346 FAMILY: TRYPANOSOMID^ 



In the second the stomach phase occurs, and is followed by invasion of the 

 proboscis, but not the salivary glands. In the third the whole develop- 

 ment occurs in the proboscis, there being no stomach phase. As far as 

 present knowledge goes, the trypanosomes of cold-blooded vertebrates, 

 with the exception of those of land reptiles and crocodiles, develop in 

 leeches. There is a stomach phase leading to invasion of the proboscis 

 and proboscis sheath, from which trypanosomes escape into the wound as 

 the leech feeds. Finally, there is T. equiperdimi, in which an invertebrate 

 host is at all events unnecessary, the infection being handed directly from 

 vertebrate to vertebrate. This trypanosome is undoubtedly allied to 

 those transmitted by biting flies, and evidence has been produced that 

 infection can be sometimes spread by the agency of these insects. The 

 flagellates of the leptomonas type parasitic in euphorbias, which have 

 both an insect and plant host, have been separated under the generic 

 name Phytomonas. 



The classification outlined above and arranged in tabular form below 

 has the advantage of convenience, if nothing more. It, however, recognizes 

 what is definitely known about these flagellates, and probably indicates 

 their phylogenetic history. The leishmania are probably derived from 

 insect leptomonas, and the trypanosomes from a crithidia or herpeto- 

 monas. Those which have a development in the anterior station may 

 have arisen by direct inoculation into the blood, while those with a 

 posterior station may have infected the vertebrate in the first instance 

 by way of the alimentary canal. It is known that certain lizards harbour 

 leptomonas in the intestine. It is very probable that this infection is 

 acquired from the insects on which the lizards feed. Similar flagellates 

 occur in the blood of lizards, and the natural inference is that they have 

 invaded the blood-stream from the intestine. If the insects, which were 

 responsible for the intestinal infection, were accustomed to suck the blood 

 of lizards, it would be possible for them to become infected from the blood, 

 in which case they might or might not lose the power of becoming infected 

 by ingesting the faeces of infected insects of their own kind. 



TABULAR CLASSIFICATION OF THE FLAGELLATES OF THE 

 FAMILY TRYPANOSOMID^E. 



A. Flagellates with only an invertebrate host. Infection is contamina- 

 tive by means of cysts. 



(a) Leptomonas. 



(6) Crithidia. 



(c) Herpetomonas. 



