350 
Crithidia gerridis 
preparations were covered and the edge of the coverglass was surrounded 
by vaseline, or similar preparations with wax feet which were then 
vaselined at the edges were made, while hanging drop preparations were 
also used. Methylene blue and neutral red were sometimes used as 
intra vitam stains but were not found of much value. 
For staining purposes, freshly teased material was fixed wet with 
formalin vapour or osmic acid, then transferred to methyl or ethyl 
alcohol and finally stained. Many preparations were mounted in 
Canada balsam, and such preparations were found of greater service 
than the ordinary uncovered films. Some preparations were also fixed 
wet with Bouin’s fluid (picro-formol-acetic) and with Schaudinn’s fluid 
(corrosive-acetic-alcohol). . 
For purposes of comparison several stains were used. Giemsa’s 
modification of the Romanowsky stain was very serviceable, though it 
has the effect here as elsewhere of intensifying the chromatin. The 
procedure often adopted was to fix the wet preparation with osmic acid 
followed by absolute alcohol, transfer it to 50 °/o spirit and grade it down 
to water. The film was then stained with Giemsa’s stain, rapidly 
dehydrated with acetone and then mounted in Canada balsam in the 
usual manner. Gentian violet also was used to stain “wet” preparations, 
and was found specially valuable in determining the structure of the 
membrane, while Delafield’s haematoxylin was of much use for the 
same purpose. Thionin preparations and some stained with iron 
haematoxylin followed by orange-G were also utilised, and the results 
obtained by each method were carefully compared. 
Occurrence of the Parasite in the Host. 
In a heavily infected specimen of Gerris paludum the Crithidia, in 
various stages, are found throughout the alimentary tract and in the 
faeces. Early stages of the Crithidia, the pre-flagellate stages, are the 
most abundant in the crop. The dilated stomach and intestine that 
follow, are often almost entirely blocked by the masses of flagellated 
forms. On one or two occasions, I have seen a few parasites in the 
Malpighian tubes, but this situation is not a common one for them. 
The rectum of the bugs contains post-flagellate or encysted forms 
adapted for resisting desiccation outside the body of the host, as well 
as free flagellates that probably perish. Both these forms are found in 
the blackish faeces voided by the Gerris. 
In a large percentage of the adult female Gerris examined, parasites 
