A. Porter 
251 
Summary. 
1. Crithidia pidicis, n. sp., is a parasite of the alimentary tract of 
the human flea, Pidex irritans. 
2. The fleas used in this investigation were bred to the third 
generation in special “flea cages” on the human body. “Wild” fleas 
were examined and yielded the same parasite. Both larvae and adult 
fleas were examined. 
3. C. pidicis exhibits pre-flagellate, flagellate and post-flagellate 
stages in its life history and these phases gradually develop, the one 
from the other. 
4. The flagellate form is very active, movement being accomplished 
by means of the myonemes of the membrane and body. 
5. Pre-flagellates (figs. 1-8) are oval bodies, from 2'3 p to 7 fM long 
by 1'5 p to 4‘5 p broad. The nucleus contains much chromatin and 
a small karyosome may be present. The blepharoplast is large. The 
flagellum develops from a chromatophile area. The rate of acquisition 
of flagellum and membrane varies in individual parasites. Division 
rosettes are frequent. The pre-flagellates have a somewhat frail ap¬ 
pearance. 
6. The flagellates of C. pidicis (flgs. 9-17) are 26 /a to Qb p long. 
Their cytoplasm is richly but finely granular, the nucleus well marked, 
the blepharoplast large, showing slight variations in shape. Chromidia 
may be present. The undulating membrane is large, well marked and 
possesses myonemes which can be detected in the living organism. The 
free flagellum is relatively short. 
7. Post-flagellates (figs. 23-36) are from p to Q p by 2'0 p 
to 4'6 p. They occur in the rectum and dejecta of the fleas. They are 
produced by concentration of the protoplasm round the nucleus and 
blepharoplast with absorption of the membrane and flagellum and 
finally the production of a thin varnish-like coat. 
8. Longitudinal division (figs. 15-22) is the method of multi¬ 
plication. It occurs in the pre-flagellate and flagellate stages. It may 
be symmetrical or asymmetric. Division is initiated by that of the 
blepharoplast, followed by that of the flagellum, membrane and nucleus 
and finally the body cytoplasm. Two types of asymmetric division 
have been observed. 
