48 SOME MINUTE ANIMAL PARASITES 



the post-flagellate stage, and serves as the cross- 

 infective agent. When it is taken into the food- 

 canal of a new host, it reproduces the flagellate 

 form in it. The post-flagellate stages (cysts) are 

 capable of resisting desiccation for a considerable 

 period, and may be distributed in the form of fine 

 dust by the wind. 



A Crithidia of much interest is Crithidia pulicis, 

 parasitic in the human flea, Pulex irritans. In order 

 to avoid errors in the investigation of this parasite, 

 fleas were specially bred on the body of a perfectly 

 healthy person, from whose blood no trypanosome 

 has ever been obtained, no matter what form of 

 examination has been employed. The fleas were 

 confined to special areas of the body by means 

 of flexible rubber and celluloid cages. The eggs 

 laid by them were hatched under the same condi- 

 tions, and the process repeated to the third genera- 

 tion of fleas, which were used for the investigation. 



When the oval post-flagellates are voided by 

 infected fleas, they are taken up by the youngest 

 fleas with the excrement and congealed blood on 

 which they live. These tiny fleas resemble small 

 but very active grubs before they reach the adult 

 stage. Adult fleas absorb the post-flagellates when 

 piercing the skin of their human hosts to suck 

 blood. In the fore-gut of the flea the parasites 

 become larger, and as they form the starting-point 

 of the life-history of the Crithidia in the new host, 

 and precede the flagellate stage, they are known as 

 the pre-flagellate forms. 



The pre-flagellate form of C. pulicis is a somewhat 



