THE PROTOZOA-THE DAWN OF LIFE 25 



parasite is almost invariably transmitted from the natural passive 

 hosts to the newly introduced animal by certain species of 

 blood-sucking invertebrates (insects and leeches). It was in 

 this way that the recently introduced cattle and horses of the 

 settlers in South Africa were attacked by nagana or tsetse-fly 

 disease, as they passed through belts of country inhabited by herds 

 of big game and the blood-sucking insects which prey upon them ; 

 for it has been shown that the nagana parasite (Trypanosoma 

 brucii] is almost certainly to be found in the blood of the native 

 buffalo, the gnu, and the koodoo. Nagana is transmitted from 

 one animal to another in South-east Africa by the blood-sucking 

 tsetse-fly, Glossina morsitans ; while Trypanosoma gambiense, the 

 cause of sleeping sickness, is conveyed from man to man by another 

 tsetse-fly, the Glossina palpates. 



Trypanosomes possess one or two flagella, both originating 

 close together at or near the anterior end of the body, one being 

 free and directed forwards, while the other turns back and is 

 attached by means of an undulating membrane to the side of the 

 body for the greater part of its length, its free end being directed 

 posteriorly. Should only one flagellum be present, it appears to 

 be invariably attached in this manner. Reproduction appears 

 most frequently to take place by binary longitudinal fission, 

 though multiple division or segmentation also occurs. There is 

 need for much further investigation of the life-cycle of these para- 

 sites, as comparatively little is known in most cases about the 

 true alternate host (transmitting agent), in which/ definite phases 

 of the life-cycle, including probably sexual conjugation, take 

 place. Thanks, however, to the researches of Minchin, Schaudinn, 

 Laveran, Bruce, Plimmer, Woodcock, and other well-known 

 protozoologists, great additions have been made to our knowledge 

 of the Haemoflagellates and of the hosts, passive and casual, which 

 they inhabit. 



The Infusoria form a complex group of the Protozoa, and 

 were originally grouped under this title from their frequently 

 inhabiting fluids containing an infusion of organic matter. Their 

 food consists chiefly of microscopic forms of animal and vegetable 

 life even more minute than themselves, and of particles of organic 

 matter that may be floating in the water they inhabit; many 

 thrive in water which contains putrefying matter, feasting upon 



