A. proteus, Rhabdostyla, and Zoothamniwn, and a high salt perme- 

 ability in A. vura and A. lacerta. Even in the latter two species, volume 

 adjustment is much slower than in marine eggs. Permeability constants for 

 incoming water calculated on the basis of swelling give, in |y.-\Watm./mm., 

 values between 0.026 and 0.031 for A. proteus, '^^ 0.023 for Pelomyxa, =" 

 and between 0.125 and 0.25 for fresh-water peritrichs. ^'■^■' These values are 

 low compared with permeability constants of 3 for human erythrocytes and 

 0.4 for fertilized Arbacia eggs. It appears that there is no close correlation 

 between the permeability of the diflFerent species for water and volume regula- 

 tion. Lower permeability to salt than to water was indicated for marine peri- 

 trichs by their tendency to swell when vacuolar output was reduced by cyan- 

 ide. ^'■^'' A high outward permeability to salt in Amoeba mira is shown by the 

 observation ^'^■' that when the animal was transferred from 100 per cent to 

 5 per cent sea water the volume increased fivefold in 12 minutes and then 

 after 90 minutes decreased to the original size. During this decrease in size of 

 3000 ij:\ only 700 fx-^ of water went out by way of the xacuoles. 1 he rest must 

 have gone out across the body surface. 



It is probable that Protozoa, like most other animal cells, are more permeable 

 to CO.. and urea than to other nonelectrolytes. Considerable shrinkage of 

 marine' peritrichs (Cothiirnia) in isotonic solutions of sucrose, glycerol, and 

 even urea, indicates a verv low inward permeability for these nonelectro- 

 lytes. ''■"'' Several parasitic ciliates failed to take up vital dyes when the _cyto- 

 pharvnx was closed, but accumulated dyes readily when it was open. '■" It 

 would be of interest to studv the permeability of those parasitic species which 

 lack oral openings. There is much variation, but Protozoa in general are cells 

 of relativelv low permeability. ■ c u 



Osmolar Concentration of Cytoplasm. The osmolar concentration ot the 

 cvtoplasm of marine and parasitic Protozoa is probably similar to the concen- 

 tration of the medium in which they live. Minimal shrinkage of Amoeba 



