Immunity in Unicellular Organisms 19 



they may single out one species of Palmellaceae from a very varied 

 flora. The Infusoria also have their likes and dislikes in the matter 

 of food. Many of the ciliated Infusoria choose Bacteria to the ex- 

 clusion of almost everything else ; others, as Nassula, have a special 

 partiality for the Oscillariae. A most striking example of this 

 is afforded in Amphileptus daparedei, a voracious Ciliate, which 

 chooses Vorticellae to the exclusion of all other animalcules ; these 

 it devours, and then becomes transformed into a cyst upon tho 

 peduncle of the Vorticellae it has devoured. This irritability clearly 

 must control and guide the Protozoa in their relations with other 

 organisms and enable them to escape the invasion of parasites. 



In this connection I must mention a very interesting observation 

 made by Salomonsen 1 and communicated to the Paris International 

 Medical Congress in 1900. He was able to demonstrate the fact that 

 almost all the ciliated Infusoria, on becoming aware of the proximity 

 of dead bodies of kindred organisms, rapidly draw away, thus 

 manifesting a very marked negative chemiotaxis. This property 

 must, it is evident, protect them from any contamination by the 

 parasites contained in the bodies of Infusoria that have succumbed 

 to infective diseases. 



We have, then, quite a number of facts which throw light on the 

 natural immunity of the Protozoa against the action of pathogenic 

 micro-organisms. Up to the present, however, we know nothing 

 concerning the existence or the possibility of an acquired immunity 

 among the lower animalculae against infective diseases. We are 

 better informed as to the resistance of unicellular organisms to the 

 action of soluble poisons, which is, in general, much more easily 

 studied than is immunity against the micro-organisms themselves. 



As a very large number of the higher animals are sensitive to the 

 toxic action of poisons of bacterial origin, the question has been put, 

 " May not the Infusoria also be poisoned by these micro-organismal 

 products?" With the object of answering this question Gengou 2 

 has studied the influence of the toxins of tetanus and diphtheria on [22] 

 the ciliated Infusoria. He was unable, however, to bring forward 

 proof that these substances exert any special toxic action on the 

 Paramaecia. These Infusoria withstand, perfectly well, doses of 



1 Compt. rend, du Congres internal, de Med. tenu & Paris en 1900. Section de 

 bacteriologie et de parasitologie. 



! " Sur 1'immunite naturelle des organismes monocellulaires contra les toxines " 

 Ann. de FInst. Pasteur, Paris, 1898, t. xn, p. 465. 



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