THE H^EMOFLAGELLATES AND ALLIED FORMS 281 



invertebrates alternately are comprised in this genus. Trypano- 

 some-forms also occur as developmental phases in the life-cycle of 

 species parasitic solely in the digestive tracts of insects. 



2. Trypanoplasma (Figs. 36, 134), with two flagella arranged in 

 a heteromastigote manner, and with the posterior trailing flagellum 

 united to the body by an undulating membrane for the greater part 

 of its length. A number of species are known, which by their dis- 

 tribution fall into three sections : (1) Species parasitic in the blood 

 of fresh-water fishes, with alternating parasitism in the digestive 

 tract w of leeches ; (2) species parasitic in the digestive tract of marine 

 fishes ; (3) species parasitic in various invertebrates. 



3. Crithidia (Fig. 135), with a single flagellum which arises near 

 the kmetonucleus, at about the middle of the body, in front of or 

 close beside the trophonucleus, and runs along the pointed anterior 

 end of the body to form the marginal flagellum of a relatively 

 short, often rudimentary, undulating membrane, beyond which 

 it is continued as a free flagellum. As an independent genus this 

 type comprises species parasitic in the digestive tracts of various 

 insects ; but the majority of the so-called species of Crithidia aro 

 merely phases in the developmental cycle of trypanosomes. 



4. Leptomonas (Herpetomonas Figs. 124, 136), with a single 

 flagellum arising at the anterior end of the body, and with no trace 

 of an undulating membrane. As an independent generic type 

 this form occurs as a parasite of invertebrates, chiefly insects ; 

 secondarily also in the latex of plants (Euphorbiacese). It occurs 

 also as a developmental form of the next genus in the invertebrate 

 host or in cultures. 



5. Leishmania (Fig. 138), with an oval body containing a tropho- 

 nucleus and kinetonucleus, but with no flagellum. As a generic 

 type this form is an intracellular parasite of a vertebrate host, 

 multiplying .there by fission and developing into a typical Lepto- 

 wowas-form. On the other hand, as a developmental phase this form 

 represents simply a non-flagellated, resting stage which may occur 

 in the life-cycle of either Trypanosoma, Crithidia, or Leptomonas. 



6. Prorvazekia (Fig. 141), with two flagella arranged in the hetero- 

 mastigote manner, as in Trypanoplasma, but with the trailing 

 flagellum quite free from the body, without an undulating mem- 

 brane. Prowazekia is therefore quite similar in its morphology to 

 Bodo, with which it was formerly confused, if, indeed, it is really 

 distinct, and it differs from Bodo only in the possession of a kineto- 

 nucleus. Several species are described, free-living or intestinal in 

 habitat. 



Considering the above six types as a whole from a morphological 

 standpoint, it is seen that there are two types of structure amongst 

 them the cercomonad or monomastigote type, represented by 



