PROTOMONADINA 



59 



all (not T. equiperdum^ which is transmitted by coitus) distributed by 

 a second, invertebrate, host, which is usually an insect for terrestrial 

 species and a leech for aquatic species. In the invertebrate the try- 

 panosome 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.gambiense, trans- 

 mitted by a tsetse fly) by the bite of the invertebrate, in others (e.g. 

 T.lewisim the rat, transmitted by a flea) by the invertebrate or its faeces 

 being swallowed by the vertebrate. The pathogenic species appear 

 always to 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 



Fig. 49. Choanoflagellata. A, Monosiga brevipes, x 1200. B, Codosiga um- 

 bellata, x 310. Both after Saville-Kent. C, Ingestion in Codosiga. f. vac. 

 food vacuole. 



capable of reinfecting the vertebrate. T. cruzt, the cause of Chagas* 

 disease in man in South America, is non-pathogenic 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 verte- 

 brate host, it passes most of its time, and reproduces, as 2. Leishmania 

 form, in the tissues. T.gambiense and T. rhodesiense, causes of sleeping 

 sickness in man when they have passed into the cerebrospinal fluid, 

 and T. brucei, the cause of African cattle sickness, are non-pathogenic 

 in antelopes. Their crithidial stage is passed in the salivary glands of the 

 tsetse {Glossina), reproduces by binary fission, and is not intracellular. 

 Choanoflagellata (or Choanoflagellidae) . Uniflagellate , generally fixed 

 forms; with a protoplasmic collar around the base of the flageflum. 

 Ingestion by attraction of particles by the flagellum to the outside of 

 the collar, adherence to this, and transference by streaming of proto- 



