Immunity in Unicellular Organisms 17 



organisms and infective microbes. All organisms which, in any way 

 whatever, penetrate into the body of an Infusorian or Rhizopod, are 

 brought into contact with the digestive endoplasm of these Protozoa. 

 If the intruders are killed and partially digested by the digestive 

 secretions, or are expelled as excrementitious matter, the Protozoon 

 remains uninjured and continues its normal and routine existence. 

 Here, then, we have an example of natural immunity, due to intra- 

 cellular digestion. On the other hand, when the foreign parasitic 

 organism resists this digestive action, it instals itself permanently in 

 the body of the Protozoon, and should it reproduce itself in small 

 numbers merely, excrete no poison and, in general, exercise no 

 injurious influence upon its host, the parasite may readily become 

 a commensal. Thus, it is not rare to find in the contents of Infusoria 

 and Radiolaria small vegetable organisms of the genera Zoochlorella 

 or Zooxanthella which not only set up no disease but, owing to their 

 assimilation of carbonic acid, may even be useful to their hosts. 

 There are cases, however, where the parasites act in a manner more 

 or less injurious to the Protozoa containing them ; in such cases 



l^fttrue and sometimes fatal infection results. 



■^ Among the infective diseases of the Protozoa the one that has been 

 most thoroughly studied is that set up by several representatives of 

 a particular genus of micro-organisms discovered by Johannes MUller 

 in 1856 and made the subject of an investigation carried out in my 

 laboratory by Hafkine\ I have already discussed these researches 

 in my work on the comparative pathology of inflammation' and need 

 here recapitulate only very briefly. Paramaecla are sometimes affected 

 by needle-shaped or spirillar parasites which penetrate, sometimes 

 into the macronucleus, sometimes into the micronucleus, reproducing 

 prolifically, giving rise to a marked hypertrophy of the aflected organs. 

 The Infusorian, in spite of this invasion, may continue to exist and 

 carry on its reproductive processes ; it is, thus, enabled in many cases [20] 

 to recover from the disease. On the other hand the Paramaecium 

 into whose body the spores of the parasite are introduced treats 

 them as it would any other ingested foreign body. Not being able to 

 digest them, owing to the resistance offered by the membrane of the 

 spore, the Paramaecium expels them just as it would any other 



1 Ann. de VInst. Pasteur, Paris, 1890, t. iv, p. 148. 



2 "Legons sur la pathologie comparee de rinflammation," Paris, 1892, p. 24; 

 "Lectures on the comparative pathology of inflammation," authorised translation 

 into English, London, 1893, p. 20. 



