THE LIVING ORGANISM 215 



structures, staining reactions, and bio-chemical 

 reactions, again give many divisions, which 

 need not here be named. 



The micro-organisms hitherto dealt with are 

 all usually referred to a subdivision of the 

 fungi amongst the plants (Schizomycetes). 

 There are in addition to these, many thousands 

 of types of botanical micro-organisms belong- 

 ing to other families, some parasitical, and 

 others chlorophyllaceous, and building up their 

 own organic materials. 



There are also many thousands of types 

 well known, in addition to many as yet 

 unstudied and unclassified, of unicellular or 

 protozoan animals, found chiefly in fresh or 

 marine water, in the fluids of soil, in higher 

 plants or animals, or in decaying organic 

 organisms or fluids. 



Many of these protozoa occurring free in the 

 blood or body-fluids or enclosed in the living 

 cells of plants or animals, are the now familiar 

 causes of many important diseases. Such 

 animal parasites, unicellular, and of micro- 

 scopic dimensions, seem especially to take 

 a large part in the causation of tropical 

 diseases, such as malaria and sleeping sick- 

 ness. It is now known that several of these 

 protozoan parasites are borne from one 

 host to another by insects, and possess a 



