382 BIOLOGY OF THE PROTOZOA 



Few stages of the life cycle are found in the vertebrate blood. 

 Here they may reproduce by longitudinal division until the blood 

 teems with them or a balance may be established whereby relatively 

 few forms can be found in the circulating blood. Such hosts become 

 carriers for many different species of trypanosomes, as appears to 

 be the case with African wild animals. 



Developmental stages, on the other hand, are well known in the 

 invertebrate hosts, the most complete account being that of Minchin 

 and Thompson (1915) for Trypanosoma lewisi of the rat in the rat 

 flea, Ceratophyllus fasciatus. Here a most unusual somatella phase 

 occurs in the stomach cells of the flea which is described on page 233. 



The young trypanosomes after leaving the stomach cell may 

 enter other stomach cells and repeat the process, or they may pass 

 down the intestine to the rectum where they, like Crithidia, become 

 attached to the epithelial cells (Fig. 122, p. 234). From here they 

 may swim off as Leptomonas forms or remain and divide as Crithidia 

 types. The rectum is, apparently, a site of multiplication, necto- 

 monad and haptomonad stages succeeding one another until finally 

 the metacyclic or transmitting types develop from haptomonads. 



It is probable that intracellular stages occur in the invertebrate 

 hosts of other species of Trypanosoma but the life history is known 

 in relatively few cases. The method of infection of vertebrate hosts 

 depends largely upon the site of accumulation of the metacyclic 

 forms in the invertebrate host. If in the rectum, as is the case with 

 Trypanosoma lewisi in the rat flea, infection of the vertebrate is 

 brought about by the contaminative method, i. e., by ingesting the 

 feces of the invertebrate or eating it whole. If, on the other hand, 

 the metacyclic trypanosomes accumulate in the salivary glands, 

 hypopharynx or other mouth parts of the invertebrate host, infec- 

 tion is inoculative. Duke (1913) suggests that trypanosomes of 

 the latter type might be described as having an anterior station, 

 and Wenyon (192(5) attempted a rough classification of the patho- 

 genic trypanosomes into those having an anterior .station and those 

 having a posterior .station in the invertebrate host. Among the 

 former a further grouping is made by Wenyon according to the 

 knowm invertebrate host and the anatomical part in which the 

 trypanosome development occurs. Thus in tsetse flies development 

 in the stomach, proboscis and salivary glands is characteristic of 

 Trypanosoma brucei (cause of nagana in cattle and of human 

 sleeping sickness in Rhodesia); T. (jambicn.se (cause of human sleep- 

 ing sickness); development in stomach and proboscis: T. congolense 

 of cattle, horses and sheep; 7 . simiae of monkeys; development only 

 in proboscis: T. vivax of cattle, sheep and goats; T. caprae in cattle, 

 sheep and goats, also T. uniforme of the same hosts. 



In tabanid flies and other blood-sucking arthropods development 

 in this anterior station is characteristic of Trypanosoma, evansi, the 



