THE HAEMOFLAGELLATES 



251 



The sub-classification of this genus, or rather the grouping and 

 arrangement of the numerous Trypanosomes at present included in 

 it, is a question of great difficulty and one in regard to which 

 hardly anything has been done as yet. 1 This is chiefly owing to the 

 fact that so little is still known of the life-history of most that 

 hitherto any attempt to group the parasites has been dependent upon 

 their adult morphology. This is not a very satisfactory criterion, 

 since, as we have seen, on the one hand, the differences in this 

 respect between different forms may be very slight ; and on the 

 other, a particular parasite may itself vary very greatly at different 

 times and under different conditions (see under " Morphology "). 

 Moreover, it may very well be that as more life-histories come to be 

 revealed, some of the forms at present placed for convenience in 



D. 



FIG. 32. 



A, Trypcinosoma gambiense (from the blood), after Bruce and Labarro ; B, T. equinum, after 

 Lignum ; C, T. evansi, from an original drawing ; D, T. equiperdum, after Lign. 



the genus Trypanosoma will have to be transferred to new ones 

 (as an example may be mentioned T. grayi). 



For the present, at any rate, a very useful aid towards dis- 

 tinguishing different species is furnished by the biological relations 

 of the parasites. For it may be assumed that here, as is usual 

 among the Sporozoa, a particular species is, in general, restricted 

 either to one particular host, or, at most, to a few allied ones. 

 Difficulty arises in considering the Mammalian forms, many of 

 which have never been observed in the true, natural hosts, but only 



unusual shape, position of kinetonucleus, etc. The occurrence, however, of an 

 allied form in Hyla, which is evidently intermediate between T. rotatoriwn and 

 the more typical, fusiform species, suggests that the former also belongs to the Hetero- 

 mastigine section. 



1 Koch, however, has attempted a classification of the Mammalian forms, which he 

 arranges in two groups, the first including such different forms as T. leivisi and the 

 large T. theileri of cattle ; the other, most of the lethal forms, which he considers 

 are not distinct species. This arrangement is very artificial and has nothing to 

 recommend it. 



