356 
Crithidia gerridis 
its lumen, and rapid longitudinal division occurs, one parasite frequently 
giving rise to four smaller daughter forms. But one also finds that 
some of these small forms ( e.g . PI. IY, Fig. 36) have attached themselves 
to the wall of the gut or to portions of debris present in the gut. 
These parasites gradually cease their active movements, their bodies 
become less mobile and their protoplasm seems to concentrate towards 
the centre (Fig. 61). At the same time, the flagellum becomes 
shorter and is retracted nearer the body (Figs. 62, 63). The nucleus 
and blepharoplast approach one another (Figs. 63, 64), and if 
smears containing the parasite at this stage be stained, the major 
part of the rounded-up or oval body takes the chromatin colouration. 
The flagellum finally degenerates (Figs. 65, 66) and disappears 
(Figs. 67—69). It does not seem entirely to be absorbed, but 
would rather appear to be cast off by the parasite, for free flagella can 
be found in fresh preparations of rectal contents of Gerris. A thin, 
gelatinous layer now appears on the outside of the rounded-up organism 
and this layer rapidly shrinks and becomes hard, forming a closely 
adherent, fine cyst wall (Fig. 69). The Crithidia now consists of an 
oval body, containing nucleus and blepharoplast, and is very chroma- 
toid in nature (Fig. 69). The cysts are op, to 6/r long by 3p to 5p, 
broad. This post-flagellate (cyst) form is an obvious adaptation for 
life outside the host; it is expelled with the faeces of the bug and serves 
for the infection of a fresh host. 
Division. 
Longitudinal division of C. gerridis may take place either towards 
the end of the pre-flagellate or during the flagellate stage as was 
pointed out by Patton (1908). 
Longitudinal division as seen at the end of the pre-flagellate stage. 
The parasites about to divide become somewhat elongated and more or 
less oval in shape (PI. IV, Fig. 16), and have a small flagellum. The 
blepharoplast first gives signs of division by elongating and becoming 
somewhat dumb-bell like (Fig. 17). It gradually constricts into 
two (Figs. 18, 19) and the halves separate. The flagellum splits 
simultaneously with the division of the blepharoplast, the division of 
the nucleus follows and the individuals show usually a bilaterally 
symmetrical form (Fig. 18). The general protoplasm becomes 
more tenuous at the long axis of the body and a split appears along it. 
