ECOLOGY, COMMENSAL ISM AND PARASITISM 365 



the number of possible transmitting agents is considerably lessened. 

 There is, obviously, always a possibility of vertebrate infection, 

 either by contamination or by inoculation, from insects which feed 

 on vertebrates, but the transmission is always difficult to prove, 

 and the fact that pathogenic flagellates live and multiply in the 

 digestive tract of insects is no proof that the insect transmits them 

 to mammals, although the inference is highly plausible. So it is or 

 has been with the transmission of pathogenic Leishmanias by bed- 

 bugs, flies and fleas or of Trypanosoma by biting flies and bugs. 

 In some cases the transmission has been demonstrated without 

 question of doubt and these will be considered in the following 

 pages. 



The family Trypanosomidae includes 7 genera which apparently 

 are genetically related and reveal an interesting series in progressive 

 parasitism. These are Leptomonas, Crithidia, Leishmania, Herpeto- 

 monas, Endotrypanum, Trypanosoma and Schizotrypanum of inver- 

 tebrates and vertebrates, and Phytomonas of invertebrates and 

 plants. These all have the same general type of structure and 

 represent the simplest forms of flagellates (Fig. 169). In all cases 

 the body in motile stages is elongate and ellipsoidal; the nucleus 

 is single and of the usual endosome-bearing type; the kinetic ele- 

 ments are more variable, but there is always a blepharoplast usually 

 connected by fibrils with a parabasal body. Rhizostyles, arising 

 from the blepharoplast are sometimes present but not invariably, 

 even in the same species. Axoplasts, analogous to axostyles, have 

 been reported for one species of Herpetomonas (II. drosophilae, 

 Chatton and Leger, 1911). The flagellum is of the usual type with 

 axoneme or axial filament originating from the blepharoplast and 

 periplastic sheath. In some forms (Herpetomonas, Leptomonas, 

 Crithidia, Leishmania and Phytomonas) the kinetic complex (kin- 

 etoplast) is anterior to the nucleus; in the fully-developed forms of 

 Trypanosoma, it is posterior (Fig. 169, D). In all cases a rhizoplast, 

 or endoplasmic portion of the axial filament is present. In forms 

 with the anteriorly placed blepharoplast this is relatively short, 

 but where the blepharoplast is posterior to the nucleus it may be 

 almost as long as the cell as in Crithidia forms and the trypanosome 

 form of Herpetomonas muscarum (Fig. 169, B), here it runs along 

 the margin of the cell restrained by the periplast. In Trypanosoma 

 the axial strand becomes the margin of a delicate periplastic ledge 

 to form an undulating membrane which vibrates with the activity 

 of the free axial filament of the flagellum. 



Other structures of the cell are less constant and of less importance 

 — cytoplasmic granules of the nature of volutin (see p. 72) are 

 sometimes very abundant; mitochondria and Golgi bodies have 

 received scant attention and play no part in taxonomic or parasitic- 

 discussions. 



