320 FAMILY: TRYPANOSOMID.E 



having at various stages of their development all four types of flagellate. 

 They are passed from invertebrate to vertebrate or from vertebrate to 

 invertebrate. 



It will be seen that to complete the series there should be a form with 

 two hosts corresponding with the genus Crithidia, but no such flagellate 

 is known at present. 



This grouping of these nearly related organisms in the genera defined 

 above may be represented in tabular form (Fig. 153). 



As will be explained below, the type species of the genus Leptomonas 

 is a flagellate of the nematode worm Trilobus gracilis. Unfortunately, 

 this flagellate has never been re-examined in the light of present-day 

 knowledge, so that there is some doubt as to its true nature. For the 

 present purpose it is regarded as having the structure of the members of 

 the genus as described above. The type species of the genus Herpeto- 

 monas is H. muscarum, a common flagellate of the house-fly. It is assumed 

 here that the trypanosome forms which may occur in association with 

 it are actually stages in its development, and recent work supports this 

 view, though some observers believe they belong to a distinct parasite 

 of flies. If this latter view should prove correct, then a new name would 

 have to be found for the genus Herj^etomonas as here defined. It would, 

 however, in no way invalidate the scheme of classification. Knowledge 

 concerning many of these flagellates is still very imperfect. It is probable 

 that some of those which are only known in an invertebrate host will 

 ultimately be found to have a vertebrate one also. On the other hand, 

 those which are only known in a vertebrate host have undoubtedly an 

 invertebrate host as well, though it is at present unknown. There is only 

 one exception to this rule in Trypanosoma equiperdum, the cause of 

 dourine of horses, which passes from vertebrate to vertebrate without the 

 intermediary of an invertebrate, though the possibility of an alternative 

 method of transmission through an invertebrate cannot be excluded 

 entirely. 



The matter is still further complicated by the claims made by some 

 observers that certain of these flagellates, which in nature appear to be 

 purely insect parasites, are experimentally inoculable into vertebrates, 

 and produce in them a condition somewhat resembling that produced 

 by flagellates which naturally have both hosts. It will be realized how 

 closely all the forms are related to one another. Moreover, some of them 

 appear to be actually in a process of transition from one genus to 

 another. This relationship is further illustrated by their behaviour in 

 culture media. For instance, a flagellate which is only known in the 

 trypanosome stage in the blood of a vertebrate will in such a culture 

 develop into a multiplicity of crithidia, leptomonas, and leishmania forms. 



