GENUS: TRYPANOSOMA 445 



The characteristics which are of use in distinguishing trypanosomes 

 are the following: 



1. MOVEMENT. — When viewed alive under the cover-glass, trypano- 

 somes vary very much in the movements they perform. Some are 

 sluggish and do little more than wriggle and twist about in a limited area. 

 This is true of many of the larger trypanosomes, like those of fish and frogs. 

 On the other hand, amongst smaller types there is a similar variation. 

 T. gamhiense is moderately motile, and may travel some distance across 

 the field of the microscope. T. lewisi is more active, while T. vivax takes 

 its name from its remarkable motility. It darts about amongst the red 

 blood-corpuscles and quickly passes out of the field. 



2, MORPHOLOGY. — The morphological features of trypanosomes 

 depend on the size and shape of the body, variations in the size and posi- 

 tion of the nucleus and kinetoplast, and the degree of development of the 

 undulating membrane and flagellum. All these features, as also others, 

 have to be taken into consideration in describing the characters of any 

 trypanosome. There may, however, be considerable difficulty in doing 

 this, as they vary at different stages of development, and anything like a 

 complete cycle is known only in a few instances. Thus, T. lewisi varies 

 remarkably at different stages of development in the rat and the flea 

 (Fig. 197). Similar variations occur in the case of T. cruzi of man (Figs. 

 207, 209), T. rotatorium of the frog (Fig. 237), and indeed, in all trypano- 

 somes in wdiich anything approaching a complete life-history is known. 

 As in most cases only one stage in the development has been seen, and 

 that in the blood of the vertebrate, knowledge of the exact morphology 

 of these forms is very incomplete, and has often led to different stages of 

 one and the same trypanosome being described as distinct species. 



The general shape of the body of a trypanosome is that of a curved, 

 flattened blade (Fig. 150). One margin of the body is generally convex 

 and the other concave. The ends are tapering. The nucleus lies most 

 usually near the centre of the body, and the kinetoplast near the posterior 

 end. The axoneme commences at the blepharoplast, and after traversing 

 the cytoplasm for a short distance passes along the border of the undulat- 

 ing membrane, which arises from the convex edge of the body as a thin ridge 

 of cytoplasm. At the posterior end of the body, the membrane terminates 

 and the axoneme may or may not be continued into a flagellum. Though 

 on first appearance many trypanosomes seem to differ structurally from 

 this type, they are, however, all traceable to it. The variations which 

 occur may be considered as arising in one of two ways. Firstly, there may 

 be an increase in the length of the convex border, giving rise to forms which 

 are more and more curved till a complete spiral may be reached (Fig. 236, 2). 

 Secondly, there may be a great increase in the width of the flagellate, 



