GENERAL COXSIDERATION OF DISICASE. 



63 



Protozoa. 



Protozoa are microscopic single celled animals. They arc 

 very simple in structure, being composed of a mass of proto- 

 plasm with or without a cell membrane. The cell membrane, 

 when present, consists of concentrated protoplasm. The pro- 

 tozoa having a cell membrane are constant in shape, as the Try- 

 panosoma Evansi, and those not possessing a cell membrane 

 vary in shape from a sphere to an irregular flat mass and are 

 capable of changing their shape whenever occasion demands. 

 Protozoa are larger than bacteria. 



i'l — Tr.vpanosoma Evansi in 3 blooflsmear from a horse affected with surra. 



Protozoa require food similar to the foods of higher animals. 

 Particles of food are inchjsed or incorporated by them prepara- 

 tory- to digestion. Digestion is accomplished by means of fer- 

 ments elaborated and secreted by the protozoa. The digested 

 foods pass bv osmosis into the protozoa, the undigested por- 

 tion being extruded by rearrangement of the cell protoplasm. 

 Respiration takes place by exchange of gases through the 

 surface protoplasm of the protozoa. They reproduce bv fission, 

 budding, ctmjugation or sporulation. 



Protozoa are universallv distributed. They all require con- 

 siderable moisture. In fact most of them live either in fresh or 

 salt water. A few only are parasitic. 



The specific action of pathogenic protozoa in the produc- 

 tion of disease is not understood. Some mav have a mechanical 

 efl:'ect only out the evidence concerning others indicates that 

 most of them produce an injurious chemic substance. 



Helmintbes. 



This branch of the animal kingdom contains manv species 

 that are parasitic. .Structural, animal parasites are much simpler 



