TRYPANOSOMA 69 



horse disease in South America ; T. cruzi, the cause of a disease in 

 children in the same continent, and so forth. The same species has 

 sometimes been described from different parts of the world and 

 given more than one name, so that there is much confusion. Not 

 all trypanosomes are carried by tsetse flies. Many, perhaps all, 

 have a wild host in which they are harmless, though in the un- 

 accustomed bodies of men or domestic animals they are highly 

 dangerous. Formerly no treatment was of any avail against them ; 

 recent research has produced several synthetic drugs, mainly 

 organic compounds of arsenic and antimony, which can cope with 

 at least the African species, but the best way to combat them is to 

 avoid the attacks of the insects which transmit them. Thus the 

 clearing around places frequented by human beings of the bush 

 which is the haunt of Glossina has led to a decrease in the number 

 of cases of sleeping sickness. 



PLASMODIUM 



MALARIA PARASITES 



A much more widespread though less dangerous type of 

 disease than sleeping sickness is malarial fever or ague. This is 

 brought about by a minute protozoan parasite known as Plas- 

 modium,'^ belonging, like Monocystis, to the Sporozoa. The 

 dangerous stage of the parasite corresponds to the trophozoite 

 of Monocystis. It lives in the red blood corpuscles, and is at first 

 a round body with the appearance of a ring (Figs. 41, 42), owing 

 to the presence of a large (non-contractile) vacuole in its middle. 

 It has a single nucleus and no mouth, and must absorb food from 

 its surroundings through the surface of its body. As it grows, it 

 loses the ring-like appearance and forms in its cytoplasm granules 

 of pigment, which is no doubt derived from the haemoglobin of 

 its host. At the same time P. vivax puts out pseudopodia. When 

 the parasite is ready to reproduce, it is known as a schizont. Its 

 reproduction, called schizogony, takes place by multiple fission. 

 The pseudopodia are withdrawn and the nucleus divides repeatedly 

 tiU there are present some sixteen smaller nuclei. These lie in the 

 outer part of the body, and most of the cytoplasm now gathers 



1 It is unfortunate that this name is also in use to denote a type of relation 

 of nuclei to cytoplasm — namely, that in which a syncytium is formed by the 

 fusion of free cells (p. 499) — which, as it happens, is not found in the malarial 

 parasite. 



