522 FAMILY: TRYPANOSOMID^ 



lated animals. As a matter of fact, when the whole life-cycle is taken into 

 account, all trypanosomes are markedly polymorphic. If it is understood 

 that only the blood stage in the vertebrate is referred to, the term has its 

 application. Thus T. gambiense, T. brucei, and T. rhodesiense are under 

 these circumstances polymorphic. There are two extreme types of try- 

 panosome. One is long and narrow, and has a well-developed fiagellum, 

 while the other is shorter and broader, and has no fiagellum. A form 

 intermediate between these two types occurs in which there is a short 

 fiagellum. The three types are spoken of as " the short stumpy," " the 

 long thin," and " intermediate " trypanosomes. T. evansi and its allies — 

 T. congolense, T. simiw, T. vivax, T. unifortne, T. cajprce, T. equiperdum, 

 and T. equinum — on the other hand, are monomorphic. In the case of 

 T. evansi, T. equiperdutn, and T. equinum all the trypanosomes have 

 flagella, and, unless they are dividing forms, they are of more uniform 

 dimensions than the polymorphic forms, and are very similar to the long 

 narrow forms of T. brucei. In T. vivax, T. unifor?ne, and T. caprce all 

 forms have flagella and a characteristic swollen posterior end of the body. 

 They differ from one another in size. In the case of T. congolense and 

 T. simicB, again, the trypanosomes are miich smaller than the members 

 of the other groups, and there is no fiagellum. 



In accordance with these morphological distinctions, there are dif- 

 ferences in the developmental cycles in the invertebrate host. Thus the 

 polymorphic forms [T. ga?tibiense and T. brucei) develop in tsetse flies 

 in the stomach, proboscis, and salivary glands; the T. congolense group in 

 tsetse flies in the stomach and proboscis ; and the T. vivax group in tsetse 

 flies in the proboscis only. T. evansi and its allies are not carried by tsetse 

 flies, but are transmitted by tabanids or other biting flies. No develop- 

 mental cycle has been detected in them. T. equiperdmn, which is possibly 

 derived from T. evansi, is in an anomalous position in that it does not 

 require an invertebrate host. 



When the size of any species of trypanosome is referred to, it must be 

 remembered that this is the average size obtained by the measurement of 

 several hundred or a thousand individuals. On the basis of the foregoing 

 facts, the classification of the pathogenic trypanosomes as given on p. 458 

 (Group B, I. and II.) may be extended as follows: 



I. Pathogenic Teypanosomes transmitted by Blood-Sucking 

 Arthropoda. 



1. Pathogenic Trypanosomes transmiUed by Species of Glossina. 



(a) Development in stomach, proboscis, and salivary glands of 

 the tsetse flies. Polymorphic trypanosomes (short stumpy forms 



