318 FAMILY: TRYPANOSOMID^ 



The most generally accepted view is that the trypanosomes of verte- 

 brates were originally purely insect flagellates which gradually became 

 adapted to the blood medium when the insects became blood-suckers. 

 The flagellates then passed into the vertebrate, and became adapted to 

 life in the blood-stream. Minchin (1908), however, held the opinion that 

 trypanosomes were originally intestinal flagellates of vertebrates which 

 thence passed into the blood-stream, and secondarily became parasites 

 of blood-sucking insects. In a later paper (1914) he appears to have 

 relinquished this view, and writes of the ancestral forms of the trypano- 

 somes as insect flagellates. Mesnil (1918), however, expresses himself in 

 favour of the view that trypanosomes originated from leptomonas forms 

 parasitic in the intestine of the vertebrate. It seems to the writer that the 

 evidence available points to the evolution of trypanosomes from purely 

 insect flagellates, as explained above. 



SUBDIVISION INTO GENERA. 



The grouping into different genera of the flagellates belonging to the 

 family under consideration is, in our present state of knowledge, exceed- 

 ingly difficult, and for the want of some definite scheme great confusion 

 in nomenclature has resulted. Some of the flagellates have only an in- 

 vertebrate host; others have two hosts, a vertebrate and an invertebrate; 

 while others, again, are known only in the vertebrate. According as to 

 whether they are limited to an invertebrate, or have both a vertebrate 

 and an invertebrate host, or whether, in their highest stage of develop- 

 ment, they reach the leptomonas, crithidia, or trypanosome form, it is 

 possible to group them in general in the following provisional manner, 

 always remembering that the flagellates which have no vertebrate host, 

 and which pass directly from insect to insect, do so in an encysted stage, 

 which does not occur in those flagellates which pass from insect to verte- 

 brate or vice versa. Other flagellates of the leptomonas type have an 

 invertebrate and a plant host. 



1. Flagellates of the genus Leptomonas are those which never develop 

 beyond the leptomonas stage. In the course of their life-history they 

 show only the leishmania and leptomonas forms. They are confined to 

 invertebrate hosts, and pass from one to another by means of cysts voided 

 with the dejecta. 



2. Flagellates of the genus Crithidia show, in the course of their 

 development, leishmania, leptomonas, and crithidia forms. They are 

 limited to invertebrate hosts, as in the members of the genus Leptomonas, 

 and are transmitted in a similar manner by means of cysts. 



3. Flagellates of the genus Herpetomonas are, again, purely invertebrate 



