18 Chapter I 



excrementitious matter. The Infusorian behaves in the same way in 

 regard to bacterial endospores. 



Hay bacilli, which occur so commonly in the infusions in which 

 the Paramaecia live, are digested in the endoplasmic vacuoles of the 

 latter, but the spores of these bacilli, after a more or less prolonged 

 sojourn in the vacuoles, are expelled with the excrement. 



As by far the greater part of the body of a Protozoon is made up 

 of digestive protoplasm, it is natural that infective epidemics should 

 be very rare among these animalcules. The Infusoria and Rhizopods, 

 organisms specially well adapted to live upon the lower Algae and 

 Bacteria, are, practically, never subject to bacterial diseases. The 

 infections observed in the Protozoa are due in most cases to the 

 invasion of the lower Fungi, such as the Chytridia, the Microspheres, 

 the Saprolegniae or the special organisms mentioned as occurring in 

 the nuclei of Paramaecia. Further, these infections are met with 

 most frequently in Protozoa which are incapable of carrying on true 

 intracellular digestion or which are in the encysted stage, at which 

 period the Infusoria, leading a passive existence, neither absorb nor 

 digest nutriment. As an exception to the above general statement 

 I ought to mention the epidemic in Amoebae caused by the Micro- 

 sphaera 1 and the disease in Actinophrys observed by K. Brandt 2 and 

 attributed to Fungi allied to the genus Pythium. In these two 

 instances we have to do with parasites which live and develop in the 

 interior of the active protoplasm of these Protozoa. Certainly a 

 proportion of the parasites are expelled with the excrementa ; but 

 there remain others which instal themselves in the protoplasm, 

 multiply there and cause the death of their hosts. In these cases the 

 digestive action of the protoplasm must be neutralised or paralysed 

 by the secretions of the parasite. This aspect of the question, 

 however, has so far not been considered. 



In addition to intracellular digestion and the expulsion of para- 

 sites by the excretory function, the resistance offered by Protozoa to 

 infective diseases should, in part, be ascribed to their great irrita- 

 [2i]bility. Anyone who will watch the manoeuvres of Amoebae or of 

 certain Infusoria in the midst of a rich microscopic flora and fauna, 

 will at once be struck by the preferences which these Protozoa 

 exhibit in the choice of their food. Amoebae are often seen 

 making search for Diatoms only, disdaining all other Algae, or again 



1 " Lemons sur la pathologie compares de 1'inflammation," p. 21 ; English edition, p. 17. 



2 Monatsber. d. Deri. Akad. d. Wissensch., 1881, p. 388. 



