64 THE INVERTEBRATA 



to vertebrates owing to the latter being subject to attack by the original 

 hosts. The original mode of infection was by faeces. The species of 

 each genus assume, in certain circumstances, the forms characteristic 

 of other genera. The following are the principal genera. 



Herpetomonas { = Leptomonas) . Basal granule and parabasal body 

 at one end, near the origin of the flagellum. Parasitic in the gut, 

 principally of insects, but also of other invertebrates and of reptiles. 



Leishmania. Oval bodies containing a nucleus, parabasal body, 

 basal granule and rhizoplast, but with no flagellum, infesting the 

 tissues of vertebrates, and transferred by flies of the genus Phleboto- 

 muSy in whose gut they assume the form of Herpetomonas. In Man they 

 cause kala-azar and Oriental sore. 



Crithidia (Fig. 47 F). Flagellum starts from a basal granule near 

 the middle of the long, slender body, to which the flagellum is united 

 by an undulating membrane; parabasal body placed between the 

 basal granule and the nucleus. Parasitic in the gut of insects. 



Trypanosoma (Fig, 47 E). As Crithidia^ but the basal granule of 

 the undulating membrane and the parabasal body are beyond the 

 nucleus, towards the non-flagellate end. Many species, all parasitic 

 in the blood and other fluids of vertebrates, and nearly all (not T. 

 equiperdum) distributed by a second, invertebrate, host, which is 

 usually an insect for terrestrial species and a leech for aquatic species. 

 In the invertebrate the trypanosome passes for a time into a condition 

 in which it resembles Crithidia^ and during which it is incapable of 

 reinfecting the vertebrate. Reinfection is in some species (e.g. T. lewisi 

 in the rat, transmitted by a flea) by the invertebrate or its faeces being 

 swallowed by the vertebrate ; this is probably the original mode of 

 obtaining entry to the vertebrate host. Other species (e.g. T.gambiense^ 

 transmitted by a tsetse fly) are reintroduced to the vertebrate by the 

 bite of the invertebrate. T. equiperdum, parasitic in horses, in which 

 it is the cause of " dourine", is transmitted by coitus and has dispensed 

 with the invertebrate host. 



Most, if not all, of the pathogenic species have a wild host with 

 which they are in equilibrium and in which they are non-pathogenic. 

 T. lewisi, non-pathogenic in the blood of the rat, has a period of 

 intracellular multiple fission in the stomach of the flea and then passes 

 into the rectum of the latter, where it changes from the crithidial to 

 the trypanosome form and becomes capable of reinfecting the verte- 

 brate, which it accomplishes in the manner mentioned above. T. cruzi, 

 the cause of Chagas' disease in Man in South America, is non-patho- 

 genic in the armadillo. It is transmitted by the bug Triatoma, in which 

 it probably has an intracellular stage, and becomes infective in the 

 faeces. In the vertebrate host, it passes most of its time, and reproduces, 

 as a Leishmania form, in the tissues. T. gambiense and T. rhodesiense^ 



