17(3 PROTOZOOLOGY 



lack of food in the culture has been noted by many observers 

 (Oehler, 1916; Claff, Dewey and Kidder, 1941; Singh, 1941; Beers, 

 1948; etc.) as a cause of encystment in a number of Protozoa such 

 as Blepharisma (Stolte, 1922), Polytomella (Kater and Burroughs, 

 1926), Didinium (Mast and Ibara, 1931), Uroleptus (Calkins, 1933), 

 etc., an abundance of food and adequate nourishment seem to be 

 prerequisite for encystment. Particular food was found in some in- 

 stances to induce encystment. For example, Singh (1948) employed 

 for culture of Leptomyxa reticulata, 40 strains of bacteria, of which 

 15 led to the production of a large number of cysts in this sarcodinan. 

 Encystment of Entamoeba histolytica is easily obtained by adding 

 starch to the culture (Dobell and Laidlow, 1926; Balamuth, 1951). 



The age of culture, if kept under favorable conditions, does not 

 influence encystment. Didinium after 750 generations, according to 

 Beers (1927), showed practically the same encystment rate as those 

 which had passed through 10 or 20 generations since the last encyst- 

 ment. When Leptomyxa mentioned above is cultured for more than 

 a year, no encystment occurred, but young cultures when supplied 

 with certain bacteria encysted (Singh, 1948). 



In some cases, the organisms encyst temporarily in order to un- 

 dergo nuclear reorganization and multiplication as in Colpoda (Fig. 

 73) (Kidder and Claff, 1938; Stuart, Kidder and Griffin, 1939), Til- 

 lina (Beers, 1946), etc. In Ichthyophthirius, the organism encysts 

 after leaving the host fish and upon coming in contact with a solid 

 object, and multiplies into numerous "ciliospores" (MacLennan, 

 1937). Pelomyxa carolinensis (Illinois stock) has not encysted since 

 its discovery in 1944, although the cultures were subjected to vari- 

 ous environmental changes, but P. illinoisensis has been found to 

 encyst and excyst frequently in flourishing cultures (Kudo, 1951). 

 Thus it may be assumed that some unknown internal factors play 

 as great a part as do the external factors in the phenomenon of en- 

 cystment (Ivanic, 1934; Cutler and Crump, 1935). 



The cyst is covered by one to three membranes. Though generally 

 homogeneous, the wall of cyst may contain siliceous scales as in 

 Euglypha (Fig. 74). While chitinous substance is the common ma- 

 terial of which the cyst wall is composed, cellulose makes up the 

 cyst membrane of many Phytomastigina. Entz (1925) found the 

 cysts of various species of Ceratium less variable in size as com- 

 pared with the vegetative form, and found in all, glycogen, oil and 

 volutin. 



The capacity of Protozoa to produce cyst is probably one of the 



