320 



THE PROTOZOA 



Affinities and Phylogeny of the Hcemo flagellates. Two opposite 

 views have been held with regard to the origin of the Flagellates para- 

 sitic in blood and their allies : First, that they have a double origin 

 along two distinct lines of descent, some being derived from hetero- 

 mastigote, the others from cercomonad ancestors (Woodcock, 

 Doflein, Senn) ; secondly, that the Haemo flagellates are closely 

 allied to certain forms hitherto classed as Hsemosporidia (p. 388), 

 and form with them a homogeneous group or order of the Flagellata, 

 the so-called Binucleata (Hartmann). 



The question of the Haemosporidia will be discussed below 

 (p. 389). It is sufficient here to deal with the Hsemo flagellates alone. 

 From the general survey of the " forms " or " genera " dealt with 

 in this chapter, it is very evident that Trypanosoma, Crithidia, 

 Leptomonas (Herpetomonas), and Leishmania, are very closely 



FIG. 141. A and B, Prowazekia weinbergi, Mathis and Leger (473), magnified 

 2,400 ; G, Prowazekia parva, Nagler (585), magnified about 2,250. 



related to one another. Structurally the first three types shade 

 off insensibly into one another, the dividing line between Trypano- 

 soma and Crithidia, or between Crithidia and Leptomonas, being 

 quite arbitrary, and far less definite in reality than it appears when 

 reduced to words ; while Leishmania is a resting non-flagellated 

 phase of any of the three. Developmentally the four types, or any 

 two of them, may occur as phases in the life-history of a single 

 species, so that the selection of a given form as the " adult " in- 

 dividual, by means of which the generic name is to be determined, 

 is also, in many cases, quite arbitrary. Phylogenetically an evolu- 

 tionary series can be recognized beginning with Leptomonas, and 

 passing through Crithidia to Trypanosoma, of which the central 

 feature is the gradual development of an undulating membrane, 

 which finally runs the whole length of a more or less sinuous and 



