STRUOL'L'RE AND LIFE-HISTORY OF CRITHIDIA MELOPHAGIA. 195 



as to this rigidity. The anterior end, to which the flagellum 

 and undulating membrane is attached, is naturally more 

 flexible than the posterior end, and its movements are more 

 marked. 



Movements of contraction of the posterior end of the body 

 of 0. melophagia result in a temporary concenti'ation of the 

 protoplasm around the nucleus of the organism. The body 

 then resembles a short, ^hick pear, drawn out at its anterior 

 end into a long, narrow stalk. Sometimes the body remains 

 in this condition, which is fairly common in forms about to 

 encyst, and in such forms withdrawal or degeneration of the 

 flagellum, followed by the secretion of a thm gelatinous wall, 

 completes the encystment. In other parasites from the 

 stomach, whei'e no encystment occurs, this concentration of 

 the pi'otoplasm in the nuclear region is not so marked, and 

 when relaxation occurs the organism is propelled forward with 

 a very slight jerk, and repetition of the contraction follows, 

 as has been before described. The jerking is never so 

 marked as in Herpetomonas, for the membrane has the 

 effect of producing smoothness of motion. 



Reversal of the direction of motion occurs and is very 

 rapid. The flagellum swings out, describing a semi-circle, of 

 which the body acts as the diameter for an instant, but the 

 force of the movement of the flagellum is so great that the 

 body also swings outwards in a line with the flagellum, and 

 the organism moves away, not exactly in the same course as 

 before, but in one at a very small angle to it. The path of 

 the organism is frequently parabolic in nature. 



Many peculiar movements can be observed when C. melo- 

 phagia is endeavouring to free itself from debris in the 

 lumen of the gut. Much writhing, both of the flagellum and 

 body of such a parasite, is then seen, and the organism often 

 swings round and round, the point of attachment serving as 

 the centre of rotation. If the posterior end should be 

 attached, the flagellum executes violent lashings and spiral 

 movements, these latter not being, as a rule, very noticeable 

 in the normal organism. 



