. Animal parasites and human disease. Parasites; Medical parasitology; Insects as carriers of disease. 96 TRYPANOSOMES AND SLEEPING SICKNESS form, differing from the preceding in that the parabasal body has moved back to near the middle of the body, and the flagellum is connected with the body for half its length by an undulating membrane (Fig. 18B). This type is a very common develop- mental phase in nearly all trypanosomes, but it is also the adult condition of many insect parasites. Finally there occurs the fully-developed trypanosome form (Fig. 18A), apparently es- pecially adapted in form
. Animal parasites and human disease. Parasites; Medical parasitology; Insects as carriers of disease. 96 TRYPANOSOMES AND SLEEPING SICKNESS form, differing from the preceding in that the parabasal body has moved back to near the middle of the body, and the flagellum is connected with the body for half its length by an undulating membrane (Fig. 18B). This type is a very common develop- mental phase in nearly all trypanosomes, but it is also the adult condition of many insect parasites. Finally there occurs the fully-developed trypanosome form (Fig. 18A), apparently es- pecially adapted in form and structure for life in vertebrate bodies. The method of develop- ment of this form from a crithidial type can easily be seen from Fig. 18. Only the first or Leishmania form and the last or trypanosome form normally occur in vertebrate bodies, though all of the four types are found in the digestive tracts of in- vertebrates. •'The fact that some flagellates never develop further than the Herpetomonas form, and others never further than the Crith- idia form, makes a study of this group of Flagellates very confusing, since when a Herpetomonas or crith- idial type is found in an insect gut it is very difficult if not impossible Fig. 19. Trypanosoma rhodcsi- to say whether it is an adult animal ense, from blood of monkey inocu- which never undergoes any further lated from case of human sleeping j 1 i j i sickness. Note posterior position of development or is only a develop- nucieus in short blunt forms, espe- mental phase of a trypanosome of cially in lower figure. X 2000. , , i (After Kinghom and Yorke.) a vertebrate animal. It is often very difficult to dis- tinguish different species of trypanosomes; of over 70 known species only a few can be distinguished on morphological grounds. Average size, position of nucleus and parabasal body, length of snout, and presence or absence of a free flagellum are sometimes useful in identifying them. More reliable, however, are their so-
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