HOLOTRICHA 

 b 



539 



Fig. 246. a-c, encystment in a species of Paramecium (Curtis and 

 Guthrie); d-f, encystment of P. caudatum, X380 (Michelson). 



in which encystment did not occur. Stages in encystment were 

 observed in P. hursaria (by Prowazek) and in P. pidrinuni (by 

 Lindner). In recent years, four observers recorded their findings 

 on the encystment of Paramecium. Curtis and Guthrie (1927) 

 give figures in their textbook of zoology, showing the process (of 

 P. caudatum?) (Fig. 246), while Cleveland (1927) injected Para- 

 mecium culture (species not mentioned) into the rectum of frogs 

 and observed that the ciliate encysted within a thin membrane. 

 Michelson (1928) found that if P. caudatum is kept in Knop-agar 

 medium, the organism becomes ellipsoidal under certain condi- 

 tions, later spherical to oval, losing all organellae except the 

 nuclei, and develops a thick membrane; the fully formed cyst is 

 elongated and angular, and resembles a sand particle (Fig. 246). 

 Michelson considers its resemblance to a sand grain as the chief 

 cause of the cyst having been overlooked by workers. In all these 

 cases, however, it may be added that excystment has not been 

 established. 



Genus Physalophrya Kahl. Without peristome; but cytostome 

 located near the anterior half of body, resembles much that of 

 Paramecium; although there is no membrane, a ciliary row occurs 

 in the left dorsal wall of cytopharynx; in fresh water. Taxonomic 

 status is not clear; but because of its general resemblance to 



