THE HJIMOFLAGELLATES. 267 



Section XI. Phylogeny and Evolution. 



The subject of the deinvation of the Trypanosomes is one 

 of much difficulty, owing to our very insufficient knowledge 

 of the majority of the parasites. The views put forward below, 

 which involve an apparently different interpretation of the 

 orientation of the body in different cases, are, to a large 

 extent, based upon Leger's important researches, on the one 

 hand, upon Trypan oplas ma (64 and 65), and, on the other, 

 upon certain Herpetomonadine forms (61 — 63, 68, 69). The 

 Trypanosomes, as a whole, are to be regarded as including two 

 entirely distinct families, in one of which the attached flagellum 

 becomes free at the true anterior end, and in the other at the 

 true posterior end. Before discussing the reasons for this 

 division of the Hsemoflagellates into two groups, however, 

 we may consider the principal features in the structure of the 

 Herpetomonadine parasites to which i^eference has just been 

 made. 



In a typical Herpetomonas (e. g., H. muscse- 

 domesticfe,^ H. jaculum, or H. gracilis [fig. 38 b]), the 

 kinetonucleus is situated near the anterior end; the flagellum 

 is not attached to the side of the body at all but straightway 



undergone ; and, in this connection, he is inclined to suspect fleas or bugs. 

 The difficulty in the way of this view is the rarity of the occurrence of the 

 organisms in the peripheral circulation ; the skin papulae which sometimes 

 occur are suggested as furnishing a source of infection for the Insectan host. 

 ' H. muscae-domesticae is here included as a typical uniflagellate 

 Herpetomonad. Leger has observed no signs of two flagella, either 

 in this species or in others of the genus (not considering, of course, indi- 

 viduals about to divide by longitudinal fission). Prowazek (87) inter- 

 preted this form as a bipolar Flagellate in which the body has been bent 

 up so that the two ends have come together and united, the flagella alone 

 remaining distinct. This view is the less tenable since Schaudinn's concep- 

 tion of a primitive, bipolar " Urhaemoflagellate," based upon Laveran and 

 Mesnil's unfortunate description of Trypanoplasma borreli, appears 

 to have no prospect of being realised ; for, up to the present, there is no 

 evidence of this bi-polarity in any known Flagellate. 



