PLACE OF MICROORGANISMS IN NATURE 21 



capable of living and multiplying within the living 

 animal body, sometimes to its detriment, while sapro- 

 phytes live on dead matter and may be found in nature 

 everywhere in air, soil, water. The body upon 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, 

 demand animal juices for their nutriment. Among 

 the protozoa this obligate parasitism exists quite 

 extensively, and many forms cannot live at all if their 

 normal cycle of life within the animal body be dis- 

 turbed. Indeed, we know the existence of many species 

 only because they pass through a certain development 

 in insects, then in higher animals and back again in 

 insects; that is, we only recognize them when they pro- 

 duce disease (see Malaria). The saprophytes include 

 the vast number of organisms having important func- 

 tions among the higher vegetables and the growth of 

 these in soil. It has been suggested that at one time, 

 now long past, all bacteria may 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 



