THE HAEMOFLAGELLATES 247 



here into the reasons for and against this diphyletic view, which 

 was first put forward by Le"ger (49). A complete discussion will 

 be found in his Review of the Haemoflagellates (pp. 270-278). 

 Liihe, in his account of the Haematozoa in Mense's Handbuch der 

 Tropenkrankheiten (2), has adopted it, though on somewhat different 

 lines from those taken by us. Minchin, also, has expressed the 

 opinion (Brit. Mai. Journ., 1907, ii. p. 1320) that - Trypanosomes 

 are most likely diphyletic. On the other hand, many authorities, 

 including Laveran and Mesnil, hold the view that all Trypanosomes 

 are descended from Herpetouionadine ancestors, basing their opinion 

 on the resemblance to a Herpetomonad shown by many Trypano- 

 somes in cultures, and by young individuals of T. leivisi (cf. Fig. 20). 

 In many cases, at any rate, we regard this phase as we have 

 previously said rather as a " pseudo-Herpetomonadine " condition ; 

 and in such cases do not attribute to it the phylogenetic importance 

 which is done by some, but consider it to be probably capable of 

 explanation on other grounds (see I.e.}. A fact which seems to us 

 of considerable significance is that Trypanoplasmatine forms are 

 known to occur in the digestive tract of fishes, e.g. " Trypanoplasma " 

 intestinalis in Box boops, and " T." rentriculi in Cydopterus lumpus; 

 moreover, another Heteromastigine parasite (Bodo lacerfae) is found 

 in a lizard. On the other hand, no indubitable Herpetomonad has 

 yet been described from the alimentary canal of a Vertebrate, which 

 we may assume to have been the original habitat of the primitively 

 Vertebrate parasites. 



Hence, all things considered, we come to the general conclusion 

 that the Trypanosomes which have the Vertebrate for their primary 

 host are Heteromastigine forms ; those derived from primitively 

 Invertebrate parasites, on the other hand, are probably Monadinc 

 forms. Endeavouring to use this view practically, for purposes of 

 classification, or, at any rate, of convenient partition of the Trypano- 

 somes, we have as follows : The parasites of fishes belong to the 

 Heteromastigine type ; this can be said with some degree of 

 confidence, in spite of the " Crithidial " forms assumed by the 

 parasites in leeches. Probably the same is true also of most forms 

 of cold-blooded Vertebrates. Of the Avian ones, on the contrary, 

 some at any rate (e.g. those of the type of Trypanomorpha noctuae) 

 are Herpetomonadine forms. Among Mammalian parasites the 

 various lethal Trypanosomes (e.g. T. brucii, etc.) are to be regarded 

 as Heteromastigine forms. We will only mention in passing that 

 certain movements of these forms in the living blood (cf. p. 217) 

 suggest very forcibly that the aflagellar end is the true anterior 

 extremity. Of the other known (accustomed) parasites of Mammals, 

 whose number has considerably increased of late, it is quite possible 

 that some (e.g. those of bats, which may have, perhaps, mosquitoes 

 as their alternate hosts) are Herpetomonadine forms. 



