ILEMOFLAGELLATA 75 



man bodies in small numbers, resulting from the bite of horse- 

 flies (Tabanidse) of various species. There is every reason to 

 believe that the parasites in these sores are normally parasitic 

 in the insects only, but are able to adapt themselves to their new 

 environment in human flesh and to multiply there for a time. 

 They are permanently sidetracked, however, and have no further 

 chance of completing their life history or of reaching new hosts, 

 unless a suitable fly should, by some infinitesimally small chance, 

 suck blood from the sore in which they were developing, and thus 

 rescue them. 



Several investigators have recently shown that a number of 

 typical insect flagellates, if injected into mice and rats or other 

 mammals, or even birds, may become pathogenic and even 

 cause the death of the animal. That the well-established Leish- 

 mania diseases of man and other animals originated from insect 

 flagellates can hardly be doubted, but it is possible that in some 

 cases the parasites may have adapted themselves to their new 

 type of host to such an extent as to have become quite independ- 

 ent of the insects from which they originated. Fantham suggests 

 that all forms of Leishmania and Herpetomonas may be mere 

 physiological races of a single species which is variously adapted 

 to live in a variety of different hosts, and perhaps able to adapt 

 itself anew to unaccustomed hosts under certain conditions. 



Leishmania and Herpetomonas belong to a group of the class 

 Flagellata known as the Hsemoflagellata. This group presents 

 a series of forms from the simple Leishmania, which at times is 

 a non-motile, unflagellated organism, through the increasingly 

 highly developed Herpetomonas and Crithidia to the trypano- 

 somes (see Fig. 18). Some reach only the Herpetomonas stage as 

 adults, others only the Crithidia stage, while others pass through 

 the entire series of developmental stages and reach the final 

 trypanosome stage. All of them are probably primarily parasites 

 of the guts of insects or other invertebrates, and only compara- 

 tively few of them have adapted themselves to spending part 

 of their existence in the blood or tissues of vertebrates. Ap- 

 parently only the Leishmania and trypanosome forms are 

 adapted for existence in vertebrates, since the other forms are 

 not found in them, except in rare instances when Herpetomonas 

 forms are found in the blood of Leishmania-infected individuals. 

 A number of species have become thoroughly adapted to life in 



