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Crithidia pidicis n. sp. 
human body, mainly using Pulex irntans, the common human flea. 
Some of the P. irritans contained the parasite that I have named 
Crithidia pidicis, a typical Crithidia, with its life history complete 
within the flea and to this parasite the present paper refers. 
Material and Methods. 
The fleas used for the study of C. pidicis were specimens of 
P. irritans obtained from various parts of England. The work has 
involved much time and trouble, for I used only bred fleas for my research, 
and, at considerable personal inconvenience, bred and reared the fleas 
upon my person, keeping them in conflned areas by means of celluloid 
and rubber “ flea-cages of my own designing. In order to avoid all 
possibility of contamination of the fleas by feeding on otlier animals, 
e.g. cats, dogs or rats, the fleas were bred to the third generation and 
these were the fleas used for dissection. No blood other than my own 
was used for feeding the fleas, a point on which I would lay stress. 
An interval of two days after the last feed was usually allowed to 
elapse before dissection as the preparations of C. pidicis were cleaner 
than if the fleas were dissected immediately after feeding. 
The alimentary canals of about 200 adult and larval fleas were 
carefully dissected and preparations made of their contents. Much 
time was given to the examination of fresh preparations of the Crithidia 
contained therein. The percentage of infected fleas was low, and hence 
much waste of time occurred in the breeding of “ unprofitable ” hosts 
and in examination of the same.' All other organs of the fleas were 
examined and faeces of the fleas were constantly searched. 
With regard to stained preparations, both wet and dry methods of 
fixation were used. The chief stains used were iron-haematoxylin, 
Delafield’s haematoxylin and modifications of the Romanowsky stain, 
especially Giemsa’s stain. Methylene blue was useful for intra vitani 
staining. Fixation by osmic vapour followed by absolute alcohol proved 
quite satisfactory. Formalin vapour followed by absolute alcohol also 
was useful. 
In connection with the occurrence of a Crithidia in the human flea 
bred in the manner described previously, I should like to state that 
the numerous detailed examinations made of my blood by smears, thick 
films and cultures have entirely failed to reveal the presence of any 
trypanosome. The bearing of this statement will be discussed later. 
