CRITHIDIA MELOPHAGIA 53 



excrement of its elders, or by some adult flea whose 

 proboscis pierces the skin in the neighbourhood of 

 the faecal deposit. 



Now, while there is a temporary stage in the life- 

 history of some trypanosomes in which there is a 

 resemblance to a flagellate Crithidia, yet there is no 

 evidence to warrant any necessary interdependence. 

 A Crithidia has a distinct life-history, and the flagel- 

 late form is followed by a post-flagellate, which is a 

 highly resistant form, well adapted for the perpetua- 

 tion of the species. A " crithidial stage " of a 

 trypanosome is but a transitory one. The life-history 

 of a Crithidia is complete in itself; it is a definite 

 entity, fixed, unalterable. 



Another interesting feature may be mentioned 

 here. A Crithidia is parasitic in the sheep-ked, a 

 wingless fly known as Melophagus ovinus. This 

 flagellate (Crithidia melophagia) has been able to 

 penetrate to the ovaries of the ked, and live and 

 multiply within the eggs. The result is that the 

 young keds are born infected, and a generation of 

 infected keds transmits the parasite to their offspring 

 as a matter of course. The case of hereditary infec- 

 tion of the host with C. melophagia has been worked 

 out in detail, and is of great interest. Whether the 

 eggs of Glossinae are infected with the trypanosomes 

 of sleeping sickness has yet to be demonstrated, and 

 this investigation should be a most interesting, though 

 extremely tedious, piece of work. 



Nearly allied to the genus Crithidia, so far as gross 

 external form is concerned, is the genus Herpeto- 

 monas, and this possesses an even greater human 



