374 MICROBIOLOGY 



treatises on that subject, 1 and to the special literature, which 

 is very large. 



Characterization. Protozoa 2 include all forms of ani- 

 mals which consist throughout their entire life of a single cell 

 or of a colony of single cells. Of the protozoa as a class it may 

 be said that the. individuals of each species are composed of a 

 cytoplasm and a nucleus or nuclear substance. In the different 

 species or groups of protozoa there are many variations 

 through which the individual passes in its more or less com- 

 plicated life-cycle. The cytoplasm varies in composition and 

 structure according to its environment. It is essentially pro- 

 teid in nature. It is more or less fluid but because of differ- 

 ence in the density and solubility of its substance it may ap- 

 pear granular, alveolar or exhibit distinct lines. These may be 

 observed in properly fixed and stained preparations. There are 

 usually an outer zone and a central part of the protoplasm. 

 The pseudopodia extend from the outer layer. The organs of 

 locomotion are flagella and cilia. There are many properties 

 and variations associated with the cytoplasm that have been 

 observed in the careful study of the life history of different 

 species. Protozoa always have a nucleus or nuclear substance. 

 This usually contains chromatin which consists largely of 

 nuclein and which appears when stained as small granules, 

 masses or rods. There are in the nucleus somatic and genera- 

 tive chromatin and in the flagellates another variety of chro- 

 matin which produces the locomotor apparatus and which is 

 known as the locomotor nucleus. 



1 The following books are especially valuable for the study of 

 protozoa. 



Calkins. Protozoology, New York. 



Doflein. Die Protozoen als Parasiten und Krankheitserreger, 

 Jena. 



Ziemann. Ueber Malaria und andere Blutparasiten, Jena. 



Laveran and Mesnil. Trypanosomes and Trypanosomiasis. 

 Trans, by Nobarro, Chicago. 



2 There is here, as with bacteria, difference of opinion among 

 authors as to where these organisms should be placed. See foot note, 

 page 8. 



