40 Co-mparative Animal Physiology 



proteus occurred in 0.005 M lactose ^"^^ and that of Vorticella in 0.0125 M 

 lactose, "- hence the osmotic concentration of the cytoplasm in the respective 

 organisms must be slightly less than these values. Kamada ^-' reasoned from 

 contractile vacuole responses that in a very dilute medium (approaching dis- 

 tilled water) the osmotic concentration of the cytoplasm of Parmnecimn is 

 equivalent to 0.025 M NaCl. Measurements of the vapor pressure of Spirosto- 

 mwn cytoplasm -*^*^ gave a value corresponding to 0.025 M NaCl. It is of 

 interest that the oxygen consumption of Paramecium is'minimal in 4 per cent 

 sea water (^0.024 M NaCl).'*'^ Hoptophrya, a ciliate from the gut of sala- 

 manders, is rounded and turgid in salines more dilute, and flat and wrinkled 

 in salines more concentrated, than 0.5 Ringer solution (0.06 M).^^^ Con- 

 ductivity measurements are not strictly comparable, but conductivities corre- 

 sponding to the following concentrations of KCl^'^ were found: Amoeba 

 proteus 0.01 N KCl, Paramecium 0.06 N KCl, and Spirostomum 0.0385 N 

 KCl. It seems certain from the preceding observations that the osmotic con- 

 centration of fresh-water Protozoa in their natural medium is 5 to 10 times 

 lower than that of most other fresh-water animals, and that the concentration 

 of Amoeba proteus cytoplasm is less than that of the cytoplasm of Paramecium 

 or Spirostomum. 



Function of the Contractile Vacuole. Contractile (c.) vacuoles eliminate 

 water and help to maintain a constant volume. In marine and parasitic pro- 

 tozoa the c. vacuole eliminates water of a nonosmotic source, i.e., water from 

 food and metabolic water. The low permeability of the cell surface to salts 

 mentioned previously raises the possibility that the c. vacuole may also func- 

 tion in the elimination of salt. Nothing is known of specific ionic regulation in 

 marine Protozoa, and the c. vacuoles may well eliminate certain ions differ- 

 entially. 



Evidence that the c. vacuole functions in osmotic regulation comes from 

 the distribution of c. vacuoles among protozoa in different habitats as given 

 in the following tabulation. ^^'^ 



This table shows that in all fresh-water Protozoa a contractile vacuole is 

 present. It is absent from most endoparasites except among ciliates. It is 

 present in many marine Protozoa. Kitching ^•'' suggested that the much 

 higher occurrence of vacuoles among marine ciliates than among rhizopods 

 may be a holdover from previous fresh-water habitat. It seems more likely that 

 it is correlated with the lower salt permeability and poorer volume adjustment 

 in marine ciliates than in rhizopods, as indicated above. It is also possible that 

 the c. vacuole functions in ionic regulation in marine Protozoa; the ionic 

 composition of marine protozoans is unknown. 



Further evidence for osmotic function of the c. \'acuolc comes from its out- 

 put of (luid under different natural conditions. Both the maximum volume 

 and the pulsation frequency of the c. xacuoles must be considered, to get a 

 measure of the total water excretion. Further, measurements must be made 



