iv MALARIA PARASITE 287 



fresh host before this takes place. It is probable that earth- 

 worms 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 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 (Laverania\ which lives within the red blood-cor- 

 puscles 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, the parasites undergo >a different 

 mode of development. From two different kinds of gametocytes, ovum- 

 like megagametes and sperm -like microgatnetes are respectively formed 

 (p. 275). 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 into the wound and again attack the red corpuscles. 



The study of the foregoing living things and especially 

 of Bacteria, the smallest and probably the simplest of all 



