240 
Crithidia pidicis n. sp. 
is very mobile and at times folds completely over so that the posterior 
end of the body lies parallel to the anterior end and the parasite is 
looped like a U. In ordinary quiet movement the body of the parasite 
may turn on itself, so that the posterior end seems folded just above the 
nucleus. (PI. X, fig. 13.) 
Entanglement movements were fairly common, but aggregation 
rosettes of G. pulicis were not nearly so numerous as in the case of 
C. melophagia or C. gerridis. One point about the aggregation rosettes 
of C. pulicis is the remarkable billowy effect due to the undulations of 
the membranes of the parasites. 
Movements occurring during longitudinal division will be described 
in the section dealing with division. 
Morphology. 
Crithidia pulicis, like other insect flagellates, exhibits the three 
typical phases of a Crithidia in its existence. While the general life 
history is quite typical, there are many points that apply to C. pulicis 
only, and hence I prefer to establish a new species for it, rather than 
amalgamate it with G. ctenophthalmi, the parasite of the rat flea, 
Ctenoph tha Imus agyrtes. 
The Pre-Jlagellate Stage. 
Pre-flagellates (PI. X, figs. 1-8) of G. pulicis are most abundant in 
the whole length of the alimentary canal of the larva of P. irritans or 
in the crops of young adiflts. They are oval or rounded bodies in their 
simplest form, measuring 2'3 p, to 7'0 p by 1'5 p to 4‘5 p. The general 
cytoplasm is faintly granular (figs. 1-8). The nucleus is well marked 
(figs. 1, 5, 6), and contains more chromatin than that of most other 
Crithidia. A small karyosome may be present (figs. 1, 6). The shape 
of the nucleus has a general resemblance to that of the entire pre¬ 
flagellate. The blepharoplast is large (figs. 4, 7, 8) and is either 
straight (figs. 2, 8), oval (fig. 7), or slightly curved (fig. 1). It is 
very obvious in life. In the older pre-flagellate (figs. 6-8), a clear area 
can often be distinguished in life, either to one side of the nucleus or 
below the blepharoplast, its position depending on the amount of 
elongation that the parasite has undergone. This is the area from 
which the flagellum develops. It takes the chromatin colouration in 
stained preparations. When the pre-flagellate elongates, the flagellar 
