322 THE PROTOZOA 



to arise in other directions, as in the digenetic flagellate parasites 

 of Euphorbiacese. 



The biflagellate genus Trypanoplasma , on the other hand, com- 

 prises species which, like those of Prowazekia, appear to have been 

 primarily parasites of the vertebrate digestive tract, and which 

 in some cases have established themselves in the blood and have 

 acquired an alternation of hosts (they can hardly be said to have 

 an alternation of generations), having become parasitic in an inter- 

 mediate host, always, so far as is known, a leech, in which they 

 pass through a simple type of development, consisting of little more 

 than simple multiplication by fission. Their structure indicates 

 affinities with heteromastigote types such as Bodo and Trichomonas, 

 common intestinal parasites, rather than with uniflagellate forms. 



The suggestion is, therefore, that the flagellates parasitic in the 

 blood of vertebrates have two distinct lines of ancestry : the one 

 from heteromastigote forms such as Bodo and Trichomonas, origin- 

 ally parasitic in the gut of the vertebrate and culminating in the 

 genus Trypanoplasma ; the other derived from uniflagellate cer- 

 comonad* ancestors originally parasitic in the digestive tracts of 

 invertebrates, and culminating in the genus Trypanosoma (compare 

 also Senn, 358). It must be emphasized strongly, however, that 

 any such conclusions are of a tentative nature, and can have no 

 finality, but are liable to modification with every increase of know- 

 ledge concerning these organisms. 



Bibliography. For references see p. 488. 



