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and fig. 4 shows an early stage in this process, while fig. 6 represents 
a more advanced stage, the anterior end having rounded up. One of 
us has figured and described somewhat similar changes in the post- 
flagellate stage of Crithidia gerridis. 
It is quite common to find these encysting flagellates in all stages 
of division, and fig. 5 shows two very nearly separated. After the 
anterior end is drawn up completely the flagellum is shed and in some 
of the parasites they are seen as tags attached to the bodies of the 
parasites; we have depicted this appearance in fig. 7. In fig. 8 we show 
the final stage of encystment; these bodies are round or oval and contain 
a nucleus and blepharoplast. There were only a few of these forms in 
the preparations, the majority of the parasites were still flagellates. In 
other species we have found these cysts in enormous numbers in the 
rectum and hindgut of their hosts. This, then, is a mere outline of 
a part of the life-cycle of this Crithidia ; we do not pretend that our 
observations even on this stage are complete. It would be necessary to 
examine the parasites in the fresh condition and watch these changes 
taking place. Unfortunately our material has been limited so that we 
are not able at present to undertake an extensive study of the parasite. 
It will be seen from our description and figures that this flagellate 
corresponds exactly with our definition (see below) of the genus 
Crithidia. In its adult flagellate stage the antei’ior end is drawn out, 
and it has a rudimentary undulating membrane ; we propose therefore 
naming it Crithidia ctenopthalmi. It is important to consider how the 
fleas become infected, and we need hardly say that this flagellate is in 
no way connected with T. leivisi. We have examined a number of fleas 
( C. agyrtes) from rats heavily infected with T. lewisi but have never seen 
any developmental changes similar to those described by Prowazek in lice. 
The fact that we have seen the encysted stages of this parasite in the 
adult flea suggests that the cysts are passed out in the faeces of the flea 
and are ingested again either by the adult flea or its larva. It is obvious 
that the larva is much more likely to ingest the cysts and one of us has 
found a similar Crithidia in the larvae of Ctenocephalus felis. We are 
at present breeding fleas ( C. agyrtes ) and hope later, should we have 
sufficient material, to study this flagellate completely. 
Flagellates appear to have been first recorded from fleas by 
Balfour (1906), who found them in the hindgut of Loemopsylla cleopatrae, 
Rothschild. This parasite, as far as we can judge from Balfour’s figures, 
is a Crithidia. Swingle (1907) was the next to record the occurrence 
of flagellates in rat fleas (species not named) in Nebraska; he at first 
