MAN AS FOOD FOR ANIMALS 45 



a firm membrane or shell within which they can resist dessi- 

 cation, often for long periods. Some of these internal proto- 

 zoans, especially one species of amoeba, are injurious to health, 

 and not infrequently their activities are attended with fatal 

 results. Of the others some appear to be more or less injurious, 

 and some quite harmless. Many of them live also in various 

 animals. 



Very interesting creatures are the so-called trypanosomes, 

 and often very deadly, especially in Africa where they are the 

 cause of that terrible scourge known as the sleeping sickness. 

 These pass through part of their life history in the tsetse flies. 

 Some of the American kinds are incubated by certain assassin 

 bugs. 



Of the protozoans which reproduce themselves by the for- 

 mation of numerous spores, the so-called sporozoans, by far 

 the most important are the several kinds of malarial organisms 

 which Hve on the red corpuscles of the blood and are injected 

 by the bite of certain species of mosquitoes. There are, how- 

 ever, various other sporozoan parasites in the human body 

 which attack different tissues, some with often very serious 

 consequences. Many of them are transmitted by lice, sand- 

 flies, ticks, mites, etc. 



The numerous sorts of spirochaetes form a very interesting 

 group of minute creatures. Some of these are free living, 

 some commensals or mess mates with various molluscs, some 

 harmless parasites of man and various animals, and some 

 malignant parasites, the causative organisms of some of the 

 most horrible of the diseases which infest man, as well as of 

 many diseases of milder form and lesser importance. 



