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Crithidia gerridis 
The pre-flagellate and post-flagellate stages of the parasite will be 
described fully in subsequent sections. These stages have a general 
resemblance to the Leishman-Donovan bodies of Kala-azar. 
Movements. 
The movements of the adult flagellate are very rapid but can be 
studied with comparative ease when the parasite is enclosed within a 
limited space by debris present in the gut. 
In forward progression, the flagellum is anterior and the undulating 
membrane can be seen, its chromatic (flagellar) border appearing as a 
bright line. The membrane is truly undulatory, crests and troughs 
of waves appearing along it. As the organism moves forwards, the 
flagellum executes spiral, lashing movements and waves pass down it 
and the membrane to the posterior end of the body. The effect is to 
propel the flagellate forwards. At the same time that the organism 
moves forwards, it also rolls slightly from side to side. This movement 
has been noticed by Patton (1907) in G. gerridis from Gerris fossarum, 
Microvelia and Perittopus. 
When the movements are very rapid, the anterior part of the body 
also seems to take some share in the lashing movement and the posterior 
region of the body has been seen in violent motion when the parasite 
had become attached by its flagellum to some debris and was endeavouring 
to free itself therefrom. The parasite may attach itself at times to the 
gut-wall of the host by means of the flagellum. 
Reversal of the direction of motion takes place very rapidly and 
with ease. The flagellum curves gradually in the direction required and 
the parasite moves on a seemingly parabolic path till its body is almost 
parallel to its first position. The flagellum which initiates the reversal 
of direction at times lies quite close to the body. The latter then 
suddenly swings through an angle of 180° and lies almost in a line with 
the flagellum. The parabolic path is not nearly so obvious in this 
case. 
Now and again I have watched parasites among debris. They have 
been slightly fixed at the posterior end while the anterior end has been 
free. The whole organism moved actively, its free end rotating through 
360° in less than a second, the attached end acting as a pivot about 
which rotation occurred. 
This movement would appear not to be a common one, but 
due to the position of the organism at the time. 
