2 TROPICAL MEDICINE AND HYGIENE 



and to vary from time to time without any obvious 

 reason. With closer study the reasons for such varia- 

 tions can sometimes be traced. The varying results of 

 prophylactic measures directed against such diseases, 

 though on the whole satisfactory, are often explicable by 

 the variations in these factors when the causes of the 

 variations are known. 



The distinction between plants and animals, so obvious 

 in the higher members of these kingdoms, is less definite 

 in the unicellular organisms. Such distinctions as the 

 presence or absence of chlorophyll, the absorption or 

 assimilation of nitrogen and carbon from their inorganic 

 compounds, or only from higher organic compounds, 

 are not conclusive. Those organisms most closely related 

 to the vegetable kingdom and those that appear to be 

 animal may either be motile or non-motile. In so many 

 instances is it impossible to determine whether the 

 lowly unicellular organisms are animal or vegetable, 

 that Haeckel proposes to make a separate kingdom of 

 such forms which he calls PROTISTA. 



The unicellular organisms approximating in most of 

 their characters to the animal kingdom are known as the 

 Protozoa. 



In warmer countries the diseases due to parasites 

 with characters in the main animal are of more special 

 importance than those caused by parasites of the same 

 division in cold climates. The diseases of cold climates 

 are usually due to parasites like bacilli, which are allied to 

 the vegetable kingdom. Many of these diseases are world- 

 wide in their distribution. 



The protozoa for this reason are first considered. 

 Protozoa are unicellular organisms. The cells may be 

 aggregated together in masses, or may occur singly. 

 Frequently parts of the cells are specially modified for 

 special purposes, such as locomotion, so that flagella or 

 cilia are formed, whilst in other instances a part only 

 of the cell is contractile and exhibits amoeboid movement. 



Many of the protozoa are non-parasitic; others are 



