A. Porter 
239 
Distribution of Crithidia pulicis in Pulex irritans. 
Crithidia pulicis shows three characteristic phases in its life history 
and is thus a typical Crithidia. It occurs throughout the alimentary 
tract of its host, both adult and larval fleas containing the parasite. 
The small oval or rounded pre-flagellates, having some resemblance to the 
Leishman-Donovan bodies, are most abundant in the entire alimentary 
tract of the larval fleas and in the crops of young adults, Fully 
developed flagellates, either growing or in process of division, are best 
observed in the midgut and intestine of the adult flea, while the post- 
flagellates are most numerous in the rectum of mature fleas and have 
often been recovered from the faeces of infected hosts. 
Forms other than non-flagellates have not been found in the mouth 
or proboscidal regions of the flea, nor have fully developed flagellates 
been observed in the oesophagus of adult fleas. 
Up to the present, there is no evidence of the occurrence of G. pulicis 
in organs other than those of the digestive tract. Hereditary infection, 
which has been fully demonstrated in G. vielophagia, has not been 
shown so far in C. pulicis. 
Movements. 
G. pulicis is very active in its movements which are brought about 
chiefly by the undulating membrane, the flagellum being relatively 
short compared with that of such Crithidia as G. gerridis. G. pulicis 
moves with the anterior (flagellar) end forwardly directed and progression 
is aided by waves of contraction that pass from behind in a forward 
direction along the body. The wave-like motion is due to the 
myonemes in the body and membrane, which myonemes can be seen 
in life by the aid of the paraboloid condenser. The same phenomenon 
has been observed in living Spirochaetes and in G. vielophagia. The 
myonemes in neither of these cases are artifacts, as some recent 
dogmatic writers would have us believe. Living organisms do not 
consistently display artifacts. The myonemes of the membrane are 
more marked than those of the body. 
C. pulicis is capable of rapid reversal of its direction of motion. 
This is brought about by the organism swinging rapidly in a semi-circle, 
the posterior end acting as a centre of rotation. 
Movements of flexion are more common in G. pulicis than in any 
other Crithidia that I have examined. The posterior end of the body 
Parasitology iv. ' 16 
