Chap. 21 THE PROTOZOANS 437 



more or less immune to injury from its parasites. Natural immunities occur on 

 every hand commonly because of chemical content or structure, or both. 

 Immunities to protozoans include that of wood-eating insects which are not 

 only immune but are benefited by the flagellates that live in their digestive 

 tracts. 



Trypanosomes and Sleeping Sickness. The most widely injurious of patho- 

 genic trypanosomes are those that cause the African sleeping-sickness of man 

 and domestic cattle, not to be confused with the sleeping-sickness or encepha- 

 litis, a paralysis, that has no relation to trypanosomes. The African disease 

 occurs throughout central Africa and is due either to Trypanosoma gambiense 

 or its near relative Trypanosoma rhodesiense. They are transmitted from man 

 to man or from wild mammals to man by blood-sucking tsetse flies (Glossina) 

 that inject the parasites into the blood when they bite just as mosquitoes inject 

 malarial parasites into the blood. The trypanosomes go through an essential 

 part of their life history in the body of the tsetse fly. This takes 14 days at 

 the end of which they have bored their way into the salivary glands of the fly 

 and are ready to enter the mammalian blood and cerebrospinal fluid (Fig. 

 21.10). The big game animals of Africa are the reservoirs for these parasites 

 and the only known transmitters are the tsetse flies. Like wild rats and fleas, 

 the big game animals and tsetse flies have become practically immune to tryp- 

 anosomes. Only man and domestic mammals are mortally harmed, an indi- 

 cation that for them the trypanosomes are still relatively new parasites. 



Class Sarcodina or Rhizopoda 



The Sarcodina — amebas, radiolarians, foraminiferans, and others — move by 

 means of flowing protoplasm, many of them by pseudopodia. They feed on 

 bacteria, microscopic plants and animals and next to the bacteria, algae, and 

 phytoflagellates are basic food supplies. Fresh-water species have one or more 

 contractile vacuoles; salt-water and parasitic species usually lack these alto- 

 gether. Reproduction is mainly asexual, by binary fission or by budding; 

 sexual reproduction is known in comparatively few species, such as fora- 

 miniferans. 



The Ameba 



Habitat. Fresh-water amebas live in ponds and streamsides, on decaying 

 leaves and slimy stems. All of them likely to be found there are microscopic. 

 The only way to see them is to collect pond water and plant debris, let it stand 

 several days, and then examine it bit by bit under a microscope. The rela- 

 tively large amebas of most laboratories have been grown in cultures, pur- 

 chased from specialists in rearing them. Ameba proteus and Ameba caroUnensis 

 (also called Chaos chaos), the giant ameba, are commonly used for study. 



Appearance. At first glance, through the microscope, an ameba seems to be 



