Protozoa of the Digestive and Urogenital Tracts 545 



monas tenax) and an amoeba {Entamoeba gingivalis) manage to infect 

 a significant proportion of the population. 



The life-cycles of these two parasites do not include cysts, so that 

 infections are spread by the transfer of trophozoites. Under experimental 

 conditions, a trace of moisture has kept E. gingivalis alive long enough 

 for droplet transfer, and for transfer by way of contaminated cups and 

 other utensils (93). Transfer by direct oral contact entails much less risk 

 for the parasite. 



Trichomonas tenax 



This flagellate probably was first described as Cercaria tenax by 

 O. F. Miiller in 1774 (94). Many years later, the organism was found 

 again and described as Tetratrichomonas buccalis Goodey (65). T. tenax 

 (Fig. 11: I, A-C) shows a size range of about 5-21 x 3.8-7. 6[j.. The im- 

 dulating membrane is usually rather short and the membrane-flagellum 

 may not extend beyond the membrane. Autotomy occasionally pro- 



Fig. 11. 1. A-C. Trichomonas tenax, parabasal body shown only in B; 

 x2530 (after Wenrich). D-F Entamoeba gingivalis: small specimen free 

 from inclusions (D), a smaller rounded form (E), and a large specimen 

 with many food vacuoles; x2000 (after Kofoid and Swezy). 



