362 Crithidia gerridis 
recorded from the British water-bug, Gerris pallidum, for the first 
time. 
(2) The parasite occurs throughout the alimentary tract, in the 
ovaries and in the faeces of its host. 
(3) There are three phases in the life-history of Crithidia gerridis, 
a pre-flagellate stage (PI. IV, Figs. 1—10), a flagellate stage, and a 
post-flagellate stage (Figs. 64—69), the latter being adapted for life 
outside the body of the host, and for cross-infection. 
(4) The adult flagellate has an elongate body, and possesses a 
large oval or round nucleus. There is also a smaller, usually rod-like 
mass of chromatin, the blepharoplast, near to which the flagellum arises. 
The flagellum may be as long again as the body, and is attached to it 
by a narrow, undulating membrane, in which myonemes are present 
(Figs. 22—46). 
(5) The movements of the flagellated forms are characteristic ; 
both the body and flagellum take part in the motion. 
(6) The pre-flagellate forms are small, usually oval bodies, 3/x to 7 g 
long, and from 2/r to 4<g broad. Their nucleus often lies to one side 
and is round, the blepharoplast is rod-shaped. The flagellum grows 
out as a thin, delicate thread, in close contact with the body. The 
pre-flagellate phase of the parasite is found in the crop of the nymphs 
particularly, and also in the crop of the adults. 
(7) The flagellate form has the general structure outlined in (4). 
The undulating membrane has indications of myonemes (PI. IV, 
Figs. 41, 44, 45). The flagellum arises near a chromatic dot, the basal 
granule (Figs. 22, 30, 43). In the nucleus sometimes eight large 
chromatic masses may be present, or the grains may be very small. 
The nucleus on the whole is of the vesicular type. The blepharoplast 
is usually anterior to the nucleus, only very occasionally is it lateral. 
Chromidia are scattered in the endoplasm of the parasite (Figs. 26, 
39, 46). 
(8) The post-flagellate stage is a preparation for extra-corporeal 
life. The parasites divide in the rectum, lose their flagella, round 
themselves off and form a thin gelatinous cyst wall that rapidly 
hardens (Figs. 68, 69). These small cysts pass out with the faeces of 
the host. 
(9) Longitudinal division is the chief method of multiplication. 
It may occur in all phases of the life-history and may be equal or sub¬ 
equal. The blepharoplast usually constricts first and division of it 
and of the flagellum follow one another very rapidly, division of the 
