74 ELEMENTS OF BIOLOGY 



and the supporting organism is called the host. This sort of rela- 

 tion between organisms is of tremendous importance to Man, for 

 many human diseases are actually the parasitization of the human 

 body by some other organism that either destroys body substance 

 or by reason of its poisonous metabolic products produces violent 

 disturbances which may eventuate in the death of the host. Organ- 

 isms which produce disease as a result of their invasion are called 

 PATHOGENIC, in distinction to non-pathogenic types. The metab- 

 olism of such organisms is saprophytic or saprozooic; their source 

 of energy is from materials that are already formed and partially 

 utilized by the host. 



Some species of Paramoecium are parasitic in higher animals. 

 Other Infusoria also occur as parasites, some of which are pathogenic. 

 But the most important pathogenic Protozoa are found among the 

 class Sporozoa. The particular sporozoon which is the most com- 

 mon pathogen of the group is the one causing human malaria. 

 There are several species of malarial Sporozoa, each producing a 

 somewhat different type of disease. 



Malarial Parasite. The life history of the malarial parasite, 

 all of which belong to the genus Plasmodium, is rather complicated, 

 for it requires two hosts in order to complete its life, Man and the 

 mosquito. Apparently all modern races of Man serve as suitable 

 hosts, but not all types of mosquitoes serve as the other host, only 

 the members of the genus Anopheles, a widely distributed genus, 

 however. In the blood stream of a human having the disease are 

 tremendous numbers of small sporozoa, known in this stage of 

 their life cycle as trophozoites (Fig. 23). These live in the red 

 blood cells. If a mosquito of the proper variety bites the person, it 

 withdraws some of these forms with the blood. In the mosquito all 

 of the sporozoa are destroyed except one type, the gametocytes. The 

 gametocytes are of two sorts, one destined to form numbers of 

 smaller microgametocytes, designated as sperms, and the other to 



