248 THE PATHOGENIC FLAGELLATES 



is anterior to the nucleus, but becomes posterior to the nucleus as 

 development progresses (Fig. 100) until the adult posterior position is 

 attained. Novy has laid great stress upon the fact that in trypano- 

 somes in culture the form is similar to that of herpetomonas and cri- 

 thidia, and for this reason regards the species of these genera as true 

 trypanosomes. It will hardly be allowed by anyone familiar with the 

 morphological changes of protozoa that trypanosomes under culture 

 in artificial media are in any way normal, either structurally or physio- 

 logically, and his purely hypothetical conclusion that herpetomonas 

 and crithidia "really represent cultural forms of true trypanosomes" 

 (1907), zoologically speaking, is far-fetched. The herpetomonad 

 form assumed by some types may be evidence of a phylogenetic 

 ancestral state, but it certainly cannot be accepted as evidence that the 

 more primitive, ancestral organisms are themselves trypanosomes. 



In the present state of knowledge of trypanosomes it is extremely 

 uncertain as to where lines should be drawn between species; mor- 

 phology is no aid in this, for the same species in the same animal may 

 present so many form changes that were they found in different 

 animals they would be assigned to different species without hesitation. 

 No safe limitations can be established until the life histories are known, 

 and as these have been worked out in only a few cases the difficulties 

 are not much relieved. Physiological grounds, culture relations, etc., 

 are equally unsatisfactory, but there is reason to believe that differ- 

 ences in such respects are indications of different specific relationships. 

 For the present, therefore, it is expedient to consider each new form 

 described in a new host as a distinct species until its affinities are 

 established by the full life history, and until then, furthermore, it 

 seems better not to break the genus trypanosoma into other genera as 

 Liihe ('06) has done on the basis of supposed different ancestry. This 

 supposition is purely hypothetical, and it is quite possible that we have 

 not yet found the true explanation of the anterior and posterior ends 

 of trypanosomes. 



Trypanosomes are present in all kinds of vertebrates, where they are 

 normally parasites of the blood system; they are also found in the 

 intestines of different blood-sucking insects. Liihe, with Leger, 

 believes the latter to be the definitive hosts, the trypanosomes coming 

 from ancestors like herpetomonas and crithidia, which are typical 

 intestinal parasites. Novy also takes this point of view, holding, with 

 Leger, that the trypanosome structures are special adaptations which 

 the organisms have developed as a response to conditions in the blood. 

 Minchin ('08) regards the trypanosomes as originally parasitic in the 

 vertebrate intestine, basing his conclusion largely upon the observa- 

 tions of Hintze ('02) and upon theoretical considerations of the fact 

 that trvpanosomes mav be transmitted bv leeches as well as bv insects. 

 There is much to be said in favor of his point of view, and Leger 's 



