iv MALARIA PARASITE 287 



forming eight elongated and nucleated bodies or sporo- 

 zoites, which become set free by the bursting of the 

 enclosing cyst-membrane : apparently, however, they 

 must enter a fresh host before this takes place. It is 

 probable that earthworms become infected by swallowing 

 spores which have been set free into the earth, either 

 directly, owing to injury to the host, or by means of the 

 excrement of worm-eating birds. The spore-membrane is 

 probably acted upon by the digestive juices of the worm, 

 the liberated sickle shaped and motile sporozoites passing 

 through the wall of the intestine and so reaching the 

 sperm sacs, when they enter some of its cells (G), 

 absorbing so much of the contents of the latter that there 

 is not sufficient left to form complete sperms. They 

 then become set free among the cells of the sperm-sacs, 

 surrounded by the tails of the degenerated sperms which 

 look like an investment of stiff cilia (H) ; they continue 

 to grow until the fully developed trophozoite-stage is 

 again reached and the life-cycle thus completed. 



It will be seen that in Monocystis there is no special 

 method for the parasite to be regularly transferred from one 

 host to another. In some Sporozoa, as in many parasites 

 belonging to other and higher groups, infection is provided 

 for, and the life-history is still further complicated owing 

 to the fact that two hosts, belonging to different groups, 

 are necessary for the completion of the cycle. A good 

 example is the malaria-organism (Plasmodium), which lives 

 within the red blood-corpuscles of man. This sporozoan 

 multiplies rapidly by multiple fission, thus producing very 

 numerous merozoites, which attack other blood-corpuscles 

 of the same host. Should a drop of blood, containing 

 merozoites, be sucked up by a mosquito (Anopheles), the 

 parasites undergo a different mode of development. From 

 two different kinds of gametocytes, ovum-like megagametes 

 and sperm-like microgametes are respectively formed 

 (P- 2 75)- After conjugation, the active zygotes penetrate 

 the epithelium of the mosquito's stomach, become encysted, 

 and give rise to an enormous number of sporozoites, which 

 reach the salivary ducts of the mosquito. If the insect 

 then bites another human being, the sporozoites are injected 



