THE PROTOZOA AS PARASITES OF MAN 151 



a number of other trypanosomes — T. rhodesiense which 

 causes a sleeping-sickness in South Central Africa, 

 T. equinum which causes a horse disease in South America, 

 T. cruzi, the cause of a disease in children in the same con- 

 tinent, and so forth. Many, perhaps all, of these have a 

 wild host in which they are harmless, though in the 

 unaccustomed bodies of men or domestic animals they are 

 highly dangerous. Drugs have proved of little use against 

 them, and the only 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. 



A much more widespread though less dangerous type of 

 disease than sleeping-sickness is malarial fever. 

 Parasites. "^ ms * s Drou ght about by a minute protozoan 

 parasite known as Hamamoeba or Plasmodium^ 

 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, 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, 

 puts out pseudopodia, and forms in its cytoplasm granules 

 of pigment which is no doubt derived from the haemoglobin 

 of its host. When it is ready to reproduce, it is known as 

 a meront or schizont. Its reproduction, called merogony or 

 schizogony, takes place by multiple fission. The pseudo- 

 podia are withdrawn and the nucleus divides repeatedly 

 till there are present some sixteen smaller nuclei. These lie 

 in the outer part of the body, and most of the cytoplasm 

 now gathers round them so as to form a rosette of little, 

 uninucleate individuals — the merozoites or schizozoites — 

 which surround some " residual protoplasm " containing 

 the pigment granules. Next the shell of the red corpuscle 



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 syncitium is 

 formed by the fusion of free energids (p. 119) — which, as it happens, is 

 not found in the malaria parasite, 



