Figure 1. Structure of apicomplexans as revealed by electron microscopy. Upper left: a metrocyte, or mother cell, in which the elements of the apical complex are minimally expressed. Upper right: a zoite in which all of the elements of the apical complex are seen. Lower: A detailed view of the conoid and associated elements such as the polar rings and the subpellicular tubules which extend about two-thirds of the length of the zoite. (Courtesy of C. A. Speer.)

Figure 2. The cytostome or micropore of Toxoplasma gondii as seen in transmission electron microscopy. The cytostome ingests food provided by the parasite's host cell. (Courtesy of C. A. Speer.)

Figure 3. A meront of Sarcocystis sp. in a hepatocyte of a mouse as seen in transmission electron microscopy. (A) Merozoites that have completed budding from (B) a centrally located residual body. (Courtesy of C. A. Speer.)

Figure 4. Endodyogeny in zoites of Neospora caninum, an economically important parasite of domestic animals. (A) A zoite not undergoing endodyogeny. (B) Partially formed progeny cells in which the nucleus is being divided into two progeny cells. (C) Two progeny cells completely formed and probably ready to be released. (Courtesy of C. A. Speer.)

Figure 5. The life cycle of Eimeria tenella of the chicken. This apicomplexan is typical of those coccidia that parasitize the intestinal tracts of their hosts. Sporogony takes place outside of the host, and both merogony and gamogony occur within cells of the intestine. (From Marquardt et al., (1999).)

Figure 6. An oocyst of Eimeria magna of the rabbit, Oryctolagus cuniculi, as seen in interference contrast light microscopy. The oocyst is infective if ingested by a rabbit. In the genus Eimeria, each oocyst has four sporocysts each of which contains two sporozoites. (A) Oocyst wall. (B) Sporocyst. (C) Sporozoite. (Courtesy of C. A. Speer.)