PROTOZOA IN GENERAL 279 



certain way not because of power of choice, but because they cannot 

 behave in any other way. The Protozoa are controlled in their be- 

 havior largely by tropisms. 



Economic Relations of Protozoa 



Man has not yet found a way or need to eat Protozoa directly as 

 food material, although he does draw on it indirectly by a food 

 chain including water fleas, larger crustaceans, and fish. Too, the 

 protozoans are not classed as predators on man as would be the 

 lion, but many of them are parasites. Many diseases of man and 

 animals are caused by Protozoa. Most of the diseases oi this origin 

 are more prevalent in the tropical and subtropical regions of the 

 earth. Such diseases may attain sufficient importance to render 

 large portions of continents uninhabitable by man; for example, 

 much of northern South America and Central America was, at one 

 time, ruled by yellow fever and malaria, and the same applies for 

 sleeping sickness in Africa. There are other Protozoa that render 

 water unfit for drinking or help fertilize the soil. 



Amoebic Dysentery. — Ulcers on the inside of the walls of the 

 intestine of man are caused by this disease. There results from this, 

 severe diarrhea and dysentery. From the intestine the infection, if 

 allowed to continue, will be carried to the liver where serious ab- 

 scesses are formed. The infection is usually obtained directly through 

 drinking water or eating food which has been contaminated with 

 the encysted organisms from fecal matter. About 10 per cent of our 

 population are said to be carriers of these organisms. The causal 

 agent is one of the Amoebae, Endamoeha histolytica (see Fig. 174), 

 and it can be rather successfully eliminated from human beings by 

 use of such drugs as emetine, carbarsone, and chiniofon, administered 

 by a physician. Some other Amoebae have been found in human be- 

 ings, but, so far as known, they are not pathogenic. Endamoeha coli, 

 Endolimax nana, and Endamoeha gingivalis are such examples. 



Foraminifera, which is an order in class Sarcodina, has some eco- 

 nomic importance because of the limestone which is formed by the 

 concentration of the material of the dead tests or shells. A genus 

 by the name of Glohigerina is one of the best known members of the 

 group. It is about the size of a pinhead, and as it dies, it sinks to the 

 bottom of the ocean where the mass forms the globigerina ooze which 

 hardens into solid chalk. 



