HOLOTRICHA 



745 



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

 brane. 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. 317,/). Michelson 

 considers its resemblance to a sand grain as the chief cause of the cyst 

 having been overlooked by workers. In all these cases, it may how- 

 ever be added that excystment has not been established. 



Fig. 317. a-c, encystment in a species of Paramecium (Curtis and 

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



Genus Physalophrya Kahl. Without peristome; but cytostome lo- 

 cated near the anterior half of body, resembles much that of Para- 

 mecium; 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 Paramecium, the 

 genus with only one species is mentioned here. 



P. spumosa (Penard) (Fig. 315, g). Oval to cylindrical; highly 

 plastic; cytoplasm reticulated; numerous contractile vacuoles; 150- 

 320^ long; in fresh water. 



Family 9 Colpodidae Poche 



Genus Colpoda Miiller. Reniform; compressed; right border semi- 

 circular; posterior half of the left border often convex; oral funnel in 

 the middle of flattened ventral side; cytostome is displaced to the 



