62 SOME MINUTE ANIMAL PARASITES 



intestine of the grouse and other birds, lizards, frogs, 

 toads, and mice being perhaps the best known. In 

 Trichomonas the supporting bars are fused to form 

 a rod. The organism also possesses an undulating 

 membrane and four flagella, one of which forms an 

 edge to the membrane. A Trichomonas is some- 

 times found in the intestine of man, and may be 

 associated with diarrhoeic conditions. 



When certain of the more important Flagellates 

 (from the economic point of view) are considered, 

 there would appear to be a close connexion between 

 the three genera, Herpetomonas, Crithidia, and Try- 

 panosoma. The Herpetomonas is certainly the simplest 

 form. It possesses no undulating membrane, but 

 has a long free flagellum arising near the anterior 

 blepharoplast, and a central nucleus. Somewhat 

 higher in the scale of organization are the various 

 Crithidia, which resemble Herpetomonas in the 

 general outline of their structure and life-history, 

 but possess a more complex structure, in that a 

 relatively small undulating membrane is present. 

 In Trypanosoma the body differentiation is even 

 higher, for the 'undulating membrane is large, and 

 extends almost the whole length of the body, origin- 

 ating as it does near the posterior blepharoplast. 



Interesting speculations also have centred around 

 the problem as to whether the insects or the 

 vertebrate were the primitive host of the trypano- 

 some. The line of speculation that seems to have 

 most in its favour appears to be as follows : The 

 parasitism in man or other warm-blooded vertebrate 

 has been evolved from parasitism in an insect. The 

 parasite may have begun its career originally as a 



