xiii, b, 5 Haughwout: Flagellated and Ciliated Protozoa 255 



other intestinal flagellates seems to be suggestive of what may 

 be expected in man in regard to these parasites. 



Next in order appears to be the desirability of attempting to 

 explain the conflicting opinions expressed by different authors 

 regarding the pathogenicity of the intestinal flagellates. It has 

 been shown that some workers regard these forms as harmless 

 or capable, at the most, of giving rise to nothing worse than 

 diarrhoea, while others frankly express the belief that they may 

 produce dysentery — actual lesions of the bowel. Is it possible 

 that we have here different strains of the same organism, some 

 showing and others not showing tissue-invasive powers — a con- 

 dition somewhat resembling the relation between Entamoeba 

 histolytica and Entamoeba coli? 



Another problem is that of cross-infectivity. Many of the 

 genera found in lower animals are found in man. Are the 

 species found in the lower animals capable of life in man? The 

 rule is known to apply in the case of Balantidium, it seems to 

 be the case with Lamblia, and Lynch has produced evidence to 

 show that the rat is a true host and not merely a carrier of 

 Entamoeba histolytica. 



The actual invasion of the tissues of the human intestine by 

 the flagellated parasites remains to be demonstrated, but it may 

 have occurred and we may have passed it by. It seems conceiv- 

 able, as I have suggested, that Trichomonas in the tissues may 

 so closely resemble Entamoeba as to have been mistaken for the 

 latter. Evidence that tissue invasion by these same parasites 

 occurs in the lower animals seems to be sufficiently convincing. 



However, it is still to be proved how this takes place. To 

 my mind the evidence, so far at least as the flagellates and 

 ciliates are concerned, seems to favor mechanical penetration of 

 the tissues rather than entrance with the aid of cytolytic agents. 

 But it is also a fact that necrotic changes in the tissues may 

 be associated with the presence there of the parasites. These 

 changes might be due either to definite cell-destroying ferments, 

 or might simply be the result of the action of katabolic products 

 of the parasites that happen to be toxic to the cell elements. 

 From the evidence, cytolytic ferments appear to be unnecessary 

 in many cases, certainly with many of the flagellates and pos- 

 sibly in the cases of the ciliates and amoeboid forms. 



But this should not preclude a study of the cytolytic enzymes 

 that may be produced by these organisms. If they exist, the 

 investigations should include an inquiry into the conditions under 

 which they are formed with a view to discover whether they 

 are constantly secreted or are produced only under special in- 



