CHAPTER XVIII 



THE PROTOZOA 



The General Structure of the Protozoa Pathogenic Amoebae 

 Trypanosomata Leishman-Donovan Body Spirochaetae 

 Syphilis Coccidia Malaria. 



THE Protozoa are an important group of unicellular organisms, 

 regarded as animal in nature, and sharply and definitely distin- 

 guished from the rest of the animal kingdom, to which the names 

 of Metazoa and Enterozoa are applied. The latter consists of 

 many cells, differentiated to perform different functions, and 

 arranged in two layers endoderm and ectoderm around a 

 central cavity, the enteron. 



"It is true that some protozoa consist of aggregates of cells, 

 and should therefore be entitled to be called multicellular ; yet an 

 examination of the details of structure of these cell -aggregates 

 and of their life-history establishes the fact that the cohesion of 

 the cells in these instances is not an essential feature of the life of 

 such multicellular protozoa, but a secondary and non-essential 

 arrangement. Like the budded ' persons ' forming, when co- 

 herent to each other, undifferentiated ' colonies ' among the 

 polyps and corals, the coherent cells of a compound protozoon 

 can be separated from one another and live independently ; 

 their cohesion has no economic significance. Each cell is pre- 

 cisely the counterpart of its neighbour ; there is no common 

 life, no distribution of function among special groups of the 

 associated cells, and no corresponding differentiation of structure. 

 As a contrast to this, we find in the simplest enterozoa that the 

 cells are functionally and structurally distinguishable into two 

 groups those which line the enteron or digestive cavity, and 



1 See Lankester's Treatise on Zoology, Part I, first and second Fascicles, 

 1907 and 1909; Minchin in Clifford Allbutt's System of Medicine, ed. 2, 

 vol. ii, pt. ii ; Hartog in Cambridge Natural History, vol. i. 



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