200 ANNIE PORTER. 



The Post-flagellate Stage of C. melophagia in th 

 Rectum of Melophagus ovinus. 



The prepai'ation of Crit India melophagia for life outside 



the body of the host occurs in the rectum of the sheep-ked. 



Large numbers of small flagellates (PI. 12, figs. 27-29) are 



present in the hind gut, also some forms in process of division 



(PI. 12, figs. 97, 98). The small forms attach themselves to 



the wall of the rectum and encyst there, but encystment can 



be watched when the rectal contents are expressed on to a 



slide and examined under the microscope. The flagellate 



(text-fig. 1) at first executes violent lashing movements with 



its fiagellum, and during this motion migration of the nucleus 



nearer the flagellar end of the organism frequently occurs 



(text-fig. 2). At the same time the body of the Crithidia 



shortens and thickens (text-figs. 3, 4; PI. 13, fig. 100), waves 



of contraction passing rhythmically down the body, which 



gradually may become somewhat triangular (text-fig. 5; PI. 13, 



fig. 101). The fiagellum meanwhile shortens (text-figs. 5, 6), 



and the organism may bend on itself (text-figs. 0, 7) during 



this period. Concentration of the protoplasm occurs, the 



flagellum becomes less wavy (text-fig. 7), and, little by little, 



it contracts nearer the body (text-figs. 8, 9; Pi. 13, figs. 102- 



106) and is withdrawn, the parasite becoming oval (text-tig. 



10; PI. 13, figs. 109-112). The organism at this time 



becomes surrounded by a thin layer of refractile, gelatinous 



substance, which rapidly hardens to form a closely adherent 



resistant cyst-wall. The oval bodies (PI. 13, figs. 109-114) 



so produced are post-flagellate forms, which become detached 



from the walls of the rectum, and pass out with the faeces of 



the ked, from which ffeces they can be recovered. These 



cysts, which measure from 2'5^ to 5"5 ^t by 1*5 /i to 3^, serve 



for the infection of other Melophagus ovinus. 



All Crithidia melophagia do not go through a post- 

 flagellate stage in the gut of their host. Some, after passing 

 a portion of their existence as flagellates in the gut of the ked. 



