Phylum Protozoa 363 



THE CLASS SPOROZOA 



The members of the class Sporozoa are all parasitic and are ex- 

 tremely widespread in their distribution. They occur in the organs 

 of animals of nearly all phyla. As they are parasitic, their nutrition 

 is saprozoic, with the food being absorbed directly from the host through 

 the cell membrane. They lack contractile vacuoles as the medium in 

 which they dwell is isotonic. Respiration and excretion likewise occur 

 directly through the cell membrane. 



Most of the members of the class Sporozoa have complicated life 

 cycles involving both sexual and asexual stages. Some species occur 

 in more than one host with the asexual stage in one and the sexual stage 

 in another. In the one host, the form which reproduces by asexual 

 methods is known as the trophozoite. While the trophozoite has no 

 special organs of locomotion, it is often able to move by amoeboid move- 

 ments. By repeated mitosis the cell becomes multinucleate and is known 

 as a schizont. This breaks into many new individuals or asexual spores 

 known as merozoites, each of which invades new cells of the host. 

 This process is called schizogony. As no gametes are involved, this 

 is a type of asexual reproduction. At times, micro- or macrogametes 

 may be produced. These unite to form zygotes; these, in turn, form 

 oocysts which contain spores that develop by the process known as 

 sporogony. At this stage, new hosts can be infected. 



Many of the members of the Sporozoa are extremely important 

 because of their effect upon man and his domestic animals. Plasmodium 

 sp., the causative agent of malaria, has influenced the settlement of 

 entire areas and modified the course of civilization. In many domesti- 

 cated animals, members of the order Coccidia may have a devastating 

 effect. In rabbits and poultry, Eimeria sp. (a coccidian) causes a path- 

 ological condition of the epithelial tissues. In rabbits this occurs in 

 the bile duct, while in poultry it is in the intestine. 



The Coccidians. — Representatives of the order Coccidia occur 

 in the epithelial cells of both vertebrates and invertebrates. The life 

 cycle of a typical coccidian may be illustrated by Eimeria stiedae, the 

 parasite of the epithelium of the bile duct of rabbits. The trophozoite 

 appears as a small rounded cell in the epithelium. By schizogony many 

 small merozoites are produced. These asexual spores invade new cells 

 and spread the infection within the same host. After invading the 

 new cells, each merozoite develops into the typical trophozoite. 



