CHAPTER 20 



THE PHYLUM PROTOZOA 



Of all the animal phyla, the members of the phylum Protozoa are 

 probably the most abundant and widespread. Some species are capable 

 of living in nearly any imaginable habitat. All bodies of fresh and salt 

 water have their teeming populations ; the miniature pool formed after 

 a rain in time will develop its protozoan fauna. The water between the 

 particles of soil abounds in them — a single gram of soil may harbor a 

 million protozoans. So many and so abundant are the parasitic forms 

 that there is scarcely an animal without its peculiar fauna. Despite 

 this tremendous abundance of forms, the protozoans were among the 

 last of the animals to be observed by man. The reason for this delay 

 was due to the fact that their discovery depended upon the invention 

 of the microscope. Again, the versatile Leeuwenhoek, the Dutch mi- 

 croscopist, is given credit for some of the first descriptions of these 

 remarkable organisms. 



These animals, though consisting of but one cell, must be considered 

 true organisms. They must be compared to the entire metazoan or- 

 ganism, not to the highly differentiated individual cell of the metazoan. 

 This single unit carries on the processes of digestion, reproduction, ex- 

 cretion, etc., which are also characteristic of the metazoans. For this 

 reason, some zoologists prefer to designate the protozoans as acellular 

 rather than as unicellular forms. 



Among the metazoans, there are many organs and organ systems 

 for carrying on vital processes. By definition, an organ consists of 

 many cells ; thus no structure within the protozoan is comparable to an 

 organ. Instead there are specialized cytoplasmic structures known as 

 organelles (little organs) which carry on the various physiological ac- 

 tivities of the organism. There may be a great many different organelles, 

 such as food vacuoles, for the digestion of food, water vacuoles for the 

 control of the water content, and special locomotor devices. 



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