136 University of California Publications in Zoology [VOL. 19 



INTRODUCTION 



In my work on the flagellate parasites of hemipteran insects during 

 the past five years the evidence indicating a close relationship between 

 Crithidia and the crithidial stages of Trypanosoma has been continu- 

 ously accumulating and has become more and more convincing. The 

 relationship is shown both in their morphology and in the stages of 

 their life cycles. This paper presents a comparison of the morphology 

 and the life cycle of the genus Crithidia with that of Trypanosoma in 

 its crithidial stages. 



Before taking up this comparison a brief discussion of the phylo- 

 genetic relations of Leptomonas, Herpetomonas, Crithidia, and Try- 

 panosoma will be of value in clarifying the subject. In addition a 

 short discussion concerning the hosts, their food, and their methods of 

 infection will give a fundamental conception of the problem in hand. 



The phylogenetic relation of these intestinal flagellates, Lepto- 

 monas, Ilerpetomonas, and Crithidia, to the haemoflagellates, or Try- 

 panosoma, has received the attention of many investigators and 

 consequently has been the subject of much controversy. Only a brief 

 statement of the historical side of this controversy need be given here. 

 Minchin (1908) discusses the possible sources from which the trypano- 

 somes may have been evolved, namely, from the herpetomonad-like 

 and the trypanoplasma-like ancestors. At that time as well as now 

 the evidence pointed to the crithidial (herpetomonad-like) forms as 

 the true recapitulative, developmental, primitive stage in the life 

 life cycle of Trypanosoma. Minchin (1912) stated: 



The types denoted by the generic names, Leptomonas, Crithidia and Try- 

 panosoma form a perfect evolutionary series with monogenetic parasites of inver- 

 tebrates culminating in digenetic blood parasites. It must be emphasized, however, 

 that any such conclusions are of a tentative nature and can have no finality but 

 are liable to modification with every increase of knowledge concerning these 

 organisms. 



Wenyon (1913) also discusses the whole question of the phylo- 

 genetic relationship of Leptomonas, Herpetomonas, Crithidia and 

 Trypanosoma. If the trypanosome be regarded as the highest stage 

 of development then his conclusion is that the phylogenetic order of 

 these flagellates would be : Leptomonas, Crithidia, Herpetomonas, and 

 Trypanosoma. He makes a distinction between the genera Lepto- 



