Physiology 485 



toxin has not affected fission-rate or death-rate (444), although undiluted 

 culture filtrates containing this toxin may be lethal (559). Tetanus toxin 

 (150 MLD) and botulinus toxin in various concentrations are without 

 action on P. caudatujn (445). On the other hand, a thermostable cytolysin 

 produced by Pseudomonas aeruginosa is lethal to Glaucoma scintillans 

 (60). Although ricin is inactive, certain snake venoms, in minimal con- 

 centrations of 1.4-150 [j,g/'ml, are lethal to P. caudatum. Locomotion is 

 inhibited and rupture of the cortex and disintegration of the ciliate occur 

 sooner or later. Susceptibiltiy to Crotalus atrox venom varies with the 

 species. Bursaria truncatella, P. aurelia, and Stenlor coeruhiis are killed 

 within an hour, Frontonia leucas, Oxytricha fallax, and Volvox after 

 longer periods, while certain other species are not harmed (445). This 

 apparent resistance of certain species may be largely a matter of degree. 

 For example, the MLD (minimum lethal dose) of Crotalus atrox venom 

 for Coleps hirtus is nine times that for Oxytricha fallax. Sensitivity of 

 14 species to Cobra venom shows no apparent correlation with sensitivity 

 to Crotalus venom (446). Adequate doses of antiserum completely pro- 

 tect Paramecium multimicronucleatum against lethal concentrations of 

 Cobra venom (447). 



EflFects of certain therapeutic drugs 



In addition to their action on growth of Protozoa, certain drugs 

 have shown specific effects on metabolic activities (349). From a practical 

 standpoint, such results are of interest because they help to plan attacks 

 against parasites at vulnerable points. As more is learned about food 

 requirements and metabolic activities, the development of specific drugs 

 for particular parasites more closely approaches realization. Another 

 interesting possibility is that the determination of specific effects of 

 chemotherapeutic drugs may reveal additional tools for the analysis of 

 protozoan metabolism. 



Specific effects of certain therapeutic drugs have been reported for 

 malarial parasites (349, 406) and trypanosomes. Hydrolysis of proteins 

 by Plasmodium gallifwceum is retarded by atebrin and quinine (408), 

 and the oxidation of carbohydrates also is retarded by these drugs (527). 

 The antimalarial activity of a series of naphthoquinones seems to be 

 related to their effects on succinic dehydrogenase (128, 199). Oxygen con- 

 sumption of Plasmodium cathemerium is inhibited by sulfanilamide and 

 sulfathiazole (562), and that of P. knowlesi by sulfanilamide (68) and 

 quinine (134). Surprisingly, however, sulfanilamide has no effect on 

 respiration of P. inui, and little or no therapeutic action, whereas the 

 drug eradicates infections with P. knowlesi in the same host (69). Triva- 

 lent arsenicals (halarsol, reduced atoxyl, reduced tryparsamide) are 

 powerful inhibitors of respiration in Trypanosoma rhodesiense (134). 



