102 MICROBES AND TOXINS 



system nor sense organs, even to such a degree as the sea- 

 anemone, the protozoon is capable of choice and of determina- 

 tion, these phenomena of course remaining more or less 

 mechanical. Superposition and propagation of impressions 

 exist among protozoa, and after long and minute observations 

 it has been maintained that no essential difference exists 

 between them and the most complicated metazoa : activity is 

 neither more nor less mechanical in the one set than in the 

 other. 



The chief business of all living creatures is reproduction. 

 Among protozoa there exist several principal' methods for this, 

 each presenting numerous variations. They may divide by 

 nuclear division ; they may divide by budding, and in this the 

 greatest diversity occurs in the number, size, and arrangement 

 of the buds. They may sporulate, i.e., their protoplasm may 

 break up, and each fragment consisting of a bit of protoplasm 

 and a bit of nuclear material can reproduce a creature similar 

 to the mother-cell which sporulated. When life-conditions 

 are difficillt, certain protozoa encyst, i.e., they contract inside 

 a resistant shell, and under shelter of this various modes of 

 reproduction may take place. Between reproduction by 

 division and that by sporulation intermediate forms exist 

 (Tillina and Colpidiuni), and the continuity in nature can 

 always be detected by the imagination. 



Parasitic protozoa, those which to subsist require to emigrate 

 from one host to another, most often reproduce themselves 

 by sporulation, and the reproduction is the more lavish the 

 greater the difficulties encountered by the species in propa- 

 gation. 



Parasitism tends to modify the species in a retrograde 

 direction but the losses may be compensated for by new 

 acquisitions. Locomotory organs, protective envelopes and 

 the apparatus designed for capturing and digesting food 

 become simplified or disappear altogether. But parasites are 

 in general more prolific and they acquire other organs, hooks 

 or suckers, by which they can better cling to their host. As 

 their life conditions or their habits become narrowed down, 



