THE H^EMOFLAGELLATES AND ALLIED FORMS 285 



Effects on the Host. The trypanosomes found infesting wild 

 animals in Nature are, as a rule, quite specific to a particular host, 

 and, so far as can be observed, perfectly harmless to it. If the 

 relations between host and parasite had always been of this type 

 in all cases, our knowledge of trypanosomes would be in a much 

 more backward state even than it is. Of recent years a vast 

 amount of attention has been attracted to these parasites owing to 

 the diseases of man and animals caused by certain species of trypano- 

 somes, and hence termed comprehensively " trypanosomiases." 

 The greater number of these pathogenic species belong, from the 

 structural point of view, to a type which may be called the brucii- 

 type (Fig. 12) ; such are T. brucii, cause of tsetse-fly disease ; T. gam- 

 biense, of sleeping sickness ; T. evansi, of surra ; T. equiperdum, of 

 dourine ; and many others. The structural similarity of these 

 species renders their identification a matter of extreme difficulty. 

 Of a slightly different type is T. equinum, of " mal de caderas " in 

 South America, with a very minute kinetonucleus ; but the recently- 

 described T. hippicum of " murrina " (Darling, 428) appears to be 

 a typical member of the brucii-group. T. theileri, on the other 

 hand, from cattle, is very distinct in size and appearance from the 

 members of the brucii-gjtoup. Finally, T. cruzi, the cause of human 

 trypanosomiasis in Brazil, stands apart from all the others in 

 peculiarities of reproduction and development, which have led to 

 its being ranked in a distinct subgenus, Schizotrypanum. 



The problem of the pathogenic trypanosomes has been touched 

 upon in Chapter II. From a survey of trypanosomes in general, it 

 is clear that the normal type of these parasites is one which is specific 

 to one or to a limited number of species of hosts, to which it is quite 

 harmless. The pathogenic species are to be regarded as aberrant 

 forms not yet adapted to their hosts, as an instance of a disharmony 

 in Nature. They are species which have probably established 

 themselves but recently in the hosts to which they are pathogenic. 

 As contrasted with the natural, non-pathogenic forms, their most 

 striking peculiarities are that they are not specific to one host, but 

 can flourish in a great number of different species of hosts, and that 

 in susceptible animals their power of multiplication has no limit. 

 T. brucii, so deadly to many domestic animals, is known to occur 

 also as a natural parasite of wild animals, to which it is harmless. 



Structure. The constitution of the trypanosome-body is of a 

 very uniform type in its general traits, though subject to great 

 variation in different cases as regards size, form, and minor details 

 of structure. The body is typically long and sinuous, with the 

 anterior end tapering gradually to a fine point, while the posterior 

 extremity is usually broader, and tapers more abruptly, or ends 

 bluntly ; but in different forms, even of the same species, there may 



