PROTOZOA AS CELLS 25 



Contractile Vacuoles 



Contractile vacuoles are by no means peculiar to the protozoa, 

 but they are so typical of familiar fresh- water species that we tend 

 to think of them as a protozoan property, if not monopoly. They 

 are absent in many marine and symbiotic forms, and this fact 

 early gave rise to the logical assumption that they are instruments 

 of osmoregulation. This is supported to a large extent by experi- 

 mental data (Grasse, 1952; Kitching, 1956a; Faure-Fremiet and 

 Rouiller, 1959). The contractile vacuole appears and grows by 

 the confluence of smaller droplets. Periodically its contents are 

 discharged through the cell surface. The amount of fluid that it 

 pumps out of the cell under normal conditions may within a 

 period of ten minutes equal the total water content of the cell 

 (Faure-Fremiet and Rouiller, 1959). In most species tested, the 

 rate of cyclic contraction and/or the maximum size of the vacuole 

 before discharge vary inversely, within limits, with the concen- 

 tration of solutes in the external medium ; whether this is a direct 

 effect on the vacuole and its filling mechanism or an indirect one 

 modifying permeability of the cell membrane is not known. 



In addition to an osmoregulatory function, a role in the secre- 

 tion of metabolic wastes has been suggested, but pertinent 

 experimental data are lacking. Whatever its ancillary functions, 

 the sheer volume of water (derived from ingestion of water with 

 food and from diffusion through the cell surface) transported 

 through the cell by this means necessitates some mechanism for 

 the selective retention or resorption of protoplasmic solutes. The 

 consensus among recent students of the contractile vacuole is that 

 an active phase segregation or secretion most probably occurs in 

 the zone where the first contributory vesicles appear. Analogy 

 with the nephridial organs of metazoa is clearly suggested. 

 Perhaps the analogy should not be carried too far, as the water- 

 conserving function of renal organs in terrestrial or marine animals 

 is not known to have any parallel in the activities of the contractile 

 vacuole ; however, the activity of such vacuoles in certain internal 

 symbiotic protozoa needs to be studied. 



Because of its accessibility, the protozoan contractile vacuole 

 offers an elegant experimental material, still largely unexploited, 

 to investigators concerned with intracellular water transport 



