358 
Crithidia gerridis 
(PI. IV, Figs. 44, 45, 54) are enormously long—some 60/z. to 90 g — 
compared with others. Small, short forms (Figs. 21, 29, 36, 48) 
also occur side by side with them. These small Crithidia have been 
produced by the division of young forms whose flagella were at the 
time but little developed. Stout parasites (Figs. 26, 37) are those 
as yet undivided. Repeated longitudinal division results in the pro¬ 
duction of very thin spirochaete-like individuals (Figs. 29, 59, 60) as 
before mentioned. 
The great polymorphism exhibited by most insect flagellates is 
extremely well seen in C. gerridis. 
At times it would be easy to mistake cases of sub-equal longitudinal 
division (Fig. 56) for conjugation of two flagellates, were the evidence 
of stained preparations only considered, or the division not watched 
from beginning to end in living specimens. This possibility of error 
has been pointed out by other workers on the group, and my own 
experience of C. gerridis and other Crithidia , and of Herpetomonas, 
leads me also to urge long and continuous observation of the living 
organisms as the only means of avoiding such errors. 
Mode of Infection. 
Gerris paludum, though a water-bug, very readily drowns if sub- 
mei'ged. As evening approaches, the bugs have been noticed to seek 
the safer foothold afforded by flat-leaved water plants such as Lemna 
major and Hydrocharis, and rest, almost motionless, upon the surface of 
the leaves. When disturbed, blackish patches on the leaves due to 
faeces show where the Gerris have been. Examination of such faeces 
has shown that they contain post-flagellate “cysts’’ of C. gerridis. The 
Gerris, especially young nymphs, soon return to their former resting- 
places, and some at any rate, have been seen to suck up the moisture 
on the surface of the leaves, and thereby absorb the encysted Crithidia. 
This casual mode of infection has been proved for other species of 
insects. 
The question of hereditary infection remains to be settled. Swingle 
(1908) has found developing forms of Crithidia in the ova of Melopliagus 
ovinus infested by Crithidia melophagia and I have evidence confirming 
this observation. But in the case of C. gerridis, though parasites have 
been seen in the ovaries, no stage of development of the parasites has 
been seen as yet in the ova. These facts are not without biological 
significance, for they indicate a tendency towards the evolution of 
