592 A MANUAL OF BACTERIOLOGY 



other pairs of flagella are also present, the arrangement of which 

 is depicted in the figure (Plate XXIII, I and m). The flagella 

 originate from a paired rod-like structure occupying a central 

 position in the body of the parasite. Two nuclei are present, one 

 on either side of the rods, and these give the organism a peculiar 

 owl-like appearance when viewed on the flat surface. The 

 flagellate is actively motile, is able to curl the tapering tail over 

 its back, and attaches itself by the sucker-like disc to the surface 

 of the intestinal epithelium. Reproduction appears to take place 

 only in the encysted condition. The cyst is clear, oval in shape 

 and measures 14 /* in length. The two nuclei of the parasite 

 migrate to one end of the cyst and divide so that four nuclei are 

 formed (Plate XXIII, n). The contents of the cyst then divide 

 so as to form two Giardia, and the division between the two 

 appears as a line running obliquely and longitudinally across the 

 cyst (Plate XXIII, o). 



Other Intestinal Flagellates 



Tetramitus mesnili has a wide distribution in tropical and sub- 

 tropical countries. Small epidemics of diarrhoea have been 

 attributed to it. In general appearance, shape and size it 

 resembles Trichomonas intestinalis, and like the latter has three 

 long anterior flagella, but it has no axostyle and no undulating 

 membrane. The cytostome is large and forms an elongated slit 

 (Plate XXIII, p). Encysted forms are produced (Plate XXIII, q). 



Cercomonas, Bodo and Prowazekia are of rare occurrence, and 

 it is doubtful if they be true parasites : they may be free-living 

 forms accidentally introduced into the host. The general charac- 

 ters of these three organisms are sufficiently indicated by the 

 figures (Plate XXIII, r, s and t). 



Trypanosomata l 



The trypanosomes are all parasitic in the blood of vertebrates, 

 and a blood-sucking invertebrate is almost invariably concerned 

 in their transmission. In the case of each pathogenic trypano- 

 some, some indigenous wild animal, tolerant to that form, serves 

 as a reservoir from which infection is derived. 



1 For current literature on trypanosomes and trypanosome diseases, see 

 The Tropical Diseases Bulletin. 



