44 VETERINARY BACTERIOLOGY 



Form and Size of Protozoa. The pathogenic protozoa vary 

 in size from those visible to the naked eye to those barely visible 

 with the highest powers of the microscope. Some are undoubt- 

 edly ultramicroscopic, the organism causing yellow fever, for 

 example. In form and shape the greatest diversity is to be noted. 

 Some are without definite shape, and are apparently only lumps of 

 protoplasm, others are highly differentiated and have as great 

 variety of organs (organella) as some of the higher animals. 



Histology. A true cell wall, as found in the bacteria, yeasts, 

 and fungi, is frequently not present in the protozoa. When 

 found, it is chitinous in nature. The ectoplast forms the limit- 

 ing membrane of the cell in the majority of cases. The protoplasm 

 is differentiated into nucleus (sometimes two, a micronudeus 

 and a macronucleus) and endoplasm or cytoplasm. Power of move- 

 ment is possessed by many forms. This may be due to the 

 development of pseudopodia, of flagella, or of cilia. 



Reproduction. Asexual reproduction is accomplished in many 

 cases by a simple process of fission, in others the procedure is 

 much more complex. Sexual reproduction is quite general, but 

 here again the complexity is so great as to render brief treatment 

 impracticable. The relationship and structure of these forms 

 will be considered in greater detail under the heading of Patho- 

 genic Protozoa in Section V. 



