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HANDBOOK OF PROTOZOOLOGY 



mecium culture (species not mentioned) into the rectum of 

 frogs and observed that the ciliate encysted within a thin mem- 

 brane. Michelson (1928) found that if Paramecium caudatum 

 is kept in Knop-agar medium, the organism becomes ellip- 

 soidal, later spherical to oval, losing all cell-organs except the 

 nuclei, and develops a thick membrane. The fully formed cyst 

 is elongated and angular, and resembles a sand particle (Fig. 

 157, d-f). Michelson considers its resemblance to a sand grain 

 as the chief cause of the cyst having been overlooked by 

 workers. 



Fig. 157 a-c. Encystment in a species of Paramecium (after Curtis), 

 d-f. Encystment of Paramecium caudatum. X about 380 

 (After Michelson). d, e, young cysts; f, old cyst. 



Family 5 Pleuronematidae Kent 



One or more well developed undulating membranes are con- 

 spicuously present. 



Genus Pleuronema Dujardin. Body medium large; elliptical 

 to lenticular, compressed. The ends are equally rounded. Peri- 

 stome is a long groove starting at the anterior extremity, from 

 which is expanded a conspicuous membrane. Longitudinal rows 

 of long cilia. Trichocysts are said to be occasionally present. 



