Physiology 489 



liminary treatment with vital dyes and other reagents (97), and is less 

 at 15° than at lower or higher temperatures (84a). Paramecium hursaria 

 seems to be less sensitive to X-rays than its symbiotic algae, which are 

 sometimes eliminated at certain dosages (572). Lethal effects of X-rays 

 on Tetrahymena pyriformis in pure culture have been attributed to the 

 production of H2O2 in culture media, which become toxic whether irra- 

 diated directly or prepared from irradiated distilled or tap water (547). 

 Effects of radium and X-rays on growth of populations have been 

 described for several species. Growth of Entamoeba histolytica, exposed 

 to gamma-rays primarily or to unscreened radium for 24-48 hours, reaches 

 a maximum one to several days sooner than in the controls (409). Ex- 

 posure of P. caiidatum and P. muJtimicronucleatum to X-rays for 10 

 minutes to four hours has retarded fission for 2-5 days. Longer exposures, 

 or exposures repeated at intervals of several days, may increase the fission- 

 rate (191). Bodo caudatiis, exposed continuously to gamma-rays in serial 

 transfers, shows retarded fission and no acclimatization. Such effects may 

 persist for several weeks after removal of the radium, although recovery 

 is complete after three months. In a given transfer, the lag phase is 

 prolonged almost three hours in irradiated cultures (485), and the period 

 of greatest sensitivity occurs about 2.0-2.5 hours befoie the first fission in 

 a new culture (486). Although the generation time is essentially normal 

 thereafter, irradiated populations cannot catch up with the controls be- 

 fore the end of the incubation period. Slower growth in the young irra- 

 diated poptdation is correlated with larger individual size. Irradiation 

 for part of the incubation period, so as to allow 8-11 subsequent hours of 

 growth, is followed by acceleration of growth to produce pcjpulations 

 exceeding 90 per cent of the normal density (487). The production of 

 ammonia (per culture and per flagellate) by B. caudatiis is increased after 

 exposures which produce maximal effects on size and fission-rate (31o). 



Locomotion 



Locomotion in free-living Protozoa is of two basic types: siuim- 

 ?ning, which depends upon the activity of flagella, cilia, or their deriva- 

 tives; and creeping, which is dependent upon direct contact with a 

 substratum. Creeping in Amoebida and similar organisms usually involves 

 pseudopodial activity and is termed amoeboid movement. 



AMOEBOID MOVEMENT 



Several explanations have been proposed for amoeboid movement 

 (90, 382, 499). According to one view, locomotion in Amoeba proteus is a 

 "walking" process in which extended psetidopodia become attached to the 

 substratum and then contract to pull the body forward (91). A rolling 

 movement has been attributed to Amoeba verrucosa. A given point on the 

 surface passes forward on the upper surface, downward at the anterior 



