314 FAMILY: TRYPANOSOMID^ 



and curved blade of cytoplasm which is more or less tapering at each end. 

 The surface of the body is covered by a very fine but denser layer of 

 cytoplasm, the periplast. That such a fairly strong and resistant mem- 

 brane is present Minchin (1909a), from a study of the cytology of Tnjpano- 

 soma lewisi, considers obvious from the manner in which the trypanosomes 

 retain their body form under trying circumstances. The cytoplasm has 

 a finely alveolar structure, and very frequently a distinct vacuole occurs 

 near the kinetoplast. 



The simplest flagellate type is the leptomonas, which has an elongated 

 and sometimes slightly curved blade-like body, and the various structures 

 described above (Fig. 150, 8). All the other flagellates of this family may 

 be regarded as having arisen from the leptomonas form. The first modi- 

 fication is the displacement backwards of the kinetoplast, which takes up 

 a position near, but still anterior to, the nucleus. There is a considerable 

 lengthening of the axoneme, which now, instead of passing through the 

 body of the flagellate, takes a lateral course to the convex margin, and 

 then passes along the surface of the body or on the edge of a cytoplasmic 

 ridge — the undulating membrane — to the anterior end of the body, and 

 thence into the flagellum. The free margin of the membrane, when one is 

 present, is longer than the attached margin; hence it is thrown into folds, 

 and has an undulatory movement when in action. Flagellates of this type 

 are distinguished as crithidia (Fig. 150, 20). A further change occurs with 

 continued displacement of the kinetoplast, which passes the nucleus and 

 ultimately occupies a position near the posterior end of the flagellate. 

 The axoneme then passes along the surface of the body for almost its 

 entire length or along the margin of an undulating membrane, so that the 

 trypanosome form is reached (Fig. 150, 30). These three flagellate types — 

 leptomonas, crithidia, and trypanosome — may all of them transform in a 

 converse manner to produce finally shorter and more rounded individuals 

 till the leishmania form arises (Fig. 150, i, 17, 27). The latter has a small 

 round or ovoid body containing the nucleus and kinetoplast. There 

 is no flagellum, but the axoneme can often be detected as extending from 

 the kinetoplast to the surface of the body. These leishmania forms, 

 under suitable conditions, will transform again into any one of the flagel- 

 late types from which they were originally derived ; or, if they arise in the 

 intestine of an invertebrate, as in the case of those flagellates which are 

 limited to an invertebrate host, they may encyst and escape in the faeces. 



Since purely insect flagellates are transmitted from one host to another 

 by small encysted leishmania forms, it follows that the leishmania forms 

 which escape from the cysts after they are ingested are much smaller than 

 the fully-grown flagellate stages. In the intestine of the host these small 

 leishmania forms increase in size, develop flagella, and change their shape 



