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Rh most cases they are parasites of insects alone, infection being brought about by the ingestion of cyst-infected fæces. Some species are parasitic in the latex of plants, and are taken up by the appropriate plant hemiptera. The distinctive characters (Fig. 237) are the possession of a single flagellum arising from a single kinetonucleus, itself situated in the anterior portion of the body, and the absence of an undulating membrane. Uniflagellate and biflagellate forms occur. Some authorities regard the latter as the result of precocious formation of a new flagellum in anticipation of longitudinal



division. They have shown that trypanosome forms, but lacking the well-developed undulating membrane so characteristic of the mammalian trypanosome, also occur, the changes in form being in all probability brought about by the migration of the nuclei; all stages being found between the non-flagellate leishmania, through leptomonad, crithidial, to the trypanosome form (Fig. 240).

Fig. 238.—Leishmania donovani. (After Leishman.)

5. includes human parasites, L. donovani (Fig. 238), the cause of a general disease, kalaazar, and L. tropica, the cause of oriental sore—a purely local infection. L. donovani, in the Mediterranean area, is found especially in children, and is associated with a similar disease occurring in dogs and caused by the same parasite. The type of parasite found in the vertebrate host is a small oval body multiplying by fission and contained within cells—endothelial cells or macrophages; it is very uniform in size. Each parasite possesses a tropho- and a very distinct kinetonucleus (blepharoplast).