CHAPTER XVIII 

 THE PROTOZOA x 



The General Structure of the Protozoa Pathogenic Amoebae 

 Trypanosomata Leishman-Donovan Body Spirochaetac 

 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 multi- 

 cellular protozoa, but a secondary and non-essential arrangement. 

 Like the budded ' persons ' forming, when coherent 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 precisely 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 

 those which form the outer body wall. The cells of these two layers 



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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