WATER, SALTS AND MINERALS 39 



Protozoa. Processes of osmoregulation among protozoans have been 

 linked with activity of the contractile vacuole. This organ is especially 

 characteristic of freshwater Protozoa, but it also occurs in many marine 

 flagellates and ciliates and in freshwater sponges (Table 2.3). In its simplest 



TABLE 2.3 

 Systematic Occurrence of Contractile Vacuoles in Relation to Habitat 



form the contractile vacuole consists of a vesicle which contracts rhythmic- 

 ally and discharges its fluid contents through a temporary pore in the body 

 surface. Vacuolar activity is less in marine than in freshwater forms, and 

 there is general agreement that it operates in osmoregulation by pumping 

 out water. 



From collected data Kitching (64) has shown that the normal frequency 

 of vacuolar contraction, expressed as vacuolar duration, varies from a few 

 seconds to several minutes in freshwater Protozoa, but in marine Protozoa 

 it is considerably longer (Table 2.4). When marine Protozoa possessing 



TABLE 2.4 

 Vacuolar Output for Several Freshwater, Estuarine and Marine Ciliates 



(selected data) 



contractile vacuoles are placed in dilute sea water, vacuolar frequency and 

 output increase (Fig. 2.8). It has also been observed that contractile 

 vacuoles appear in certain marine protozoans normally lacking them, 

 when they are kept in fresh water. 



When marine peritrichs are subjected to dilute sea water below 75% 

 vacuolar output is raised and body volume increases (Fig. 2.9). Cyanide 

 (a respiratory inhibitor) interferes with this vacuolar activity so that 

 individuals in diluted sea water continue to swell (Fig. 2.8). Removal of 

 the cyanide leads to recovery of vacuolar activity, and the vacuole pumps 

 at a faster rate than normal, with consequent decrease in body volume. 



