240 - The Philippine Journal of Science ms 



Anopheles are familiar. Similar differences are seen in the 

 case of Trypanosoma lewisi as it lives in the rat and as it lives 

 in Ceratophyllus. There are countless other examples, such as 

 that afforded by Euglena on its removal from the influence of 

 sunlight to darkness. Is it, therefore, wholly improbable that 

 changes may occur in the intestinal tract that may lead to the 

 substitution of a cytozoic or histozoic life for a coelozoic life in 

 the case of the intestinal parasites? Balantidium and even Enta- 

 moeba, it is well known, may live in the intestinal tract for long 

 periods of time without giving rise to symptoms, yet sooner or 

 later they may invade the tissues. What are the conditions that 

 lead to these diverse modes of life in the same organism living in 

 the same site? May we not rather seek the answer in the host 

 than in the parasite directly, and is it not something more than a 

 mere lowering of vitality? Is there any good reason why 

 changed conditions may not substitute tissue parasitism for lu- 

 men commensalism in the case of the flagellates? It seems 

 improbable that the reactions of immunity as recognized in con- 

 nection with bacterial and allied infections are to be considered 

 here, for immunity as we know it in the Protozoa seems to be 

 of an exceedingly low order. 7 Does it not seem that we are 

 dealing with chemical affinities of a different nature, chemical 

 reactions governing the regulation of the diseases of a charac- 

 teristically nonfebrile character unaccompanied by phenomena of 

 immunity? These phenomena need not be necessarily restrict- 

 ed to the metabolic chemistry of the parasite. They might be 



7 In connection with immunity problems with the Protozoa, it must 

 always be borne in mind that in many instances animals recover from 

 certain protozoal infections, such as coccidiosis, not through the develop- 

 ment of any immunity, but solely through the normal life cycle changes 

 of the parasite which develop the exogenous phases of the cycle. Once 

 this border line is passed, the organism ceases to be infective to the host 

 except through the original portal of entry. Gradually the schizogonous 

 forms develop the propagative phase, until finally asexual reproduction — 

 the only phase in which auto-infection is known to occur in such forms — 

 has completely given way to sporogony. This, of course, leaves unan- 

 swered the question as to how we may account for persistent carriers of 

 coccidial infection. This naturally presupposes either the continuance of 

 the schizogonous cycle as the source of supply of gametocytes, or the 

 reversion of the gametes or gametocytes back to asexual forms. The 

 latter performance is so wholly at variance with established biological 

 principles as to be, to say the least, rather improbable. On the other hand, 

 we are led to wonder what are the conditions that bring about the pro- 

 longed series of asexual reproductions while the organism continues to 

 produce oocysts. 



