CHAPTER XXVI 



THE PATHOGENIC PROTOZOA. AMEB.E. FLAGELLATA 



THE lowest forms in the animal kingdom, the protozoa, are 

 characterized by the simplicity of their structure as compared 

 with the higher animals, the metozoa. For the most part each 

 organism consists of a single cell composed of cytoplasm and nuclear 

 substance. Nevertheless, although unicellular and of such simple 

 morphology the protozoa are much more complete than bacteria 

 both in form and in their life cycle. 



Morphology. The cytoplasm of the cell consists usually of 

 an outer, dense portion, the ectoplasm, and an inner, more fluid 

 portion, the endoplasm, which surrounds one or more nuclei as well 

 as various granules and vacuoles. Certain of the latter appear 

 to act as digestive organs ; others show periodic contractile move- 

 ments and serve to eject waste products from the cell body. 



In many of the protozoa the chromatin substance of the nucleus 

 is massed together in a deeply staining round body called the 

 karyosome, and embedded in the karyosome is the centrosome, a 

 small body always present in metazoon cells, which plays an im- 

 portant part in cell division. In certain forms still another definite 

 portion of the nuclear chromatin, the kinetic nucleus, may form 

 the root of a flagellum. The kinetic nucleus may be distinct or it 

 may merge into another small body, the blepharoplast. Each 

 of these four bodies, the karyosome, centrosome, kinetic nucleus, 

 and blepharoplast, have their origin in the nucleus of the cell. 



When conditions become unsuitable the organisms may pro- 

 tect themselves by forming a highly resistant enveloping mem- 

 brane. Such an encysted form will withstand extremes of heat and 

 cold and long periods of drying, and then as soon as conditions 



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