THE H^MOFLAGELLATES AND ALLIED FORMS 321 



serpentine body, probably as an adaptation to life and movement 

 in a broth-like medium, containing numerous suspended bodies, 

 such, as occurs in the gut of an insect, especially a blood-sucking 

 insect, or in the blood-fluid of a vertebrate. Leishmania, on the 

 other hand, represents an offshoot from the main stem in which the 

 resting, non-flagellated phase has become the most prominent stage 

 in at least one part of the life-cycle. 



On the other hand, the Haemo flagellates of the biflagellate type, 

 Trypanoplasma and Prowazekia, stand sharply apart from the 

 uniflagellate genera. The orientation of the body, and of the undu- 

 lating membrane, when present, in particular, is entirely different in 

 the two types. The development in the invertebrate host of Trypano- 

 plasma and Trypanosoma, respectively, are quite distinct In type, 

 rieither form passing through any stages which suggest the slightest 

 affinity with the other. The only feature common to the two types 

 is the possession of a kinetonucleus in addition to the principal 

 nucleus, and it is questionable to what extent this structure can 

 be relied upon to indicate affinity. The large kinetonucleus of 

 Trypanoplasma is very different in appearance from that of the 

 uniflagellate genera ; and, according to Alexeieff (324), it is a struc- 

 ture of quite a different order from the cytological point of view. 

 Finally it should be remarked that it is only in the biflagellate 

 genera that parasitism in the gut of vertebrates is known to occur, 



With regard to the origin of the forms parasitic in blood, two 

 theories have been put forward. Leger (545) and Brumpt (389) 

 have upheld the view that they were originally parasites of the 

 digestive tracts of invertebrates, as many allied forms still are ; 

 that in many cases their invertebrate hosts acquired the blood- 

 sucking habit, whereby the intestinal flagellates became accus- 

 tomed and adapted to life in blood ; and that, finally, forms so 

 adapted passed from the invertebrate host into the blood of the 

 vertebrate itself. Minchin (476), on the other hand, suggested 

 that possibly the ancestral forms may have been parasites of the 

 digestive tract of vertebrate hosts, and may have passed from the 

 digestive tract into the blood, whence they were taken up by blood- 

 sucking invertebrates and transmitted to fresh hosts, acquiring 

 finally the power of being parasitic upon, and establishing themselves 

 in, the invertebrate host. 



It must be admitted that all evidence which has accumulated 

 of recent years is in favour of the view of Leger and Brumpt. so 

 far as the uniflagellate forms are concerned. The types denoted 

 by the generic names Leptomonas, Crithidia, and Trypanosoma, 

 form a perfect evolutionary series, beginning with monogenetic 

 parasites of invertebrates and culminating in digenetic blood- 

 parasites. From the same stem other forms of parasitism are seen 



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