A. Porter 
353 
Very frequently great clusters of G. gerridis (PI. IY, Fig. 47) occur 
when the gut contents are placed on a slide. The parasites then are 
very conspicuous and the combined movements of them recall the 
movements of ciliated epithelium. As a rule, each Crithidia has its 
own independent motion. It is worthy of note that the shimmering 
appearance of the groups is due to the twisting movements of the bodies 
of the parasites, for in these aggregations the flagella are towards the 
centre of the group and serve to attach the organisms to the anchoring 
cells or debris, and so there is little or no flagellar motion. Parasites 
exhibiting torsion of the body are shown in Figs. 30 and 54, also some 
in Fig. 47. 
Often the organisms seem to swell out and thicken in the centre, a 
somewhat “peg-top” like appearance resulting. This is not easily seen 
in fixed smear preparations though occasionally it is found (PI. IV, 
Figs. 23, 38). 
At times, forms are seen in which the posterior end of the body has 
contracted considerably so that the membrane appears of relatively 
greater length than in ordinary specimens. At the same time the 
membrane moves violently, being thrown into large waves along the 
body. The Crithidia then has an appearance very like that of a 
trypanosome, but if conditions of light and temperature become more 
favourable, normal movements are resumed, the resemblance to a 
Trypanosome disappears, and one realises that the seeming Trypanosome 
is only a Crithidia moving in an aberrant fashion. 
Morphology. 
The life of the Crithidia may be considered as having three phases: 
(1) a stage in which no flagellum is obvious as such, but in which a 
flagellum gradually develops—the pre-flagellate stage; (2) a fully 
flagellated stage; (3) a form specially adapted for resisting desiccation, 
possessing no flagellum, but existing as a cyst—the post-flagellate 
stage. 
The Pre-flagellate Stage. 
This phase of the parasite is best seen in the nymphs of Gerris 
paludum. When the crop of an infected nymph is examined, small, 
oval or pear-shaped bodies, like Leishman-Donovan bodies, are found 
in it (PL IV, Figs. 1—4). Occasionally they are somewhat quadrilateral 
in form. They have no flagellum. These are the early pre-flagellate 
