THE PROTOZOAN SUBGROUPS 337 



Order I. Coccidia 



Sporozoa (p. 336). — The Coccidia are cytozoic or cell-infesting para- 

 sites, attacking epithelium of in\-ertebrate and vertebrate animals. Re- 

 production is by schizogony and by sporogony, the asexual and sexual 

 generations alternating in the life cycle. In species parasitic to domestic 

 animals the fertilized cell produces sporoblasts covered by a sporo- 

 cyst membrane. 



Life History. — The life cycle is similar to that of the malaria organ- 

 isms except that no arthropod intermediate host is required for the 

 sexual reproduction. Infection with Coccidia is with the encysted stage 

 (oocj'st) by way of the mouth. Hence the parasites are almost exclu- 

 sively found in the epithelium of the aliment ry canal and organs con- 

 nected with it. Reaching the stomach and duodenum, the oocyst is 

 acted upon by the digestive juices and the sporozoites contained in the 

 cyst are hberated. These enter the epithelial cells of the mucosa. Within 

 the cells they lose their spindle form and enter the stage of the tro- 

 phozoite in which they grow to a size depending somewhat upon that 

 of the invaded cell. By the process of schizogony the trophozoite di- 

 vides into a number of small protoplasmic masses which are the mero- 

 zoites or asexually formed spores. These invade other cells and in the 

 same manner grow into another generation of merozoites. Bj^ many 

 repetitions of this cycle a large number of cells are invaded and de- 

 stroyed, and the death of the host animal may follow as a result. After 

 a number of asexual cycles some of the merozoites do not grow and di- 

 vide into another generation of merozoites, but develop into stages which 

 begin the sporogonous or sexual cycle. In this process the female tro- 

 phozoite instead of dividing develops into an egg or macrogamete. The 

 male trophozoite, by division, forms minute male repi-oductive elements 

 or microgametes. By their motility the microganietes reach and fer- 

 tilize the macroganietes which, becoming surrounded by a resistant 

 membrane, arrive at the stage of the oocyst. Within the oocj'st a num- 

 ber of spores may be formed, each inclosed in a protecting membrane 

 and constituting a sporocyst. By division each sporocyst forms two 

 or more sporozoites, and thus the sexual cycle is completed. Where the 

 parasites are in the epithelium of the alimentary tract or its comaiiu- 

 nicating organs, the oocysts pass to the exterior with the feces. In other 

 cases it may be that they can only reach the outside after the death and 

 disintegration of the host. 



The effect of coccidiosis upon the animal is brought about by the ex- 

 tensive destruction of cells resulting from the repeated production of 

 merozoites by schizogny. This progressive reproduction and cell de- 

 struction would in every case result in the death of the animal were it 

 not that the number of schizogonous generations is limited. The cell 



