PART IV. 

 PATHOGENIC PROTOZOA. 



CHAPTER L. 



GENERAL CONSIDERATION OF PROTOZOA CLASSIFICATION- 

 MORPHOLOGY AND REPRODUCTION. 



Definition and Morphology. Protozoa represent the lowest and most 

 simple form of animal life. The organisms, however, vary much 

 in their morphology. While unicellular and occasionally united in 

 colonies of unicellular individuals, they may have a body with a 

 variety of parts, or small organs called organella, serving different 

 purposes. In this respect protozoa when compared with the lowest 

 forms of vegetable life (the bacteria) are considerably higher in 

 structural differentiation. In a summary manner protozoa may be 

 defined as unicellular animal organisms. Calkins gives the following 

 more elaborate definition : "A protozoon is a primitive animal organ- 

 ism, usually consisting of a single cell, whose protoplasm becomes 

 distributed among many free living cells. These reproduce their 

 kind by division, by budding, or by spore formation, the race thus 

 formed passing through different form changes, and the protoplasm 

 through various stages of vitality collectively known as the life cycle." 



Protozoa, like the lowest forms of vegetable life, are very prevalent 

 in nature, but they do not find the conditions necessary for or favor- 

 able to their nutrition and multiplication as easily as^the bacteria. The 

 lowest forms of animal life being highly differentiated and representing 

 a great variety of morphologic features, the phylum protozoa has 

 been divided into "several subphyla, numerous classes, subclasses, 

 orders, families, and genera. 



Protozoa, like bacteria, live either in the outside world or as para- 

 sites on or in other organisms. They may exist as harmless commen- 

 sales or they may be pathogenic parasites. 



The number of protozoa known to be pathogenic to man and 



the higher animals is comparatively small. As these belong to a 



few families only, medical and veterinary studies of protozoa, aside 



from a general survey of the phylum protozoa, may be confined to 



34 



