PROTOZOA 17 



production or digestion, but in a few cases it facilitates syngamy 

 by keeping gametes together. Encystment is less common among 

 species which live in the relatively equable conditions of the sea, than 



Fig. 17. 



Fig. 16. 

 Fig. 16. Paramecium caudatum from the ventral side, x 375. After Doflein. 

 CV, contractile vacuoles: in the anterior one the main vacuole has just 

 disappeared by discharging, and is about to be reconstituted from the 

 accessory vacuoles, which are at their maximum size ; in the posterior one the 

 accessory vacuoles having reformed the main vacuole are themselves re- 

 forming ; f.v. food vacuoles ; me. meganucleus ; mi. micronucleus ; v. vestibule. 



Fig. 17. The cycle of action of the contractile vacuole and its canals in Para- 

 mecium. From Lloyd, after Putter. 



in freshwater and parasitic forms. Cysts which do not subserve re- 

 production may be resistance cysts, against drought, alterations of 

 concentration, or the appearance of poisonous substances in the 



