PLACE OF MICROORGANISMS IN NATURE 21 



which a parasite lives is called the host. There are a 

 few of the parasites that can carry on a saprophytic 

 existence for a short time (facultative parasites), while 

 others (obligate parasites), such as the organism of 

 influenza, require animal juices for their nutriment. 

 Among the protozoa this obligate parasitism exists 

 quite extensively. Many forms cannot live at all if 

 their normal cycle of life within the animal body be 

 disturbed. The saprophytes include the vast number 

 of organisms having important functions among the 

 higher vegetables and the growth of these in soil. It 

 has been suggested that at one time, now long past, 

 all bacteria might have been saprophytic. 



The general remarks concerning parasites apply 

 alike to protozoa and bacteria, but in medicine there is 

 at the present time more interest in the bacteria. For 

 this reason only a few diseases caused by protozoa are 

 important. 



In order that the positions these unicellular forms 

 occupy in the living world may be known and used 

 for reference to large works, their biological classifica- 

 tion is given here. The lowest of the orders among the 

 plants is called Thallophyta. This is divided into 

 Algse, Lichens, and Fungi. The Fungi are divided 

 into Hyphomycetes (moulds), Blastomycetes (yeasts), 

 and Schizomycetes (bacteriaceae or bacteria). This 

 family is divided into Cocci, Bacilli, and Spirilla. 



Protozoa, the lowest animal class, present the orders 

 Sarcodina, Mastigophora, and Sporozoa, which con- 

 tain nearly all the forms of interest in this work. 



