MONOCELLULAR ANIMALS-THE PROTOZOA 



127 



before the cause was discovered. By 1909 

 it was established that the tsetse fly was 

 not only a mechanical vector but an inter- 

 mediate host in which the parasite went 

 through a definite part of its life cycle. 



The trypanosome is sucked up into the 

 gut of the fly during its blood meal ( Fig. 7- 

 20). Here it undergoes some changes in 

 morphology and eventually makes its way 

 into the salivary glands, a common proce- 

 dure among parasites of blood-sucking in- 

 sects. Some time later, if the fly bites 

 another person, the parasite is injected 

 alone with the saliva. It remains free in the 

 blood for a time but finally makes its way 

 into the fluid surrounding the brain and 

 cord. In this stage the metabolic products of 

 the parasite have a paralyzing effect on the 

 person, eventually causing "sleep" from 

 which he usually never awakens. 



Vast regions of Africa are denied man be- 

 cause of the ravages of this disease. Like so 

 many diseases, the parasite is ineffective 



against the local wild animals, which, how- 

 ever, act as reservoirs, always keeping the 

 parasite circulating in goodly numbers. For 

 this reason the wholesale destruction of the 

 tsetse fly is the only satisfactory control 

 measure; this is not at all impossible now 

 with the recent discovery of such effective 

 insecticides as DDT. 



The future development of a large fertile 

 area of Africa must await the control of this 

 disease. This has become an urgent need in 

 the face of an expanding world population. 

 It was once thoucrht that the white man 

 could not thrive in the tropics, but data col- 

 lected during the past war, when large 

 numbers of white men lived in tropical 

 countries for several years, have disproved 

 this conjecture. There is no reason why he 

 cannot be as successful in the tropics as else- 

 where, once we gain control of the tropical 

 diseases to which the white man is very re- 

 ceptive. Much of the backwardness of other 

 races living in the tropics is due to the mur- 



DifFlugia 



Plagiopbrys 



Arcella 



Actinopbrys sol 



Fig. 7-21. Various types of fresh-water sarcodinids. 



